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The Settlement and Metamorphosis of Marine Animals

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The Settlement and Metamorphosis of Marine Animals 32 N.Z. Ec0L0GIcAL S0c I ETY creased beyond that which we would expect that the very old are but little affected by in the wild, is surely significant. the force of natural selection. Hence, dele- terious mutations may not only find a foot- Envir'onmental senescence arises from the ing here, but may also accumulate. Further- many injuries and stresses an organism en- more, natural selection must tend to push counters throughout life; but how does hereditable degeneration further and fur- innate senescence 'arise? In the simplest ther towards the end of the reproductive case the life table of a stable population sub- period. Here then, is one way of accounting ject only to constant random mortality for some aspects of innate senescence. But shows that each succeeding age group is as few animals in the wild ever survive the smaller than its predecessor. If, instead of risks of living long enough to reach an being potentially immortal, members of this advanced chronological age (as we can see population die through some sudden innate when we compare life tables for wild and degeneration after reaching a certain age, captive animals of the same species) it is the effect upon the population will depend understandable why senility is so rare a upon the age at which this occurs; if it hap- natural occurrence. pens far enough along the survival curve there will be so few animals left alive that REFERENCE its occurrence will probably pass unnoticed MEDAWAR, P. B., 1952. An unso,lved problem of b'i- (rather like senility in wild animals). So ology. H. K. Lewis, London. The Settlement and Metamorphosis of Marine Animals Professor E. Percival Thorson (1) in discussing the choice of simple, straightforward processes taking substratum by marine larvae, points out place without reference to the substrate. that further detailed knowledge of life cycles Cole et al. (2) in studying settlement of indicates that larvae possess more adapta- oyster larvae have shown that although free- bility than has been hitherto assumed. dom from silt is essential, abundant settle- For instance, when plankton is collected ment occurs on cultch (shells or tiles) by two-net, it is not unusual to take larvae fouled with other attached organisms,. par- of ophiuroids or echinoids, which are in pro- ticularly recently attached oyster spat. cess of metamorphosis, but in Lyttelton Much less settlement takes place on clean Harbour, completely metamorphosed ophiu- shells. Whether settlement is encouraged roids or nearly completed metamorphosed by secretion into the water of some attrac- spatangoids are taken near the bottom. S pe- tive substance by spat or by fully settled cimens taken nearer the surface are less young has not been determined. changed or not at all. Observations made by Moore (3), 'Pye- That some larvae of benthic animals will finch (4), Knight-Jones (5) and others, readily metamorphose is shown by fully ma- show that cyprids (one of the larval phases ture larvae of Phor~onis metamorphosing in of barnacles) move over a surface, appar- sea-water in a glass dish by no obvious stim- ently testing it. They settle readily in ulus. It may be that the actinotrocha does cracks and scratches and in angles. Settle- metamorphose in open water and that the ment was found normally to be followed by adhesive surface exposed collects detritus metamorphosis into the young adult. sufficient to weight the animal and drag it The larvae of Elminius modestus settled as down. t hough gregarious (vide oyster spat), aggre- According to these observations, it is legi- gating near previously settled forms and it timate to regard some settlements and meta- is suggested that an olfactory sense may morphoses as not being conditioned by the playa part in bringing them together. nature of the substrate, but at the came time Miller et al. (6) showed that the settle- recent work shows that often they are not ment of the polyzoan larvae of Bugula (one . N.Z. EC0L0GIcAL S0cIETY 33 of the organisms which foul ships' bottoms) both errant and sedentary, as well as the is facilitated by the presence of a slime film brachiopods before-mentioned. on non-toxic substrates. However, although they prefer slime-coated to slime-free non- At the end of the breeding seasons,which toxic surfaces they will, if a slime film is not overlap, of the two species, eight times as many young Terebratellas were found as available, attach themselves equally abund- Tegulorhynchias. Terebratella larvae range antly on clean, non-toxic paints. relatively far and wide in contrast to those Wilson (7) found in regard to the meta- of Tcgulorhynchia. Tegulorhynchia is well morphosis of the larvae of the polychaete, down underneath stones, while Terebratella Ophelia, that substrates may be attractive, most commonly grows as a border to these neutral, or repellent, with gradings within colonies. these catagories. Recently it has been found It has been noticed that the larvae did not that the presence of micro-organisms is a normally metamorphose completely when primary factor in inducing the larvae of settled on a clean surface or one exposed to Ophelia to settle and metamorphose. These seawater for only three or four days. Settle- micro-organisms, perhaps mainly bacteria, ment on clean glass leads to abnormality which are the food of the larvae, must be and death. The mantle rudiment undergoes neither too few nor too many. Wils,on says, partial reversal and the setae of Tegulor- "it is possible that not only the relative hynchia project horizontally, those of Tere- abundance of micro-organisms present influ- bratella being too small to be seen. Detailed encesthe larvae testing the sand grains, but examination shows that the gut is shortened, that different kinds are more attractive than but does not move through a right angle, others." thus remaining longitudinal instead of lying transversely, the development of the mus- Wilson, studying wider aspects of the con- culature is affected in some way not yet ditions affecting metamorphosis and settle- clear and no matter how contraction takes ment inquired into the nature of the change place, the mantle is unable to enclose the of seawater in the west and to the west of anterior lobe and to form the two valves. the English Channel, using the early devel- opment of Echinus eseulentus as test mater- While Ophelia larvae would not settle on ial. He says that, other things apart, the unsuitable substrates, nor would the bar- water off Plymouth leads to a much poorer nacle cyprids, nor the polyzoan >larvae,the metamorphic response than occurred. in the brachiopod larvae will settle usually and late 1920's and 1930's. He says that this is in great nmbers on a clean glass surface, the first time that there has been recognised but the change is partial or not at all. the fact that a bottom fauna may be pro- These facts, concerning worms, barnacles, foundly affected by the character of the polyzoans, sea-urchins, brachiopods, bring supernatant water in which the larvae us closer to the environment than ever.. We would pass their planktonic lives. A slight are required continually to examine more change in the nature of the water could minutely the nature of the life history, in cause a bottom fauna, or some part of it, to order to appreciate the relations between be wiped out. animal and environment, and to account for My own observations on Terebratella in- presencesand absences. conspicua and on Tegulorhynchia nigricans showed that their larvae would settle on REFERENCES naturally slimy surfaces of rocks in appro- (1) THORSON, 0., 1946. Medd. f. Komm. f. Dan.Fisk. priate places in the Inner Harbour at Lyt- Hav. 4 (1). (2) COLE, H. A., KNIGHT-JONES, E. W., 1949. Fish. telton and in the laboratory. Both species Invest. Lond. 11. 17 (3) are commonly found attached to Tegulor- (3) MOORE, H. B., 1935, J. MM. bioi. As.,. U.K. hynchia and only very rarely is Terebratella 20 :279. found attached to Terebratella. I have no (4) PYEFINCH, K. A., 1948, J. Mar. bioi.Ass. U.K. knowledge of the attachment of Tegulorhyn- 27 :464. chia to Terebratella. Very commonly, the (5) KNIGIITJONES. E. W., STEVENSON, J. P., 1950. shells of Terebratella are clean, whereas the J. Mar. bioi. Ass. U.K. 29 :281. shells of Tegulorhynchia carry sponges,poly- (6) MILLER, M. A., et ai. 1948. Bioi. Bull. 94 (2). (7) WILSON, D. P. Various papers in J. Mar. bioi. zoans, hydroids, harpaeticoids, polychaetes, Ass. U.K. to 1955..
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