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Robin Craw's recent analysis of biogeographical studies in New Zealand (1980) is timely. Like Craw, I am impressed with Sir
Sir
Such a model of speciation is central to Darwinian, as well as neo-Darwinian theory. In more recent times it has been referred to as the “Dumbell” model of speciation (Stebbins, 1969) and in paleontology as “phyletic splitting” (Simpson, 1947). Fleming has invoked this model to account for the origin of a number of species within the cockle genus Bassina (Fleming, 1958) and, 17 years later, of the endemic parrot genus Nestor (Fleming, 1975). In keeping with such a theory Fleming (1958) describes speciation as “… the splitting of one species into two separate species …” and this is “… preceded by some form of spatial, generally geographic, isolation, such as is entailed in the formation of geographic races”. Darwin (1859) himself said that species are “… only strongly
common ancestor. He saw as central to his theory the hypothesis that species arise by slow accumulation of differences as a result of natural selection. He maintained that the extreme imperfection of the geological record largely explains why intermediate forms which would make up the complete sequence are not found. Hence he remarked “He whorejects these views on the nature of the geological record, will rightly reject my whole theory” (1859, p. 342).
An alternative model of speciation (Fig. 1b) argues that species arise from small populations which have become geographically separated from an ancestral species. Like gradualism this also can properly be described as an allopatric model. This founder model of speciation differs from the gradualist Darwinian model in other ways apart from the size of the population involved. The founder model maintains that species remain stable through time and hence the two species do not diverge, but the ancestral species remains the same and the small population undergoes rapid change, and a speciation event results. It is possible, of course, that the small population involved is not a peripheral isolate of an ancestral population but the ancestral population itself. That is the species range and population size is reduced drastically and a small relic population survives and a speciation event results.
Let us return to the Nestor parrots, the Kaka and Kea to exemplify the alternative model. Fleming (1975, 1979) postulates that during the early Pleistocene glaciation a sea barrier separated two Proto-kaka populations in the North and South Islands. The two populations became differentially adapted to the alpine South Island conditions and the more temperate North Island. These populations consequently speciated into the Kea and North Island Kaka, and the South Island subspecies of the Kaka is the result of reinvasion from the north.
An alternative view, and one which has not been previously considered is that both the Kea and the South Island “subspecies” of the Kaka may be derived from small isolates of the Kaka, which is now restricted to the North Island but was once distributed over both islands. It seems plausible to suggest that a small population of such a species was trapped on the western side of the Southern Alps and consequently speciated. We now recognise this form as the Kea. As evidence for this, subfossil deposits of Keas are much less common than Kaka (P. Millener, pers, comm.), indicating that the former has always been restricted to alpine areas. A population from the same ancestral species may have been isolated in the northern parts of the South Island and have given rise to the South Island Kaka population of today. Alternatively, as Fleming suggests, it may represent a founder population derived from the North Island. These relationships are represented in Fig. 2 and illustrate the founder effect hypothesis.
This founder effect model of speciation has resulted in a recent revolt in paleontology against the ruling paradigm of “phyletic gradualism”. The ideas of punctuated equilibria (Eldredge and Gould, 1972; Gould and Eldredge, 1977) and species selection (Stanley, 1975, 1978) argue that the fossil record, in many groups at least, is compatible with the theory that species arise in small populations which became geographically separated from the ancestral species. Species are argued to arise quickly in
Another entirely distinct line of evidence indicates that, at least in some groups, species arise by the chance separation of a few individuals (perhaps even a single female carrying male sperm) from an ancestral population. The work of Carson and his co-worker (1970, 1976) provides compelling evidence that a large number of species of Drosophila endemic to the Hawaiian Islands have arisen in this way.
I believe therefore that there is good evidence that species can arise via small isolated populations. Equally I find the available evidence that large populations can speciate vague, and open to the possibility that founder-effects are involved. Species possess various systems which result in little genetic change through time and which are responsible for the characteristic stasis of species (Lerner, 1954; Carson, 1975; Paterson, 1978). My own view can be well summarised by Powell's (1947) comment when discussing the evolution of New Zealand land snails. “It would seem therefore that the numerically superior populations more or less isolated by topographic boundaries preserve their individuality by pressure of numbers, and that new forms arise primarily and under exceptional conditions through the accidents of small numbers becoming isolated from the main areas of distribution”.
In view of the intimate Darwinian connection between adaptation and speciation the question must be asked:
A decidedly expansionist ideology seems to be basic to the view of “adaptive radiation”. Huxley (1955, p.389) describes the process in this wav:
In keeping with the view that speciation is a goal of evolution, species are described as groups of organisms which are reproductively isolated from other groups. The isolation is “achieved” by the creation of “Isolating Mechanisms” and these are responsible for “protecting the integrity of species”. Since “Biological Isolating Mechanisms” (Mayr, 1942) need not be called into action if the populations involved are geographically separated, the question of the status of such populations is largely academic. Dobzhansky (1950), in fact referred to Isolating Mechanisms as belonging to two major categories geographic and reproductive.
This “Isolationist” view of species has recently come under critical attack. Paterson (1978, 1980, 1981) has argued that this is inappropriate and misleading and termed it the “Isolation Concept”. He has introduced an alternative Recognition Concept which views a species as a group of organisms which are tied together by a common communication system between males and females. Paterson has termed this the Specific-Mate Recognition System (SMRS). Hence, according to this view, the so-called “protection of species integrity” is an incidental consequence or effect (sensu Williams, 1966) of a change in the SMRS of a population. Speciation, according to this concept, is not a goal or purpose of evolution
Fleming's (1975) study of divergence in the scrub cicadas (Genus Kikihia) appears to use the same reasoning as Dobzhansky. It is argued that subspeciation occured in South Island populations of these cicadas which were restricted to coastal areas of scrub vegetation during glacial stages. Apparently these populations are almost exclusively allopatric and they are described as subspecies or races (Fleming, 1975; 1979), despite the fact that they differ in song. Since as Fleming (1975) states “The complex and stereotyped ethology of cicadas, in particular the specificity of the males' songs and of the females' response to them, play a notable role in speciation”, a cogent argument could be advanced for the specific distinctness of these populations in terms of the Recognition Concept. One cannot help but feel that in dealing with such allopatric populations authors consider their status is a matter for arbitrary designation, or that Isolating Mechanisms are “not necessary” since the populations are allopatric.
In New Zealand many species of indigenous birds are described as having separate North and South Island subspecies. Again, this appears to be a matter of convention rather than a considered biological opinion based on evidence. I believe that the status of these bird populations is of considerable importance both for evolutionary biology and for the conservation of threatened species.
The Recognition Concept might be usefully employed here. This concept is predictive in that, if one can determine the nature and characteristics of the SMRS of these geographically separated populations, it is possible to predict whether males and females of these groups would recognise each other as mates. I would suggest that studies of the details of courtship behaviour including analyses of male songs, etc. may enable a determination of the status of these populations. This does require, however, a knowledge of which characteristics of calls are important in mate recognition and which are not.
Consideration of the Recognition Concept also leads to an alternative view of the nature of speciation. If a species is essentially a group of organisms which is tied together by a common communication system, the SMRS, then speciation results when the SMRS of individuals of a daughter population so changes that these individuals no longer recognise members of the parental population as mates. The SMRS and meiosis are then recognisable as the two major components of sexual reproduction. Meiosis results in the production of haploid gametes from a diploid organism, while the SMRS has the function of achieving the efficient meeting of these gametes (Fig. 3).
A significant change in the SMRS results in the production of a new sexual cycle and speciation. Seen in this way speciation, in any form of sexual organisms, is not itself adaptive but an incidental consequence of the existence of sex.
As Sir
I thank
Tuatara 25(1), p. 18 (Winstanley)
Fig. 2 has been rotated 180°.
As the figure is printed, the caption should read:
Terminal appendages of the New Zealand Corduliidae. Females above, males below.
Left to right: Procordulia smithii, Procordulia grayi, Hemicordulia australiae, Antipodochlora braueri.
Scale line represents 1mm.
Being a soft-bodied, succulent, fleshy water bag in a dry environment poses problems, which range from simply drying out, through being stepped on, to being leapt on with greed by some heartless predator. On the face of it, a land mollusc seems a highly unlikely proposition, but the terrestrial molluscs have evolved satisfactory answers to the daunting array of potential disaster which confronts them, and have done it so successfully that they can be found in large numbers in an extraordinary variety of habitats all over the world. These habitats do not include only the moist areas; snails are common in many arid areas, and can be spectacularly successful in withstanding desiccation. A famous example is that of a specimen of the South African desert snail Eremina desertorum, glued to a card in the British Museum in 1846, which emerged from its shell and happily crawled off into the middle distance when exposed to moisture four years later. This ability to wait through dry conditions has often been noted — the West Australian camaenid snails are good at it, as are the South Australian helicellids, many African helicidae, and the prosobranch land snails of the Khasi Hills of Assam. Many deserts of the world have their populations of land snails, all waiting patiently for the rains to come.
The ability to retain water is the key factor for a snail living on land. Snails are inveterate hypochondriacs, constantly fretting about their water balance, and modifying their entire life style to keep that balance within allowable limits. Obviously, there are basic structural adaptations involved, but modifications to behaviour patterns, such as a nocturnal habit, the ability to seek out moist micro-habitats, and the ability to aestivate are also important (Hyman, 1967; Solem, 1978). This paper, then, will look at the hazards faced by the land molluscs, the modifications which allow them to deal with those hazards, the selective presures which have influenced the development of the slug form, and the evolution of one group of slugs in particular, the Athoracophoridae.
The Class Gastropoda is divided into three subclasses, the Opisthobranchia, Prosobranchia, and Pulmonata; the prosobranchs and pulmonates both have land representatives. The prosobranchs retain the open mantle cavity found in marine forms, and when they retreat into the shell during dry conditions they seal the opening with an operculum. As a water retention device, this is superb; in fact the seal is so effective that accessory notches, slits, grooves or separate tubes have evolved to allow air to circulate past the barrier of the operculum (Rees, 1964). However, the arrangement of the prosobranch mantle cavity allows high rates of water loss when the snail emerges, unless conditions are very humid. In the land prosobranch, the gills are lost and respiration takes place on the inner surface of the mantle. The kidney secretes a stream of almost pure water from the posterior of the pallial cavity, and this moistens the cavity, allows respiration, and stops the tissues drying out — at a cost. The arrangement is only marginally efficient. The snail can be active in periods of very high humidity, but when ground level humidity drops below about 95 per cent, the snail must retreat behind its operculum and wait for wet weather to return. This obviously cuts down the opportunities the snail has for feeding and mating.
The pulmonate snails lack an operculum to seal the shell aperture. Instead they secrete an epiphragm, a membrane of mucus or calcified
First, the pallial cavity, instead of being open, is enclosed, and the mantle collar is fused to the neck of the snail. This creates a large internal chamber communicating with the outside through a pneumostome, and this allows water loss over the breathing surface to be controlled. A bonus is the ability to retain reserve water inside the mantle cavity; this reserve may be as much as one-twelfth the total body weight (Blinn, 1964). Having a savings account of water allows a much more flexible approach to changing conditions. A pulmonate can remain active when a prosobranch is forced by falling humidity to withdraw into its shell, and this enables the pulmonates to exploit a wider variety of habitats.
Second, the pulmonates have evolved various methods of reducing water loss during excretion and respiration. A new organ, a ureteric groove or tube, develops in pulmonates, and there is direct evidence that in some species at least water resorption from kidney filtrate does take place (Vorwohl, 1961; Martin et. al. 1965). In many pulmonates, the kidney opens inside the pallial cavity some distance away from the pneumostome, and the excretion products must then be flushed out. In the order Sigmurethra there are various combinations of ciliated grooves and-or closed tubes that arise from the kidney, pass posteriorly to the hindgut, then reflect forward and continue partway to or actually reach the pneumostome. The evolution of this tube permits water conservation, and having it exit direct to the exterior through the pneumostome means that excretory products can be got rid of without having to use extra water to flush them out of the pallial cavity. (Solem, 1978).
Third, the pulmonates are well equipped to make good use of the water which they have. The physiological mechanisms which allow this are revealed in the relative impermeability of the mantle collar to water (Machin, 1966), and in the ability to withstand considerable water loss over long periods of time. Helix can survive water loss of around 50 per cent body weight, Arion 60 to 66 per cent, and Limax 75 to 80 per cent. In such a desiccated state Helix can survive 10 to 11 months, and Helicella over a year (Hyman, 1967).
Fourth, an array of other modifications also comes into play in resisting desiccation. These include the development of retractable tentacles, and the use of slime to cut down evaporation from moist surfaces.
Superimposed on all these structural modifications are behaviour patterns which cumulatively reduce the effects of desiccation and predation. A generally nocturnal life style, retirement to crevices in dry conditions, aestivation, burrowing (in slugs), and withdrawal into the shell during periods of lowered humidity are all effective. Taken together, this battery of adaptations and behaviour patterns has given the molluscs major successes in conquering the land.
A slug is any snail in which the shell is completely lost, or buried in the mantle. If land snails are an unlikely proposition, then slugs seem doubly unlikely — but there are some 500 species of terrestrial slugs, and approximately 1000 species of land-dwelling “semi-slugs” in which the shell has become reduced to the point where the animal cannot withdraw
It has been known for over a century that all land slugs are not closely related, but have evolved independently from several different snail groups. These groups are not randomly distributed among the land snails. Solem (1974) recognises 60 families of land snails divided into six orders. Two of these orders, the Onchidiacea and Soleolifera, contain nothing but slugs. Animals in these orders show many peculiarities in structure and their relationships with other land snails are uncertain. All other slugs are found in the Order Sigmurethra, which contain a mixture of shelled, semi-slug, and sluglike species. Even here, the distribution of slugs is not random. Of the 14 families making up the Suborder Holopodopes, there is only one family of slugs, the Aperidae, and two families of semi-slugs, the Rhytididae and Bulimulidae. Of the 10 families constituting the Order Holopoda, only the Helminthoglyptidae contain semi-slugs. In contrast, 18 families comprise the Order Aulacopoda. Five of these, the Philomycidae, Parmacellidae, Limacidae, Testacellidae, and Athoracophoridae, contain only slugs; one, the Arionidae, contains only slugs and semi-slugs; two, the Helicarionidae and Urocyclidae, possess shelled, semi-slug, and slug species in great variety, and a further three, the Charopidae, Succineidae and Zonitidae, have mainly shelled and a few semi-slug species. The remaining seven of the 18 families contain only shelled forms. The Orders Orthurethra and Mesurethra contain no slugs (Runham and Hunter, 1970). In both these groups the excretion products are released well within the pallial cavity and are subsequently flushed out, and there is no water-resorbing ureter (Solem, 1978). The pallial region, then, provides one of the major clues to why slug evolution is restricted to a few groups of land snails. All slugs have a long, closed ureter opening directly to the outside, thus allowing resorption of water from kidney products, and avoiding the expense of flushing those products from the mantle cavity. Slugs and semi-slugs have evolved only in the Superorders Sigmurethra and Systellommatophora, the two major groups with a closed ureter. Probably the existence of a water-conserving ureter was a preadaptation for the evolution of slugs.
What, then, are the selection pressures which favoured reduction of the shell in so many pulmonate families? First, slugs are common in those areas where microhabitats with plenty of moisture exist. Such a microhabitat, which maintains high levels of humidity through the year, means that a shell is no longer so vital. Second, a shell requires calcium. This tends to preclude snails from calcium-deficient areas. Third, building, transporting, and maintaining a shell requires energy. Fourth, a bulky shell prevents a snail from getting into crevices, which may make it difficult for it to avoid both desiccation and predation. These factors in combination are quite sufficent to allow a slow reduction in shell size where conditions are right and in those groups which possess adequate water conservation mechanisms.
Of course, the loss of the shell does mean that slugs are more likely than snails to be eaten by something large and unfriendly. However, they have developed other defences. Many of the Arionidae are brightly coloured — and distasteful. The Athoracophoridae in particular have developed cryptic colouration to a high degree. The pattern of grooves they show on the back closely resembles the veins of a leaf, and Athoracophorus
bitentaculatus shows a range of colour patterns which mimic the colour changes which occur in fallen leaves, from yellowish-green through to chocolate brown, all in the same population. One variety of the Australian athoracophorid
Slug evolution is a gradual process, and there are plenty of living species to illustrate every stage. The transition from snail to slug involves a large number of inter-related changes. Reduction and loss of the shell is necessarily accompanied by changes in the mantle cavity, the excretory system, the free muscle system, the arrangement of the heart and respiratory surfaces, and by a reduction of the visceral hump with major reorganisation of the internal organs. Consider first the pallial cavity of a stylommatophoran snail. The roof is heavily vascularised for gas exchange, the hindgut runs along the parietal-palatal margin to the pneumostome, the heart and kidney abut the posterior margin, and in the Sigmurethra the ureter runs posteriorly and then recurves forward along the hindgut to the pneumostome. If this already complex arrangement becomes compacted, provision must be made for maintenance of the respiratory area. Heavier vascularisation can provide this up to a point, as in the amphibulimine Bulimulidae (Van Mol, 1972), but compaction may well go on to such a degree that the amount of pallial cavity roof available is inadequate, even with heavy vascularisation. In this case, vascularisation can invade mantle lobe or shell lap, it can be concentrated in pouch-like lobes that hang from the remaining roof margin (Solem, 1978), (figure 1), or, in the final stages of compaction the respiratory surface may expand into a closely packed feltwork of blind diverticula, as in the Athoracophoridae (Figs 4 and 5). The last case will be covered in more detail later. As compaction proceeds, the other organs abutting the mantle
As the pallial cavity becomes reduced, the stomach moves forward, and the oesophagus is shortened. In the Thailand helicarionid Muangnua it is almost non-existent, and in the Athoracophoridae it has migrated onto the top of the buccal mass and is extremely short (Fig. 3) (Burton, 1962, 1980). Other space adjustments can be seen in the reproductive system, where a major evolutionary trend involves the progressive fusion of the male and female ducts, with the male duct becoming a ciliated cleft in the wall of the female duct. This allows the female duct to be relatively narrower, as it can distend during the passage of eggs until the male duct in the wall of the female duct is virtually obliterated. This topic will be further explored later.
Finally, the free muscle system, so essential in the snail for retraction of the animal into the shell, undergoes radical reduction. The elaborate tail fan needed to retract head and foot in the snail disappears, and only a small buccal retractor, tentacle retractors, and a penis retractor muscle remain (Solem, 1974).
The Athoracophoridae are a family of slugs found in New Guinea, eastern Australia, the Bismarck Archipelago, the Admiralty Islands, the New Hebrides, New Caledonia, New Zealand, and the subantarctic
The family comprises two subfamilies, the northern Aneiteinae and the New Zealand and subantarctic Athoracophorinae, and contains only slugs — no semi-slugs or snails need apply. The shell is reduced to a number of calcareous deposits embedded in the remains of the mantle collar, and in many specimens even these are missing. There is no sign of a visceral hump, and the pallial complex has become flattened and compacted to a high degree, with a highly convoluted and elongated ureter and a compact, high-surface-area lung of a type found in no other mollusc. There is no question that the Athoracophoridae as a group have advanced far down the road to slugdom. Obviously, there can be no fossil evidence to show the details of the path they have taken, and all the clues must be gleaned from a study of comparative anatomy. Nevertheless, the Athoracophoridae are a varied group, and a close study reveals that the clues are plentiful. The New Zealand and subantarctic species in particular show a rich web of inter-relationships and a gradation of answers to specific problems. The conceptual tool which allows fruitful exploration of the problems posed by the Athoracophoridae is Solem's work on the effects of compaction (Solem, 1966, 1972, 1974, 1978).
The Athoracophorid structures that first arouse loud shouts of “aberrant!” among malacologists are the lung in particular and the pallial complex in general. The lung consists of a dorsal cavity opening to the exterior through a pneumostome; a feltwork of thin-walled diverticula radiate ventrally and laterally from the floor of the cavity. A blood sinus encloses these diverticula, and blood drains from the sinus directly into the atrium of the heart (Figs 4a, 4b, and 5). The efficiency of this system is not known, but there is no question that it provides a large respiratory surface crammed into a small, flattened space. Thus the athoracophorid slug is able to do without the complex mantle lobes and folds seen as respiratory surfaces in other slug families (Fig. 1). Further, the pallial complex as a whole shows a high degree of modification and compaction (Burton, 1981a in press). The complexly-folded ureter is a case in point. The Athoracophoridae lack a mantle cavity as such — the lumen of the lung could be regarded as a remnant of the mantle cavity, but it is reduced in size, and has lost all the functions of a mantle cavity apart from respiration. As the lung does not store pallial water, the respiratory surface is on the floor of the cavity rather than the roof. The function of pallial water storage has apparently been taken over by the ureter, which, particularly in the New Zealand and subantarctic slugs, has become complexly folded and much expanded. (Figs 4a, 4b, and 5). It seems possible that the water resorption properties of the ureter are not highly developed in the Athoracophorinae. Significantly, these slugs possess a tubule connecting the ureter with the intestine, and this tubule may allow water stored in the ureter to pass back to the gut and be reabsorbed in times of water-stress. The tubule enters the intestine just before it joins the rectum, and presumably the water can be drawn out of the rectum as the intestinal contents pass through. The northern representatives, the Aneiteinae, lack this tubule, and the arrangement of the pallial complex shows other primitive characters. In these slugs the anus, renal aperture, and pneumostome are closely grouped on the dorsolateral aspect of the back.
This close association is recognised as a basic gastropod condition (Hyman, 1967; Purchon, 1968). In the New Zealand and Subantarctic Athoracophorinae this association is broken to various degrees (Figs 6 and 7). The anus migrates away from the pneumostome (Palliopodex verrucosus) then leaves the mantle area altogether (Pseudaneitea gigantea), then migrates down to the margin of the back (P. aspera, Athoracophorus
bitentaculatus). The pneumostome, which is no longer associated with the anus, is free to move medially, and the renal orifice migrates anteromedially. Once the close association seen in the Aneiteinae is broken, each orifice is free to move to the most advantageous position (Burton, 1980). The shift in anal position may be associated with the desirability of keeping the dorsum free of faecal material. Even in the Aneiteinae, which have the anus inside the mantle area, the breadth of the mantle area makes it possible for the slug to extrude faeces on to the side of the dorsum rather
Athoracophorus bitentaculatus, are dorsoventrally flattened, and in all these species the anus has migrated to a position at the margin of the back, so that faecal material never soils the dorsum. Clearly, in this respect the Aneiteinae show the primitive condition, and that in the Athoracophorinae is derived. The process of flattening and compaction has obviously been at work in the group.
Another example of the results of compaction is seen in the reproductive system. Examination of athoracophorid reproductive systems sheds some light on the evolution of pulmonate and opisthobranch reproductive systems in general. Since the late 1880's it has been traditional to state that a monaulic (having combined male and female ducts as a single gonoduct) pallial “spermoviduct” is a primitive stage, and that the advanced condition is shown by the separation of the male and female ducts. Plate
Palliopodex verrucosus from the Auckland Islands, Pseudaneitea ramsayi from Three Kings Islands and Athoracophorus bitentaculatus on the mainland) being monaulic, and all the others showing the diaulic conditions (Figs 8, 9, and 10). In the light of the controversy outlined above, which came first? On balance, it seems most likely that in the Athoracophoridae the diaulic condition is primitive, the monaulic condition is derived, and the three monaulic species arrived at the condition independently. First, the northern Aneiteinae, demonstrably more primitive than the Athoracophorinae in the arrangement of the pallial complex, all show the diaulic condition (Solem 1959, Oberzeller 1870, Burton 1980). Second, the New Zealand and subantarctic species show a lot of variation in their size and in their body cross section, i.e. in their state of compaction. They range from large rounded slugs such as Pseudaneitea gigantea and P. papillata to small, highly compacted, very
P. ramsayi and Athoracophorus bitentaculatus. P. gigantea in particular shows a number of primitive characteristics, such as a dorso-lateral anal position close to the pneumostome, a smooth skin, and a well-developed central radula tooth, a condition recognised as primitive in other groups (Solem, 1959); its reproductive system shows the diaulic condition, as do those of all the other large, rounded species (Burton, 1980). The three monaulic species all show the effects of compaction; they are small and flattened, and can on this account be regarded as advanced. However, they are not closely related, and the land masses on which they occur have never been in contact (Fleming, 1979). Third, the fusion of gonoducts as part of a general process of compaction seems more likely than their separation, as fusion effectively gives two ducts in the space of one.
In the three monaulic athoracophorid species, the oviducal gland forms the wall of the oviduct, rather than being a discrete gland attached to one side, and the prostate gland is a diffuse structure emptying into the male duct at intervals along its length. This arrangement of the prostate gland is presumably necessary because prostatic fluid can escape from the male duct along its length and must be replenished at intervals. The monaulic arrangement is most similar to that seen in the Aneiteinae, which have an elongate, lobular prostate gland feeding into the vas deferens at intervals,
Similarly, taking the compaction hypothesis into account, some evolutionary trends could be inferred in the reproductive systems of the diaulic Athoracophorinae. Such trends include the development of a discrete prostate and a distinct oviducal gland, and a general shortening of the reproductive tract (Fig. 10). It must be stressed that this figure does not imply an evolutionary relationship between the species named; each species simply illustrates a different stage in a possible evolutionary sequence.
Thus the process of compaction has been carried almost to an extreme in the Athoracophoridae, and in Athoracophorus bitentaculatus in particular. In this species it has resulted in a slug so flattened that it can easily fit into small crevices and spaces between leaves denied to fatter slugs, and thus gain protection from desiccation and predation. Second, compaction and flattening confers on the slug a much higher foot area-weight ratio, which improves its ability to climb up vertical surfaces and allows it to browse on trunks and leaves at night.
The Athoracophoridae as a group are highly advanced in some respects. Solem (1974) has described slugs in general as representing the acme of land snail evolution. As Athoracophorus bitentaculatus in particular has carried the process of compaction to its logical extreme, it can be regarded as a supreme example of the current state of the art in the land mollusca.
The XVth Pacific Science Congress will be held in Dunedin, New Zealand, 1-11 February 1983, Its theme is to be “Conservation, development and utilization of the resources of the Pacific”.
A session is planned on the diversity, distribution, abundance and management of vertebrate populations in the Pacific region. Joint sessions will be arranged with related disciplines. Speakers are now invited to offer papers (with title and short summary) on such topics as:
For further information, please write to Dr C. W. Burns, Section Convener, (Ecology and Environmental Protection), C/o Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand.
References and sources of material.
A. coriaceus was described by Stephani in 1916 from a specimen collected by T. Kirk in New Zealand (Stephani 1912-1917) and now lodged in the Stephani Herbarium at Geneva (T. Kirk 112; G. 19821). This specimen, along with notes made by Stephani, was kindly made available for inspection. Two other specimens which were examined came from the Herbarium of E. A. Hodgson. One of these (H 1623; MPN 17066) was collected by E. A. Hodgson at Kiwi Valley, Wairoa, New Zealand in 1923 and determined by A. coriaceus; the other (H 7622; PRB 17145; MPN 17072) was collected in the Manawatu Gorge, New Zealand by A. coriaceus. The account below is based mainly on living plants collected from roadside banks on the south side of Tarata Saddle, Taranaki in November 1979 and kept growing at Massey University. Voucher specimens are lodged in the Herbarium of Massey University (MPN 17073).
Morphology of the gametophyte.
The thallus is perennial, dioecious, firm in texture and of a subglaucous to green colour. It is firmly attached by rhizoids either to the soil or to old thalli which it has overgrown. The thallus may be ligulate and up to 1.5cm long and 7mm broad, but commonly it is repeatedly branched, the broad branches spreading outwards to form a partial rosette up to 2cm broad and 3 (-4)cm long (Figs. 1 and 2). The border is irregularly lobed and finely crenate. Older parts of the thallus gradually die off as the thallus extends. In January at the margin of some thalli there appear plate-like, multicellular gemmae which are readily detached and are dispersed in water. Often they attach by rhizoids to the upper surface of older parts of the thallus. Male plants are usually flat and have a few scattered antheridial cavities, each containing one large antheridium, with a body up to 270 microns wide and 300 microns long and a stalk of 3 (-4) tiers of cells. In female plants the edges of the thallus are often upturned and connivent, so forming a shallow cup, on the floor of which are one or more embedded archegonia and later sporophytes.
Anatomy of the thallus.
In a transverse section of a young thallus the upper epidermis is distinctive, for it is composed of thin-walled papillate cells, but with age it may erode (Fig. 3). Cells of the lower epidermis may have a thickened outer wall and may carry rhizoids. The compact ground tissue in well-developed female thalli is 10-12 (-16) layers deep in the central region and is gradually reduced to some 3-6 layers towards the margin, but in male thalli the central region is only 6-8 layers deep. The interior cells often show pitting on the walls and trigone thickenings at the angles. A single, bright green, disc-shaped chloroplast lies close to
Nostoc as in A. laevis.
Morphology of the sporophyte.
The capsule is 1-1.5cm tall and 0.4-0.5mm broad; it is surrounded at the base by an involucre 2mm high. When first it opens, the upper part is golden in colour but later it turns brown. There are stomata in the capsule wall. Other epidermal cells mainly have the outer and radial walls conspicuously thickened but on two sides of the capsule there is a longitudinal line, two cells wide, of thin-walled cells. Dehiscence starts a short distance below the apex in one of the lines of thin-walled cells and extends somewhat in a vertical direction (Fig. 1). The opening may remain as a narrow slit or the sides may flatten out to a spatula shape, which later may become bent or twisted. Only occasionally does splitting take place on the other side also and extend right to the apex, in which case the valves often twist spirally (Fig. 2). The pale yellow
Comments.
As far as is known A. coriaceus is endemic to New Zealand. It differs from A. laevis in the papillate epidermis, the form of the chloroplasts, the plate-like gemmae, the generally shorter capsule which often opens on one side only, and the surface marking of the spores.
Anthoceros affinis Schiffn. in the brief original description, which was based on a plant collected in Auckland, is stated to be perhaps only a
A. laevis differing only in that the spores are very smooth (Schiffner, 1889). However, it is now known that the degree of marking of the spore coat of A. laevis is extremely variable (Proskauer, 1958; Hassel de Menendez, 1962). By courtesy of the Curator of the Cryptogamic Herbarium of the New York Botanical Garden, the Phaeoceros laevis folder of New Zealand specimens from the Herbarium of W. Mitten was made available for examination. Amongst the specimens is one determined by J. Proskauer as Phaeoceros ref. affinis on the basis of immature spores adhering to partially decayed old sporophyte fragments. Earlier it had been named Pellia carnosa (Hook. and Tayl., 1844) and included under Anthoceros laevis by Mitten (1855). In my opinion the specimen is not sufficiently distinct from A. laevis to justify the establishment of a separate species.
Phaeoceros novazealandicus (Pears.) Prosk, was originally described as Aspiromitus novazealandica (Pearson, 1923). By courtesy of the University of California (Berkeley), two accession numbers, UC 213698 and 213699, of Aspiromitus novazealandica (both listed as type and collected at Taupo in 1904 by Setchell) were able to be examined. Proskauer (1951) has already stated that this species is a typical member of the yellow-spored section of Anthoceros. In my opinion the specimens with capsules correspond with A. laevis. However, a figure of a male plant with large solitary antheridia (Pearson, 1923) seem to belong to Megaceros leptohymenius, a species which often grows intermixed with A. laevis and superficially resembles it.
By courtesy of the New York Botanical Garden the sheet of specimens of Anthoceros colensoi from the Herbarium of W. Mitten was available for examination. The lectotype (Colenso 2069) is annotated by J. Proskauer as Megaceros giganteus (Lehm, and Lindenb.) Campb. It was
The bluffs of the North Otago Heads, which shelter much of the town and harbour of Oamaru from southerly winds, are fringed by volcanic rocks which form a reef exposed at low tide. The reef and its tidal pools have a diverse mollusc fauna characteristic of the northern Forsterian zone. The intertidal reef extends outwards to a depth of about 18 metres to the south-east of Cape Wanbrow. This submerged reef is the habitat of the commercially exploited rock lobster, Jasus edwardsii, which supports a seasonal (June to December) local industry.
This checklist provides a list of 78 species of mollusc which have been taken from commercial lobster pots in this area between 1970 and 1980.
The pots, referred to locally as “craypots”, have a welded steel rod frame covered by plastic-coated netting and have an entrance on top. Some are woven from wire and cane, including the original type which used native supplejack, Ripogonum scandens, but these are seldom used now by commercial fishermen here. The pots are baited with fish or fish skeletons and are set on the grounds in 10-18m of water. They are lifted, cleared, rebaited and reset daily when weather conditions allow. Inevitably, many molluscs are captured in the pots and some unexpected species turn up from time to time. They may move in to consume the bait, be transported in as shells mobilised by hermit crabs (Eupagurus spp.) seeking the bait, or possibly be washed in by water turbulence (e.g. bivalves).
Some of the unexpected finds include Early Miocene fossils — ?Polinices sp., Penion sp., and Cucullaea sp.; presumably eroded from the submerged continuation of the adjacent Awamoan formation in the Old Rifle Butts locality. However, the occurrence of frequent specimens of Struthiolaria obesa, indicates an underwater Early Pliocene stratum which is not present in the terrestial Waitaki series (Notosaria nigricans has been recorded and the asteroids Asterodon dilatatus, Pentagonaster pulchellus, Patiriella regularis, Astrostole scabra and Coscinasterias calamaria have been taken in the pots. Notable amongst the latter were two sets of 4 to 8-armed Patiriella regularis. Many species of fishes, Crustacea, and miscellaneous invertebrates, including sea-spiders ?Achelia sp., also have been noted.
Nomenclature is based on Powell (1979). Abundance is significant and is designated after each species as follows: (P) Plentiful; (U) Uncommon; (R) Rare.
Chitonidae.
Acanthochitonidae.
Haliotidae.
Fissurellidae.
Trochidae
Turbinidae.
Turritellidae.
Trichotropidae.
Calyptraeidae.
Naticidae.
Cymatiidae.
Muricidae.
Buccinidae.
Volutidae.
Turridae.
Amphibolidae.
Cucullaeidae.
Arcidae.
Glycymeridae.
Ostreidae.
Pectinidae.
Kelliidae.
Carditidae.
Mactridae.
Mesodesmatidae.
Tellinidae.
Veneridae.
Hiatellidae.
Pholadidae.
Myochamidae.
Octopodidae.
Many of the shells listed above are far removed from their hitherto noted habitats (Graham, 1962). For instance, some intertidal species can be found in this deeper water, while other normally deeper water species have been taken in this comparatively shallow habitat. Since most are dead shells inhabited by hermit crabs, their occurrence in the pots may be more indicative of crab movements than of molluscan distribution. Dead shells of Struthiolaria papulosa gigas have been found, many in perfect condition, yet I have never taken this species alive in North Otago by any method. It should be noted that the identification of Maurea pellucida ?pellucida is based on the specimens having more incised sculpture and deeper colour than M. p. forsteriana. In general, most molluscan shells from the pots are eroded and encrusted with Porifera, Bryozoa or the Cirripedes Balanus decorus or B. trigonus. All gastropods in this condition have the slipper shell, Maoricrypta monoxyla, living within their apertures.
I am very grateful to Mr Douglas Mills who has continued to bring specimens ashore for me since my retirement from the fishing industry and I thank those other Oamaru fishermen whose curiosity has added to the species recorded.
Experiments involving encounters between the snail Helix aspersa and the diplurid spider Porrhothele antipodiana confirm that not only does this spider kill snails, but it also spends many hours feeding off the body of the snail. Spiders can maintain body weight for at least two months when fed exclusively on a diet of snails. It is suggested that snails could be quite important as a source of food and fluid for these spiders over the drier months of summer and autumn.
In an investigation of the prey of Porrhothele antipodiana (Laing, 1973) it was reported that snails* formed 7.5 per cent by number of the prey remains found in the tunnels of this spider. At the time of publication, the question of whether P. antipodiana consumed the body of the snail was left open. Subsequent experiments and direct observations on encounters between snails and spiders have been carried out in an attempt to answer the foregoing question.
Snails appear to be an unlikely prey for spiders; their slimy covering and their protective behaviour of retreating into their shell would seem to offer a spider little chance of feeding on them. This was also the view of Bristowe (1941) who believed that these molluscs were immune from attack by spiders on account of their shells and slimy covering; in fact he went as
* Identified as Helix aspersa by the National Museum, Wellington.
Mature female spiders were selected for the experiments. They were kept in individual plastic containers 20cm x 20cm and 8cm deep. A covering of soil was provided on the bottom of each container. Glass lids were attached with lumps of plasticine to give a small air gap at the edges of the lid.
The snails were all Helix aspersa; this being the common species found in the open country and urban areas where P. antipodiana flourishes. Both the snails and the spiders were weighed before each encounter. (Note: snail
Of the five spiders used in the experiments, all killed snails and three proved to be especially proficient snail killers. Weighings of the snails after the spiders had finished with the bodies revealed that a substantial weight loss was always involved (Table 1). In conjunction with the weight loss of the snails, the spiders involved always showed a weight gain (Table 2). There can be little doubt therefore, that the spiders did ingest some of the snail.
In addition to the changes in body weight of the spider, evidence of ingestion could be obtained from measurements of its abdomen. These always increased after a spider had fed on a snail. (Table 3).
Observations on four instances of snail predation confirmed that feeding was a protracted event. (Table 4).
Having determined that P. antipodiana could both kill and eat snails, the next step was to feed a small group of spiders on a snail-only diet for an extended period of time. The three spiders which had killed snails regularly were used for this experiment. Each spider was offered a snail twice weekly. Weighings of the spiders were carried out weekly.
A control group was not used in this particular experiment as it was known from previous experience that P. antipodiana individuals kept in the same containers but without food were in a very poor condition at the end of four weeks. A body weight loss of up to 20 per cent was common
The three spiders fed solely on snails did not, in fact suffer any undue weight loss after eight weeks (Fig. 1) and their general condition was very good when the experiment was terminated.
One of the central questions concerning snail predation was: How can a spider kill and eat a snail which has retreated into its shell? Detailed observations were made to answer this question. These revealed that the following phases were important in the process:
A diagrammatic portrayal of these phases is given in Fig. 4.
It was clear that the spider was faced with several problems when trying to kill a snail. First, a large snail would drag the spider with it as it retreated into its shell after the strike had been made. Sometimes the spider lost its hold during this movement and would not pursue the snail into its shell. Second, the snail retreated deep into its shell and sometimes the spider lost its hold at this stage. Third, the snail usually began to produce foam from its pulmonary aperture soon after the strike had been made. This production of foam was the main deterrent to the spider: more spiders relinquished their hold when they began to be covered in the foam than at any other stage. In some instances the foam production was so extensive that a week elapsed before the last traces of foam had been removed from a spider's palps and legs.
The snails seemed to be quite resistant to spider venom. Even a bite from a large spider which lasted for several minutes, and which would have killed any large insect or even a mouse, appeared to have little effect on most snails. The time for death to occur if the spider managed to hold on with its fangs, seemed to be in the vicinity of 30 minutes. At about this time the spider was able to partly withdraw the snail's body from the shell, so presumably prior to this the snail was still providing some resistance to the spider.
Six other species of spiders were tested with small snails to determine if snail eating was widespread amongst spiders. The results of these investigations are given in Table 5. In the case of Cantuaria sp. a deeper container was provided so that this spider could construct a burrow and operate in its normal feeding manner.
From the limited number of species tested it would appear that snail eating is not widespread. In fact most spiders appeared to find the initial contact with the snail a disturbing event and would not proceed with the strike, thus confirming the results of Bristowe's experiments (1941).
The investigations reported here indicate that snails are definitely used as food by P. antipodiana The question remains, however, how important are snails in the diet of these spiders? It is probable that snails could be useful as a source of fluids to P. antipodiana due to the high fluid content of their bodies. The protein mass in the foot masculature would also be useful to the spider, providing it could be digested. Maintenance of body weight by the three spiders over two months indicates that this is probably the case.
That only Porrhothele and Achaearanea ate snails is worth noting, for these two spiders also incorporate both slaters and millipedes in their diet. This itself is an unusual feature because these animals, like snails, are not taken as prey by the majority of spiders.
There are at least two, and probably more, of the Australian mygalomorphs which feed on snails. Raven (1978) recorded finding empty snail shells in the webs of the diplurid Bymainiella boycei The same observer has also reported (1979) the finding of an individual Cethegus (F. Dipluridae) actively feeding on Helix aspersa. Raven was also able to elicit a feeding response from the theraphosid Selenocosmia crassipes by offering it live Helix aspersa.
It seems very likely that further investigations of the mygalomorphs will reveal a widespread use of snails as prey items.
Snails would be valuable as prey over the drier months of late summer and autumn, and the eight-week time span of the feeding experiment described in this paper relates especially to this feature: eight weeks is the length of time we might expect a spider such as P. antipodiana to have to survive possible dehydration. The ability to accept unusual prey of high water content could well be a significant advantage during the dry months.
The size of the snail most often eaten was in the 10-25mm shell diameter range (Table 6.) Searches carried out around Wellington through summer and autumn confirmed that snails of this size were indeed present in the localities occupied by P. antipodiana.
It is clear that the behaviour of P. antipodiana when killing a snail is very important. In particular, this spider allows itself to be dragged into the outer portion of the shell and be covered with mucus. Moreover, it usually stays there until the snail's resistance ceases, thus revealing a tenacity uncommon in spiders. Very little appears to be known about the foamy mucus and its probable value in deterring predators.
The discrepancy between the weight loss from the body of the snail caused by the spider killing and eating it (Table 1) and the weight gain shown by the spider after feeding on the snail (Table 2) is quite marked. This could be due partly to evaporation of fluids from the dead snail during the protracted feeding period. It could also be due partly to seepage of digested snail remains onto the substrate rather than all digested fluids going into the spider's mouth. From the information given in Table 3 it seems that the abdomen of the spider is capable of accommodating large quantities of digested food. Whether its elasticity is great enough to cope with the large volume of material from a snail's body is unknown.
I would like to thank the following: Dr F. Climo, National Museum, Wellington, for his helpful comments on the snails; Dr
Published by Bordas, Paris. 504pp, 1979
The recent publication of Guide de la Nature en France (1979) (Bordas, Paris) is, in my view, an outstanding event both in the field of travellers, trampers and nature enthusiasts' “Guides”, and in conservation. Here in handy format (in spite of its 504 pages!) is the ideal companion to either a day's walk or a long tour. Here also is concise information — effectively blended with good local maps (although not road-rail maps) and splendid photographs — to tempt anyone with the slightest interest in landscape or wildlife to plan new excursions. It is a book to integrate local natural history information with that provided by standard plant or bird identification books or other accounts of geology or general natural history.
The Guide is aimed at a wide range of enthusiasts for the countryside; it is in the highest degree practical! Who could resist its advice to take one of the “randonnees a bicyclette” (“reméde aux problèmes du pétrole”), or the tours by air, canoe trips, or even “randonnées à skis” so imaginatively described?
The text is the product of a distinguished and experienced group of scientists, ecologists, journalists and photographers assembled under the direction of the Groupe Paul-Emile Victor — the group includes as well as Paul-Emile Victor, such eminent writers and conservationists as Alain Bombard and Jacques-Yves Cousteau. (The group exists “pour la defense de l'homme et de son environment”).
It is of particular interest that New Zealand has just recently begun to develop guides with emphasis upon landscape and natural history; two fairly newly issued volumes are the:
AA Book of the New Zealand Countryside (Lansdowne Press: first published 1978, 3rd edition 1980); and Wild New Zealand (Reader's Digest: 1981).
The New Zealand volumes are also splendidly illustrated, as might be expected in view of the distinguished work being produced by a number of landscape and natural history photographers. Neither is in the compact format which distinguishes the French volume, and makes the latter so useful for ready reference and transport. But it is in the sheer assembly and condensation of information in digested form that the French Guide is so outstanding and surpasses anything yet achieved here. In part this doubtless reflects the comparatively advanced stage which has been reached in the ecological investigation of the European country and landscape. Understanding of our own landscape and ecology is still developing actively and this will naturally be incorporated in guides to the countryside in years to come. Our knowledge of New Zealand's plant and animal life has advanced dramatically in the past half century and this is reflected in the excellent local coverage which has been possible in recent books on our landscape, including the two named above. Perhaps the main criticism concerning these and other recent New Zealand volumes of this type is that travel brochure eulogies of the landscape still creep in — a feature happily absent from the French volume which nevertheless never fails to convey its enthusiasm for the diversity of scenes and landscapes which it describes.