Infestation by Bankia avstralis Caiman (Figs. 3 and 4)

X-ray photographs of the first short-term block proved negative, and the first indication of Bankia attack was given by the second-month long-term block, where X-ray photographs revealed a light infestation of young animals ranging in size from 0·3 cm. to 1·8 cm. in length. By a careful comparison of the photograph and the surface of the test block, it was possible to see the minute respiratory holes of these young animals. In all cases, X-ray photographs failed to reveal the presence of Bankia in the short-term blocks. Isham, Moore, and Smith (1951) also record that "in panels exposed for only one month ship worms were neither large enough or numerous enough to furnish material adequate for growth rate determination." However, in February, when the long-term blocks clearly indicated a heavy set, it was possible in the present instance, with the aid of a hand lens, to make a fairly accurate count of respiratory holes on the short-term blocks.

Bankia respiratory holes are readily distinguishable from those of Limnoria, and counts of the holes give a reasonably accurate estimate of numbers present after the first few months. Counts from X-ray photographs give a check on the numbers obtained from counts of the surface holes. In large specimens of Bankia australis, the pallets often protrude slightly from the surface holes, and counting is then easy. The burrows are lined with lime, and usually go a short distance straight into the wood before turning to run with the grain. The fact that the shell valves and pallets are also mainly composed of lime greatly facilitates the count, as these calcareous structures show clearly in X-ray photographs. Accurate measurements of burrow lengths cannot be obtained after about the tenth month because of the number of intimately intertwining burrows.

Fig. 2.—Monthly incidence of Limnoria quadripunctala Holthius in the long-term test blocks. Temperatures shown by triangles. Full explanation of diagrammatic presentation of monthly temperature records on page 3 of text. *Approximate numbers.

From the second month (May, 1949), Bankia was clearly indicated on the X-ray plates. The burrow length for this month was from 0·3 cm. to 1·8 cm. Pallets could not be distinguished in specimens 0·3 cm. in length, were faintly visible in the 0·6 cm. to 0·8 cm., but were distinct in all specimens above this size. The smallest Bankia australis recorded on the X-ray plates was 0·3 cm. in length. There appeared to be no significant difference in growth rate between animals in the horizontal and vertical blocks. Over the first nine-month period (April to November, 1949), there were constant small reinvasions of the blocks by larval

Fig. 3.—Monthly incidence of Bankia australis Caiman in the long-term test blocks. Water temperatures shown by triangles.

stages. Numbers settling increased markedly from December, 1949, to April, 1950. The peak was February, when approximately 2,00 settled, with another 750 already established.

Figure 4 gives the growth of B, australis based on measurement of burrow lengths; the principal curve gives the length of the longest burrow in the block for each month. The other data for each month shows the groupings of the shorter burrows as a vertical line, with the average in each group marked by a filled-in circle, These latter groups are here considered as representing monthly sets, page 19 although—e.g., November—there are ten such groups which exceed the number of months and indicate more than one heavy set in some months. The principal curve, in general, is that of an ordinary growth curve, but shows a deceleration of growth rate in September and again in November (i.e., winter and spring). This is also noted by Isham, Moore, and Smith (1951), who have interpreted this as consequent from overcrowding, but in terms of the actual set found on the short-term blocks, the total population is still short of its maximum. In any case, the burrows measured are those of animals which have lived through the period of sharply increasing growth-rate (August and October), and did not benefit to the same degree as did those measured in the September and November blocks. This deviation of the curve accordingly must be considered as due to individual variation and not having the significance of the variation in Isham, Moore, and Smith's curve.

Fig. 4.—Growth curve of Bankia australis Caiman based on burrow lengths. Principal curve gives lengths of longest burrows. The symbol shows groupings of shorter burrows. Burrows measured at the end of each month.

results are a true indication of the growth and incidence of these borers in Port Nicholson. The sudden rise in numbers in February, followed by a marked decrease in March, is difficult to explain. A steady rise in numbers is more to be expected, with perhaps a flattening-off of the curve of total numbers about March with the decrease in temperature. The March and April, 1950, blocks were exposed to the same heavy settling as the February block as well as later settlings. A possible The incidence of Bankia australis over the thirteen-month period shows one or two unusual features which seem most reasonably to be accounted for by fluctuations in local conditions. Further study is necessary to see if these preliminary explanation seems to be that the adult Bankia in the February block liberated large quantities of spawn, and the resulting larvae settled mainly on that block. There has obviously been a decline in the survival rate, and for some reason spawn liberated in March and April failed to survive in high numbers on these blocks. Calcareous encrustations such as barnacles, oysters, the tubes of marine worms, and encrusting bryozoa form an armour against the entry of borers into wood, especially when their establishment precedes larval lodgement (Watson et at., 1936). This may have been a factor which prevented survival from large settlings on the March and April blocks, as on these other species were well established. However, these species were present also on the February block, and it is hard to see why they failed to reduce the numbers of larvae settling in February if they were one of the chief factors responsible for reduced settlement in March and April.

Several factors are known to influence "ship worm" attack, and different factors are thought to influence different species of "ship worm." Caiman (1919) claimed that pollution or muddy water rendered timber less likely to attack. These factors were present to a minor degree only, and seem unlikely to have influenced the numbers of settling larvae in the present experiment. The greater number of animals in the horizontal unit of the test blocks is in accordance with the findings of Shillinglaw and Moore (1947), who report that non-resistant cross and diagonal bracing below mid-tide level are very susceptible to attack. As all the blocks were placed at approximately the same depth, no indication of the vertical working range of Bankia australis can be given at the present time. It is generally agreed that, where the substratum is suitable, certain types of fouling organism are indicators of likely areas of borer attack. These include barnacles, hydroids, and mussels (San Francisco Bay Marine Piling Survey, 1923; Watson et al., 1936). Where these animals are already present in an area, borers have usually made their presence obvious also. Species of all the indicator groups enumerated above are present on the wharf piles in the vicinity of the present experiment; and, as indicated by the above description, borers have also made their presence obvious. In an area where barnacles, hydroids, and mussels suddenly make their appearance and where no borer troubles have been previously experienced, then one may page 21 expect borers, since salinities and temperatures that suit borers are usually within the ranges of tolerance of these other animals. The sudden appearance of barnacles, hydroids, etc., suggests a change in local salinities and temperature conditions which will suit the borers. Environmental conditions vary within the harbour area, and further experiments are necessary to give a more complete picture of borer activity in Port Nicholson. Most species of Bankia show preference for water of high salinity (Watson et at., 1936), and one would predict that B. australis may be less active on the eastern side of the harbour near the Petone foreshore, as the movement of the Hutt River water in the harbour on this shore must lower the salinity of the water.