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copyright 1965, by Victoria University of Wellington
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During
The present Wellington Harbour plankton collecting programme commenced on
The fauna discussed in this paper is macroplanktonic. It includes the Coelenterata represented by Hydrozoa and Ctenophora; Chaetognatha of the genus Sagitta; Crustacea including both permanent and temporary planktonic forms; Chordata represented in the Urochordata by the family Salpidae, and in the Vertebrata by the eggs and larvae of various fish. Particular attention has been given to both decapod and stomatopod larvae which form a major component of the plankton. Other groups not mentioned above formed only a small fraction of the total volume of plankton samples, and these have been disregarded.
It was not possible to identify all the taxa to the specific level, as the systematic status of many New Zealand marine organisms has not yet been determined. Many crustacean larvae reared in the laboratory to juveniles still eluded final identification, as juvenile stages frequently lacked the adult characters used in classification.
In the plankton, short period fluctuations of considerable magnitude were observed. In some cases these resulted in the absence of species normally abundant. However, the broad seasonal trends in the Wellington Harbour plankton have emerged with quite startling clarity, and some quantitative work has been done to assist in the description of these trends. In addition a number of relatively common and distinctive species were identified and selected as seasonal indicator species. An annual plankton calendar has been prepared from monthly and seasonal changes in the abundance of these species.
The decapod larval terms "zoea" and "megalopa" are here employed in the sense suggested by D. I. Williamson (
Plankton samples for analysis were obtained with a 36 gauge extra heavy grit gauze two foot diameter cone net, with a 2lb "Agee" preserving jar tied into the collar. The sampling gear was towed behind a launch using from 50 fathoms to 60 fathoms of 8lb three-quarter inch hemp. While taking a sample, the towing speed which varied between ¾ knot and 1½ knot was adjusted to correct for current, wave, and wind resistance to both the gear and the launch. This allowed the net to filter at depths between one fathom and three fathoms for the greater part of the ten to twenty minute towing period. By turning the launch in a tight circle once during each tow, the speed of the net in relation to the water was reduced allowing the net to sink and filter at depths of five or six fathoms for about five minutes.
On completion of every tow the plankton adhering to the mesh on the inside of the net was washed into the jar with rinses of sea water. The bulk of the sample was preserved in 5% formalin solution in the field, while a selected sub-sample was kept for the study of living material. The date, time of day, cloud and wind conditions, state of tide, and surface water temperature, were all recorded for each plankton tow.
The volume of plankton in each sample was recorded in a graduated measuring cylinder after settling for ten minutes. Where the duration of a tow was greater or less than the standard of 20 minutes the total volume of plankton was adjusted by simple proportion. From the number of samples in each month the mean monthly volume was estimated.
For comparison of samples with wide variations in volume the following sub-sampling technique was employed to obtain a constant representative volume (l0cc) of each sample.
Samples were diluted to approximately two litres with fresh water and well stirred. Sub-samples of 50cc were then removed and allowed to settle in a 50cc measuring cylinder. The excess water was siphoned off and successive sub-samples were added to the measuring cylinder to give exactly 10cc of settled plankton. This 10cc sub-sample was diluted to approximately 100cc with water and poured into a very large, shallow Petri dish which was placed on top of a circular piece of black paper divided into ten equal sectors with white ink. Three randomly selected sectors were counted for each of the species considered in the analysis. If the results of these three counts were erratic, further sectors were counted. When the total volume of a sample greatly exceeded 10cc, a further 10cc sub-sample was taken and treated in the above manner.
The number of individuals of each species in a 10cc sub-sample was thus obtained by simple proportion from the number counted in three or more of the sectors, and the result was adjusted to the total volume of the sample by simple proportion.
The counted and estimated animals were then divided into the following five major elements: Pleurobrachia pileus (O. F. Müller); Obelia geniculata (L.); Octophialucium funerarium (Quoy and Gaimard) together with Phialella quadrata (Forbes); Salpidae; and Crustacea. These five elements representing either individual species or groups of species were found to have considerable seasonal variation in numbers. This is reflected in their respective contributions to the total volume of the plankton samples (Text-fig. 8).
The species constituting the bulk of the volume together with the species selected as seasonal indicators were used to compile the annual plankton calendar (Text-fig. 7).
Unidentified larval Crustacea were reared further in the laboratory. Two or three larvae were placed in a covered finger bowl filled with approximately 200cc of fresh sea water. The larvae were fed every second day with teased particles from the mantle and gonads of the marine mussel Perna canaliculus (Gmelin). To reduce invasions of protozoans, fungi, and bacteria to a minimum, the amount of mussel supplied at each feeding was no more than required by the species, the amount being learned by experience. The early juvenile stages of several brachyuran species consumed their own weight of mussel in only three feedings over a six day period. Anomuran larvae were found to be considerably less voracious, taking up to seven or eight feedings to consume a similar quantity. The sea water was drained and renewed on non-feeding days. These frequent water changes eliminated the need for aeration and ensured the regular removal of faecal pellets and any unconsumed food material.
Throughout
Very encouraging results were obtained with this refrigerating apparatus, and the use of mussel as food was found to be generally successful in the rearing of larval and juvenile Brachyura, larval Stomatopoda, and many later stage anomuran larvae. This feeding method gave little success with early stage anomuran zoeae.
Decapod crustacean larvae found in the plankton were in some cases identified by comparison with larvae hatched from eggs of ovigerous females obtained from beach collections. Females carrying eggs about to hatch showing larval eye spots were kept in a small sea-water aquarium until the eggs hatched. The hatched larvae were kept alive for as long as possible by the described laboratory rearing method.
Drawings of entire specimens were made on squared paper with the aid of a wide field "Spencer" binocular microscope fitted with a squared micrometer in one eyepiece. Coloured chromatophores are indicated by the code shown in Text-fig. 1.
Two adjacent bays in Wellington Harbour were selected as research stations. Station One extends from Point Halswell south-east across Kau Bay to Kau Point in a zone approximately 300 yards wide (Text-fig. 2). Station Two extends Station One from Kau Point south across Mahanga Bay to Point Gordon and is approximately 200 yards wide (Text-fig. 2).
The littoral zone of both stations (Kau Bay and Mahanga Bay) consists mainly of rough shingle interspersed with rocks, but large rock outcrops extend to well below the low water mark at Point Halswell, Kau Point, and Point Gordon. From the low water mark to a depth of approximately one fathom there are extensive colonies of Macrocystis pyrifera (L.) and other algae clinging to a rocky substrate. Macrocystis pyrifera is concentrated mainly at Point Gordon, Kau Point and in Kau Bay. From one fathom to approximately five fathoms the sea floor shelves steeply, and is covered with coarse sand and gravel on which thick patches of red-green algae and large sponges grow. The sand and gravel merges into soft grey mud below five fathoms. Both plankton stations lie for the most part over the muddy substrate at depths between seven and nine fathoms.
Station One is directly exposed to the prevailing northerly winds. Tows were not usually attempted in winds from this quarter above force four on the Beaufort Scale. Station One is however sheltered by steep hills from the less frequent southerly winds. Station Two is also exposed to the north, but its northern end is sheltered by the rocks of Kau Point.
The tidal flow pattern for Wellington Harbour has been outlined by Brodie (
With heavy flooding in the Hutt River and the Kaiwharawhara and Ngauranga streams the plankton stations were discoloured by muddy water. Under such conditions no significant change was seen in the plankton samples.
Effluence from the Ngauranga Freezing Works appeared to have no marked effect on the plankton samples. Effluence was mainly particles of fat which surface drifted in the direction of the stations during prolonged wind from a northerly quarter.
The accuracy of the monthly mean temperature is limited by insufficient recordings, but from the information available broad seasonal patterns can be
Mean monthly surface water temperatures varied from year to year. This variation was due to the difference in air temperatures as shown by the meteorological records. Thus the milder winter of
The plankton fauna of Wellington Harbour discussed in this study can be divided into two sections as described below.
The numerical abundance of the species discussed is based on the data presented in Text-fig. 7. All quantities may be equated with a single 20 minute horizontal plankton tow with a 36 gauge two foot diameter cone net at depths between one fathom and six fathoms. The numerical equivalents of the expressions "rare", "few", "common", "very common", abundant" and "very abundant", lie within the ranges set down in the key to Text-fig. 7. When the above expressions have been used in this sense they have been italicised.
The species comprising this first section are present in their adult form all the year round and numbers do not fluctuate significantly during the year. These include Isopoda, Amphipoda, Gopepoda, and Chaetognatha.
Small numbers of Isopoda occur but these have not received any special attention.
Amphipoda (Hyperiidae) of several species occur in the majority of samples, but these are rare in all months of the year (Text-fig. 7).
Copepoda are the most plentiful component of the permanent plankton, and many species are present in the Wellington area. Copepoda are particularly significant in the winter months (June, July, August) when other crustacean species are comparatively scarce (Text-fig. 7). Parasitic Gopepoda (Galigidae) mainly male have been taken only rarely.
The arrow worm Sagitta serratodentata var. tasmanica Thomson, is found in the majority of samples throughout the year, but it is never numerically significant (Text-fig. 7). This species is considered to be of oceanic origin, as greater numbers occurred in samples taken during a flooding tide.
This section includes the larger number of species. Their planktonic existence is a phase of their life history and is therefore seasonal. Included in the temporary plankton are Salpidae and Ctenophora of which adult and juvenile forms are present for only part of the year, medusae of Hydrozoa, eggs and larvae of fish and larval Crustacea.
The salps, Thalia democratica (Forskäl) (Text-fig. 3, A, B) and Ihlea magalhanica (Apstein) in both their solitary and aggregate forms, constitute a major part of the late summer and autumn plankton. They indicate the presence of oceanic water at the plankton stations. Thalia democratica occurs regularly and very commonly in January. From February until May the species is present in most samples but in greatly reduced numbers (Text-fig. 7). Ihlea magalhanica is the rarer species and occurs only in January and February (Text-fig. 7).
Pleurobrachia pileus (O. F. Müller) (Text-fig. 3, C) is dominant in the winter plankton. However, this is one of the most variable planktonic species. In Pleurobrachia pileus first appears rarely in April, and few are sampled in May. The species is common in June and abundant in July and in August when it may dominate many samples. In September, October and November, P. pileus is common in all samples, but occurs only rarely in December and January. In February and March the species is absent (Text-fig. 7).
The most abundant medusa throughout the year is that of Obelia geniculata (L.) (Text-fig. 3, D). The hydroids are found in great numbers on the Macrocystis pyrifera colonies at Point Gordon, Kau Point and in Kau Bay. The hydroids start to liberate medusae in April, when small medusae measuring between 1.6mm and 2.5mm in diameter occur abundantly in the plankton. From May through to December both large and small medusae are very abundant. These are very common in January. Few medusae are recorded in February and March, when the majority of these are large and measure over 5mm in diameter (Text-fig. 7).
The medusae of Octophialucium funerarium (Quoy and Gaimard) (Text-fig. 3, E) and of Phialella quadrata (Forbes) (Text-fig 3, F) occur in significant numbers in the plankton. These are readily separated by the presence of eight radial canals in Octophialucium funerarium, and four radial canals in Phialella quadrata. The medusae of Octophialucium funerarium occur rarely in occasional August samples, and this infrequent occurrence continues from September into December. Few medusae are recorded in January, and they are rare in February and March. During the next four months occasional very large medusae were caught below six fathoms, but were absent from the surface samples. Phialella quadrata is both smaller in size and more common, and occurs throughout the year. This medusa is few in number in September, common in October through to December, very common in January, and rare for the remainder of the year (Text-fig. 7).
The eggs, larvae, and juveniles of fish of many species are abundant in the spring plankton (July to October), but very few of these could be identified. Fish eggs are present throughout the year (Text-fig. 7). They are very abundant in July, August and September, and abundant in April, October and November. Except in January when few are recorded, fish eggs are common or very common for the remainder of the year.
The following juvenile fish have been identified:
Larval Crustacea belonging to many genera and species are conspicuous in the plankton. This Class has been made the subject of a special study.
It was not possible to identify euphausid larvae without dissection, and the significance of this group in the harbour plankton did not warrant this detailed study. Several species first occur in September and are present through to December, but in these months they are few in number. They are common in January and February, but rare in March and April. Euphausid larvae are absent for the remainder of the year (Text-fig. 7). An euphausid larva, probably of the fifth "Furcilia" stage is illustrated in Text-fig. 4, A.
Two different cirripede nauplii were recorded from the harbour, but neither could be identified. A large sized species (Text-fig. 4, B) occurred for three months in the spring (August to October), but it was never abundant except in September (Text-fig. 7). Other much smaller nauplii and metanauplii, probably of two species, occurred both in spring and autumn, but never in quantity.
In Europe cirripede larvae occur with explosive outburst in the spring at the time of diatom increase (Pyefinch,
Two species of Stomatopoda are recorded for Wellington Harbour. These are Squilla armata Milne Edwards, and Heterosquilla spinosa (Wood-Mason). Two larval forms were also obtained from the plankton, and these were reared through to a juvenile stage in the laboratory and referred to the above two species.
First and second stage "Alima" larvae of Squilla armata are common in March and April and very common in October of each year. Although later stage larvae are present all the year round (Text-fig. 7), the high proportion of early stage larvae in the above months suggests that in each year, S. armata liberates larvae in a spring peak and a less pronounced autumn peak. A stage two "Alima" larva of S. armata is illustrated in Text-fig. 4, D.
The larvae of Heterosquilla spinosa first appear rarely in August, and few are recorded from September into January. They are absent for the remainder of the year (Text-fig. 7). A high proportion of early stage larvae occur in spring (August to October) which is probably the only spawning season of the year for H. spinosa. A stage five larva of this species is illustrated in Text-fig. 4, C.
Natant decapod larvae of both oceanic and shallow water origin occur regularly in the Wellington Harbour plankton. These larvae could not be identified with certainty beyond genus and the only reference of value is the reports of the "Terra Nova" (
Oceanic larvae of the following genera have been obtained:
The larvae of Sergestes sp. occur rarely from July to September, but are absent for the remainder of the year (Text-fig. 7). The larvae of two species of the genus Pandalus occur rarely throughout the year (Text-fig. 7). Early stage Pandalus larvae are more common in October and November, and again in April
Larvae of shallow water decapod Natantia were common within the harbour, and again these larvae could not be placed with certainty in any genus.
Adults of Jasus edwardsi (Hutton, J. lalandei) are found at Kau Point. Small numbers of first stage Phyllosoma larvae attributed to this species (Text-fig. 5, A) are rare in the September plankton but are absent for the remainder of the year. Later stage Phyllosoma larvae were not found. Dr Pike (Mar. Dept. pers. comm.) has found that "berried" females of J. edwardsi from Island Bay, Wellington, begin to liberate their larvae early in August, and about half the females are spent by the beginning of October. The appearance of first stage Phyllosoma larvae in Wellington Harbour in September agrees with these observations. Jasus edwardsi is the only known representative of the Palinura recorded in Wellington Harbour. Jasus verreauxi (Milne Edwards) is not known to breed as far south as Wellington (Pike, pers. comm.).
Larvae of about 18 species of Anomura are found in the Wellington Harbour plankton. Many of these have been identified in the following families:
The larvae of this section (Anomura) are more abundant than those of any other section of the Decapoda, and occur with greater regularity.
The family Galatheidae is represented by two or three species probably belonging to the genus Munida Leach. These occur in the plankton rarely from August to November and in March and April. No larvae were obtained in the intervening months. As the larvae were taken mainly on the flood tide they were probably carried into the harbour by the incoming oceanic water.
From Lebour (1930) the larvae were readily identified as belonging to the Family Galatheidae and probably to the genus Munida, but they possessed features differing from those of the typical Munida larvae and rather more characteristic of the genus Galathea Fabricius.
The first species, of which a stage five (final stage) zoea larva measuring 6.5mm in length has been illustrated (Text-fig. 5, B) has a rostrum with a much broader base than is usual for Munida, and it is rather more typical of Galathea. The antennal scale is also more like that of Galathea, and is shorter and broader than in known Munida larvae. On the other hand the telson is longer and narrower and has more posterior setae than is usual in either Munida or Galathea larvae.
A stage four larva obtained from the Wellington plankton is very similar to the illustrated stage five larva. However this larva measured only 3.8mm in length. A length increase of 2.7mm is hardly possible in successive larval stages, so it is
The second species from the Wellington plankton has a rostrum and antennal scale rather more typical of Galathea than of Munida, but the long slender shape of this larva and the form of its telson identify it with the genus Munida.
Hence, when obtained, the adults of these larvae will also have characters that differ appreciably from the typical Munida gregaria which is so plentiful in the outside deeper water fauna. The early post larva of this species occurs in dense swarms in the Tory Channel area and around Banks Peninsula in December. Its larvae should therefore be present in abundance some three months earlier (September and October). The breeding season of M. gregaria and the two (probably three) species from the Wellington Harbour should therefore coincide. The larvae of M. gregaria have been described by Rayner (M. gregaria appears to be the only species of Munida recorded for New Zealand, the Wellington larvae must belong to an unrecorded species. However when fully identified these larvae could conceivably belong to a genus other than Munida.
During the Victoria University of Wellington deep water biological investigations in Cook Strait (1954-1957) adults of eight or nine galatheid species representing the genera Munida, Galathea and Uroptychus Henderson, and possibly several unknown genera were collected. None of these have yet been described. From the Bay of Plenty, Pike (pers. comm.) has recorded the genus Munidopsis Whiteaves, as well as species of Munida, Galathea, and Uroptychus which were not obtained during this University's work in Cook Strait. Hence there are four or more genera and twelve or more species of Galatheidae occurring in New Zealand waters which have not been described as adults. Of these the larvae of Munida gregaria only are known (Gurney, Zoo. Rec.
The zoea larvae of Petrocheles spinosus Miers, Petrolisthes novaezelandiae Filhol and Petrolisthes elongatus (Milne Edwards), with their enormously long rostrum and posterior carapace spines, are among the most distinctive decapod larvae in the plankton. The larvae of these three species have been described in detail (Wear,
Petrocheles spinosus larvae are present from July through to January of each year. However these larvae are rare (Text-fig. 7) and occur only in some samples.
The zoea larvae of Petrolisthes novaezelandiae are found in the plankton throughout the year. Few larvae are recorded in August, but they are common in September and October, very common in November, December and January, and common from February through to April. However these larvae are usually rare in May, June and July (Text-fig. 7). Occasional tows may catch P. novaezelandiae larvae in great abundance, especially during November and December.
Petrolisthes elongatus zoea larvae first appear in September, but they are rare from this month until the end of November. Few larvae are recorded in December and January, but they are common in February and rare in March. From April to August they are not recorded (Text-fig. 7).
Two species of axiid larvae were recognised in the Wellington plankton.
The first species (Text-fig. 5, C) is rare in August and October, and is common only in September. It is absent for the remainder of the year. This larva compares with a larva described by Gurney (
The second species is represented by a few larvae in October and November samples from Wellington Harbour, but in December they are rare. This larva is not recorded for the remainder of the year. This species is very similar to two species described by Gurney (Iconaxiopsis sp., and the second species from Wellington Harbour is sufficiently similar to these to warrant its inclusion in the genus Iconaxiopsis.
There have been no adult Axianassidae described from New Zealand waters.
One of the most common and conspicuous larval decapod species in the summer plankton is the striking, long-necked, Lucifer-like larva to which the name of "Trachelifer" was given by Brook (Jaxea nocturna (Chierghin), and attributed to the genus Jaxea Nardo. Larvae caught in Wellington Harbour (Text-fig. 5, D), when reared, moulted to a juvenile of the genus Jaxea having adult features of this genus.
The larvae of Jaxea sp. first appear rarely in September and few are recorded in October and November. They are abundant in December and January, common in February, and rare in March. From April to August these larvae are not found in the plankton (Text-fig. 7). A single 20 minute horizontal plankton tow in late December or in January may produce over 5,000 "Trachelifer" larvae. During these two months considerably fewer larvae were found in occasional samples from other areas in the harbour. The abundance of first and second stage larvae caught at Stations One and Two suggests that adults are present in considerable numbers in the substrate of Kau Bay and Mahanga Bay.
No adult species of Jaxea have yet been described from New Zealand waters. However, Pike (pers. comm) has obtained an undescribed Jaxea in 40 fathoms to 90 fathoms in several North Island localities. I also obtained seven adults from Evans Bay, Wellington Harbour, in Mustelus antarcticus). The characters of the juvenile reared from Wellington larvae compare closely with those of the adult species obtained by me and by Dr Pike.
Callianassid larvae from the Wellington Harbour plankton (Text-fig. 6, A) were identified as Callianassa filholi Milne Edwards by hatching first stage larvae from an ovigerous female. Later stage larvae were obtained from the plankton and reared through to the juvenile stages. A few of these larvae first appear in the July plankton. They are very common in August, abundant in September, and very common from October through to late December. They are still common in January but rare in February and March, and absent from April to June (Text-fig. 7).
C. Devine (pers. comm.) in his M.Sc. thesis studies on C. filholi at New Brighton, Christchurch, in C. filholi are known to occur on the sandy beaches of the eastern side of the harbour. However in August and September, early stage larvae occur abundantly at Stations One and Two, usually in large swarms, and it is improbable that these are liberated on the eastern side of the harbour. Relatively few larvae were found in plankton samples from other
C. filholi favouring a sub-littoral habitat in Wellington Harbour must now be considered.
Callianassid Crustacea have been recorded from most of the oceans and from habitats which range from 400 fathoms to between the tide marks (de Man, C. australiensis was strictly intertidal, and Dahl (C. subterranea in the Gullmar fjord, Sweden, is restricted to depths of 30 to 35 metres.
Our knowledge of the systematics of the New Zealand Paguridae is still confused, and relies mainly on the work of Filhol (
Characters of pagurid larvae are given by Gurney (commonly from September to December and again in April and May, but are rare for the remainder of the year. A fourth species is common in October and November only, and the fifth species is rare and occurs only during the spring (September to November). However the family Paguridae is represented in the plankton all the year round (Text-fig. 7). One of the three more common species is illustrated (Text-fig. 6, B) and is a larva of the "Pagurus bernhardus" type (MacDonald, Pike, and Williamson,
Three species of zoea larva were reared in the laboratory to the megalopa stage, which in the Paguridae is known as the "Glaucothoe" larva. One of these survived to the first juvenile hermit stage but this lacked sufficient adult characters for identification.
"The systematic position of the Dromiacea has, on the evidence of adult anatomy, been a subject of considerable difference of opinion, and they have been associated on the one hand with the Anomura and on the other hand with the Brachyura" Gurney (
Gurney (
A dromiid larva occurs rarely in the Wellington Harbour plankton in spring and autumn. It is considered to be of oceanic origin as it is more plentiful in samples from Palliser Bay and Island Bay. This larva is certainly a dromiid, but there have been no adult dromiids described from New Zealand waters. However Hale (Petalomera wilsoni (Fulton and Grant) from South Australia,
Petalomera larvae are unknown, but they will undoubtedly follow the general dromiid pattern. These Wellington larvae may well belong to P. wilsoni.
The characters of this larva support the separation of the Dromiacea from the Brachyura, and further, on the basis of several "pagurid" characters it supports Pike and Williamson (
This larva will be the subject of future work.
With the exception of the Dromiacea, all brachyuran larvae conform to a general morphological pattern which is remarkably uniform throughout the whole group (Gurney,
The high degree of endemism among New Zealand Brachyura and the lack of literature concerning the larvae of these endemic groups makes identification impossible without either hatching the eggs of identified ovigerous females, or rearing the larvae to a juvenile stage which can then be referred to an adult species. There is an urgent need for further work in this field.
The zoea and megalopa larvae of about 32 brachyuran species can be distinguished in the Wellington Harbour plankton. This is more than the number of adult species recorded for the Wellington area. The majority of these larvae could not be identified. However, the few genera and species identified belong to the following families:
Among the fauna are several families and genera in common with the Plymouth fauna of the United Kingdom, possessing larval stages comparable with those belonging to corresponding Plymouth groups (Lebour,
Larvae in the Wellington Harbour plankton were identified as Cancer novaezealandiae (Jacquinot and Lucas) after rearing the megalopa larva to a juvenile crab in the laboratory. These larvae first appear rarely in July or in August, but in September and October they are common. Few larvae are recorded in November, and in December they are again rare. From April through to June this species is not recorded (Text-fig. 7).
The zoea larvae are similar to known larvae of the genus Cancer in having dorsal, rostral, and paired lateral carapace spines which are all well developed. The smooth dorsal and rostral spines are long and straight, and the lateral pair
Larvae of two species of this genus are found in the Wellington plankton. These larvae are rare in August, few in September and October, and common in November. Few larvae are recorded in December and January, and by February and March they are again rare. From April to July no Pinnotheres larvae were found in the plankton (Text-fig. 7).
The zoea larvae of the genus Pinnotheres are characterised by their very small size, their great reduction or absence of carapace spines, and their tri-lobed telson (Text-fig. 6, D). They are very similar to the zoea larvae of the genus Halicarcinus (Text-fig. 6, E) but readily separated from these by the shape of the telson.
One of the Wellington species cannot be distinguished from the zoea larva of P. novaezealandiae Filhol described by Bennett (P. schauinslandi Lenz, as this is the only other adult of the genus Pinnotheres recorded for New Zealand (Bennett,
The larvae of H. edwardsi Filhol, and two other species of this genus occur in the plankton throughout the year. These larvae are common in July, very common in August and September, common in October, and very common in November. Few are recorded during December, but they are once more common in January and February. However, few of these larvae are recorded between March and June (Text-fig. 7).
One zoea larva from the Wellington Harbour plankton was identified as H. edwardsi by comparing planktonic larvae with larvae hatched from an ovigerous female by J. C. Yaldwyn. These hatched larvae were kindly lent to the author by Dr Pike. The remaining two planktonic larval species were very similar to the larvae of H. edwardsi, and undoubtedly of the genus Halicarcinus. A late stage zoea larva of Halicarcinus sp. is illustrated (Text-fig. 6, E).
Zoea larvae of the genus Halicarcinus are very similar in their primary characters to those of the genus Pinnotheres. However Halicarcinus zoeae are distinguished by their long, rectangular, forked telson without lateral lobes (Text-fig. 6, F). The zoea larva from Spirits Bay which Gurney (Pinnixia". However, Bennett (Halicarcinus.
The zoea larvae of Hemigrapsus edwardsi (Hilgendorf) are rare in the September plankton, few in October, and common from November into January. Few larvae are recorded in February and March and the species is again rare in April, but absent for the remainder of the year (Text-fig. 7).
The larvae were identified by rearing a final stage zoea through to a juvenile crab. Quite remarkable success was achieved with the rearing of this species. On
Zoea larvae of Heterozius rotundifrons Milne Edwards (Text-fig. 6, H) are few in the plankton from November through to February. They are rare in March and April and absent for the remainder of the year (Text-fig. 7). Larvae were reared from identified ovigerous females and subsequently identified in the plankton.
The plankton calendar (Text-fig. 7) has been compiled from seasonal variation in the numerical occurrence of the described species in Wellington Harbour plankton samples from
Several of the species discussed were occasionally present in quite small numbers, but by virtue of their large size they were found to make an appreciable contribution to the volume of plankton. Such species were Thalia democratica, Ihlea magalhanica, Octophialucium funerarium, Phialella quadrata, and Pleurobrachia pileus. When present, these species together with the smaller but very abundant medusae of Obelia geniculata cause major volumetric increases.
An attempt was made to calculate the number of individuals of each species required to make up a given volume, but the very considerable size range of some species made this impracticable. For example the great size differences between solitary and aggregate forms of the Salpidae, between juveniles and adults of Pleurobrachia pileus, and between small and large medusae of Octophialucium funerarium and Phialella quadrata, made any such calculations misleading. However an approximate relationship between numerical abundance and volume with regard to Obelia geniculata, Octophialucium funerarium, Phialella quadrata, Pleurobrachia pileus, and Salpidae, may be obtained by correlating Text-figs. 7 and 8.
The important features of the plankton in each month are described below, and these are largely based on the data presented in Text-figs. 7, 8 and 9. All quantities are equated with a single 20 minute horizontal plankton tow, using a 36 gauge two-foot diameter cone net at depths between one fathom and six fathoms. The numerical equivalents of the expressions "rare", "few", "common", "very common", "abundant", and "very abundant" are given in the key to Text-fig. 7. Where these expressions have been used in this sense they have been italicized.
Sea temperatures vary between 17°C. and 19°C. The plankton volume is generally about 100cc, or more when Thalia democratica is abundant. Obelia geniculata occupies less than 10% of the volume, while Salpidae, especially Thalia
democratica, make up over 40% of the volume of most samples. Hydromedusae make up about 20% of the volume. Pleurobrachia pileus is rare and may be absent. About one-third of the volume comprises larval Crustacea, with 33 decapod species recorded for this month. Of these the larvae of Jaxea sp. and Petrolisthes novaezelandiae are usually the most common, but other species are strongly represented.
Sea surface temperatures vary between 17.5°C. and 20°C. but are consistently higher than in January. The volume of February samples is rarely more than 10cc, and all species are poorly represented. Pleurobrachia pileus is usually absent, and the only stomatopod larvae recorded are those of Squilla armata. Twenty decapod species are found in February, but these are represented by few individuals excepting the larvae of Petrolisthes elongatus, P. novaezelandiae, Jaxea sp. and Halicarcinus spp. which are all common.
Sea surface temperatures in this month are between 16°C. and 17°C. Plankton volume is about 15cc, with greater numbers of Thalia democratica being mainly responsible for this small increase. Larval Crustacea have increased with first stage larvae of Squilla armata, Munida spp., Paguridae and some brachyuran species recorded. Twenty-six larval decapod species are present with Petrolisthes novaezelandiae and Paguridae the most common.
Sea surface temperatures are between 14°C. and 16°C. but may fall to 12°C. by the end of the month. Plankton volume is about 50cc. Large numbers of Obelia geniculata medusae and to a lesser extent Pleurobrachia pileus are together responsible for this increase. Phialella quadrata and Thalia democratica are recorded, but usually in small numbers. Fish eggs are abundant, but fish larvae are still rare. Twenty-two larval decapod species were found in April, with Petrolisthes novaezelandiae and Paguridae still the most common of these.
Sea surface temperatures are between 12°C. and 14°C. The volume of May samples is extremely variable. Catches of up to 500cc may be taken towards the end of the month with vast numbers of Obelia geniculata medusae and Pleurobrachia pileus comprising over 90% of this volume. Thalia democratica occurs only rarely, usually towards the beginning of the month. Decapod larvae are poorly represented and only 15 species are found. Pagurid larvae are the most common of these.
Sea surface temperatures fall from about 12°C. at the beginning of the month to 10°C. at the end of June. The volume of June samples is generally more than 130cc, and occasional tows may produce more than a litre of plankton mainly comprising a great abundance of Obelia geniculata medusae and Pleurobrachia pileus. Copepoda are now abundant and occasionally make a large contribution to the volume of plankton. June is the poorest month for larval Crustacea. The most common crustacean larvae are those of Squilla armata. Only nine decapod species occur and all of these are very poorly represented.
Sea surface temperatures range between 10°C. and 11°C. but slightly lower temperatures are occasionally recorded. Vast numbers of Obelia geniculata medusae and Pleurobrachia pileus still persist and may comprise over 90% of the volume
Phialella quadrata was very common in July samples of very abundant and fish larvae are now common. As in June Copepoda are abundant. Of the decapods, 18 species are found in July, but these are represented mainly by a few first stage larvae occurring towards the end of the month.
Sea surface temperatures are the lowest for the year in early August and readings below 9.5°C. are relatively common. However towards the end of August there is a rise to approximately 11°C. The volume of plankton is usually greater than 100cc, and is comprised mainly of Obelia geniculata medusae and Pleurobrachia pileus which occur in abundance. Octophialucium funerarium and Phialella quadrata are present but their abundance is variable. Fish eggs are very abundant and fish larvae very common. Copepoda are very common. The spring increase of larval Crustacea makes up about one-third of the volume of plankton. Twenty-eight species of larval decapod can be recognised in August samples, the most common larva being of Callianassa filholi.
Sea surface temperatures between 11°C. and 13°C. are recorded in this month. The average volume of plankton is about l00cc, and over one-third of this volume is made up of Copepoda and crustacean larvae. Obelia geniculata medusae are still very abundant and Pleurobrachia pileus is common, but these species do not occur in the vast numbers characterising the winter months. Large numbers of fish eggs and larvae are usually present. In September larval Crustacea reach a maximum which is sustained through to late January. Stomatopod larvae are common and cirripede nauplii and metanauplii occur in abundance. Larval decapod Crustacea are very numerous and 44 species occur. Of these Callianassa filholi dominates all samples and may be very abundant. Other species are well represented. Jasus edwardsi Phyllosoma larvae occur in September only.
Sea surface temperatures are between 13°C. and 15°C. Average tows produce between 50cc and l00cc of plankton. Obelia geniculata is very abundant and Pleurobrachia pileus is common, but together they comprise only about 50% of the volume. Fish eggs and larvae are still very common, but these do not occur in the abundance characterising the previous three months. In comprising about 45% of the volume, larval Crustacea are more abundant than in any other month of the year. Squilla armata larvae are very common with early stage larvae dominant. Forty-six larval decapod species are recorded with Callianassa filholi the most abundant species.
Sea surface temperature recordings vary between 15°C. and 16.5°C. Between l00cc and 200cc of plankton is produced by an average tow in this month. The bulk of this volume is made up with Obelia geniculata, Pleurobrachia pileus, Octophialucium funerarium, and Phialella quadrata. Crustacean larvae are still very abundant, but these comprise less than one-third of the volume of plankton. Decapod Crustacea are represented by the larvae of up to 45 species, with Petrolisthes novaezelandiae, Callianassa filholi and Halicarcinus spp. the most abundant.
Sea surface temperatures vary between 15°C. and 17.5°C. but most of the recordings are around 17°C. The volume of plankton in this month is similar to November, and varies between l00cc and 150cc. The abundance of Obelia geniculata medusae falls off rapidly towards the end of the month, and by December Pleurobrachia pileus is usually rare. Ihlea magalhanica occurs rarely, but Thalia is still absent. Larval Crustacea are abundant but comprise less than one-third of the volume of plankton. Forty species of decapod larvae have been isolated in December samples. Petrolisthes novaezelandiae, Callianassa filholi and Jaxea sp. are dominant among the Decapoda. Up to 5,000 larvae of Jaxea sp. are recorded in some samples. In December brachyuran zoeas occur in peak abundance for the year.
Although usually conforming to the described monthly pattern, the plankton is characterised by short-period fluctuations which are non-seasonal.
In several cases plankton samples were neither quantitatively nor qualitatively representative of the month in which they were taken. The following volumes were recorded for successive samples from Station One in
The sample taken on 5 July produced only a small fraction of the volume normal for these months, and Obelia geniculata medusae and Pleurobrachia pileus which usually dominate the June and July plankton were very scarce. The larvae of several brachyuran species present in the majority of the above samples were absent on 5 July.
A similar example occurred on Jaxea sp. were rare, and Salpidae, Octophialucium funerarium and Phialella quadrata were absent.
The larvae of species occurring seasonally were occasionally identified in samples taken outside the expected seasonal boundaries. A single stage one larva of Petrocheles spinosus was recorded on Jaxea sp. were recorded on
During the spring and summer months occasional plankton samples were dominated by many hundreds of early stage larvae of any one of several crustacean species. These monospecific swarms were probably produced by large numbers of adults liberating their larvae simultaneously, and the larvae were not dispersed by tidal action. This is supported by the observation that species occurring sparsely in the plankton for several weeks may be suddenly represented by an abundance
Identified decapod Crustacea forming such larval swarms are as follows: Hemigrapsus edwardsi, Petrolisthes novaezelandiae, Callianassa filholi, Jaxea sp. Of these, Callianassa filholi and Jaxea sp. swarm quite frequently in the months of their greatest abundance.
Surface water temperature probably influences numerically dominant species thereby controlling many of the fluctuations in the volume of plankton. A critical temperature seems to be between 15°C. and 16°C. When the temperature falls consistently below this level Obelia geniculata medusae and Pleurobrachia pileus occur in great abundance and cause a sharp rise in the volume of plankton. As the temperature rises above 16°C. in early summer these two species become rare or absent. The converse probably applies to the Salpidae, especially Thalia democratica which makes a large contribution to the volume of summer and autumn samples. Salpidae do not occur until early December when the surface water temperatures are consistently above 15°C. or 16°C. but in late May when the temperature again falls below 15°C. Salpidae are absent.
Minor variances from the described monthly pattern were noted during the research period. For example in Jaxea sp. and Callianassa filholi larvae appeared in the plankton two or three weeks earlier than expected, and Pleurobrachia pileus became absent at the end of November, which was over a month earlier than in
It is not possible to interpret the causes of early or delayed larval liberations, shortened or protracted liberation periods, or the many irregular variations characterising the plankton samples without continuous plankton recording over an uninterrupted period of time correlated with meteorological and hydrological data.
Skerman (
Skerman's data and the data obtained during this current research programme suggests that marine animals within the harbour environment are influenced by sudden short-period variations in temperature. During laboratory rearing experiments, adult Crustacea collected from the littoral and sub-littoral margins of Wellington Harbour, and juvenile Crustacea reared from planktonic larvae, survived abrupt temperature changes of plus or minus 16°C. without obvious detriment. This showed that many species possess a wide eurythermal capacity.
Skerman contends that the short-period temperature fluctuations in the harbour environment may be outside the eurythermal capacity of the shelf forms beyond the harbour entrance—a factor which may determine the composition of the harbour population. From laboratory experiments involving living plankton, the fact that the larvae of adult Crustacea known to occur only beyond the harbour entrance (Sergestes sp., Pandalus spp., Munida spp., and possibly Petalomera sp.) survived abrupt temperature changes over a wide range suggests that short-period temperature variations are not the limiting factor determining the harbour population. A more probable limitation would be an insufficiently long period of sustained optimum temperatures to induce some species to breed, or to complete their larval metamorphosis.
Wilson (Plymouth) showed that the nature of the water itself had a very marked influence on the development of larvae (polychaetes and echinoderms), and it is possible, therefore, that the oceanic larvae which come into the harbour environment are unable to develop in this water mass. Many species living on or in different types of substratum are unable to metamorphose unless the required substratum is found (Wilson,
Dakin and Colefax (
The annual range of sea surface temperatures for New South Wales is from 23°C. in February to 15°C. in August. The temperatures are therefore generally higher than in the Wellington Harbour where slightly lower summer maxima (20°C.) and much lower winter minima (9°C.) are recorded.
In the New South Wales plankton the abundance of Thalia in all months except June, July and August is probably a result of the relatively high sea temperatures. Thalia occurs only in late summer and autumn in the cooler Wellington waters.
Obelia geniculata and Pleurobrachia pileus which dominate the Wellington Harbour winter plankton are not discussed by Dakin and Colefax (P. pileus. The winter "bloom" of Obelia geniculata and Pleurobrachia pileus in the Wellington plankton suggests that these species favour lower temperatures than occur in New South Wales waters. It is probable that the distribution of Obelia geniculata follows the southern circumpolar, cold-temperate distribution of Macrocystis pyrifera outlined by Knox (
The zooplankton of the Wellington Harbour follows essentially the same seasonal pattern described for that of New South Wales. However, the zooplankton of New South Wales developed in late spring or in early summer (November or December) and tended to sustain itself at a high level during the summer and into late autumn. The peaks are in November-December and in March (Dakin and Colefax,
From a consideration of the evidence presented in this study, it is clear that the majority of zooplanktonic species occur in a seasonal cycle. This cycle is independent of the seasonal trends in the total volume of the plankton. Total volume is largely dependent on the abundance of Obelia geniculata and Pleurobrachia pileus which tend to have temperature optima differing from the majority of species in the plankton.
As the major components of the Wellington Harbour plankton are known to occur elsewhere in New Zealand waters, it is possible that the plankton of other coastal or harbour areas of New Zealand shows a similar seasonal pattern to that described for Wellington Harbour. There is a need for further work in this field which is largely still untouched, and studies of this type from other New Zealand localities would prove especially valuable.
I am grateful to Professor L. R. Richardson for suggesting this study, for placing the Zoology Department's plankton collection at my disposal, and for his guidance throughout this work. I am indebted to the academic staff, technicians and students of the Zoology Department who assisted in collecting plankton from Wellington Harbour. I wish to thank Dr P. M. Ralph for identifying planktonic coelenterates, and Dr R. B. Pike (Marine Department) for assistance with literature on decapod larvae and for encouraging discussions and constructive criticism.