Settlement and Metamorphosis in Barnacles and Decapods

Settlement and metamorphosis are two crucial processes in organisms with a biphasic life cycle, forming the link between the pelagic larva and benthic juvenile-adult. In general, these processes occur during the final larval stage. Among crustaceans, settlement behavior and the cues that trigger settlement and metamorphosis have been studied in greater depth in barnacles than in decapods, likely a result of the former losing the ability to move after they join the benthic juvenile-adult population, undergoing metamorphosis. Both barnacles and decapods respond to different environmental cues associated with the adult habitat, such as substratum, biofilm, and the presence of conspecifics. In the absence of cues, larvae can delay their metamorphosis for a period of time. This ability to prolong the development can be advantageous because it increases the probability of settling in a suitable habitat. However, delayed metamorphosis has also associated costs (e.g., smaller size, lower growth rate, and higher mortality), which may be carried over to subsequent development stages, with consequences for recruitment.

Population studies on a polymorphic prosobranch snail ( Clithon(Pictoneritina) oualaniensis Lesson)

The prosobranch gastropod Clithon (Pictonentina) oualaniensis Lesson which is widely spread in the Indo-Pacific region, shows a high degree of variability as regards shell colour and pattern. The present report is based on some 72000 snails collected round the coasts of Ceylon, in Malaya and Singapore, and in Hong Kong; and on some preliminary breeding results (Nugaliyadde, appendix 4). Clithon has an axial (transverse) pattern which most commonly consists of fine transverse lines (f.t.l.); less often, there are coarse transverse lines (c.t.l.) or more widely spaced patterns such as zebras or tigers (which are rare in Ceylon, but common in the eastern provinces). F.t.l. and, to a lesser extent, c.t.l. are often complicated by the presence of small triangles or tongues the density of which increases with age. Superimposed on the axial is often a spiral pattern which generally consists of three roughly equidistant spirals. The ratio of axial:spiral patterns is about the same over the whole area and about 62:38. The spiral patterns in Ceylon include spiral tongues, ladders and yellow spirals; these are fairly distinct in certain populations, but grade into each other to a varying extent in others. In the eastern provinces, ladders are completely absent, but there are other spiral patterns which may be their equivalents. Between them, these simple axial and spiral patterns account for the great majority of all animals. In addition, there are certain rarer types which are mostly sharply classifiable (dilution, purple-tipped tongues, purple spirals, black and a few others). Preliminary breeding experiments show that axial as compared with spiral patterns correspond to distinct genotypes, spirals being dominant over axial patterns. Within the axial patterns, there are numerous intergrades between f.t.l. and c.t.l.; the tongue pattern can vary from absence to a density which covers most of the shell; it is largely an age effect like greying of hair in man. In the same shell, these patterns can change into each other gradually or sometimes abruptly following a temporary cessation of growth (including attempted predation by hermit crabs). They are prob ably largely, if not entirely, non-genetic in origin; this is compatible with the few breeding data so far available. Zebras and tigers which are almost confined to the eastern provinces are probably genetically distinct separate entities. Both population studies and breeding experiments make it probable that the spiral group includes a significant element of genetic segregation. Purple spirals are probably due to a single (? recessive) gene, and a simple genetic basis is probable for most or all of the rarer variants. Within a given province, populations are remarkably uniform. The rarer variants are generally homogeneously distributed within a province (except purple spirals which are much commoner on the east than on the west coast of Ceylon). There is more variance between populations as regards the ratio of axials to spirals, and as regards the various spiral patterns. But the variance between populations, both in space and in time, is not significantly greater than the error variance, and it thus appears that what variance there is lacks permanence; i.e. there is oscillation round essentially stable mean values. There is thus no need to invoke permanent differences in gene frequencies or permanent differences in the environment as between populations. On the other hand, there are consistent and major differences between provinces, and these must clearly be genetic in nature. They may have arisen during the spread of Clithon from its original home which, presumably, has been coastwise and slow. Clithon has probably no pelagic larva and, as localities where it can exist are separated by many miles of open coastline where it cannot, its spread must have been at the mercy of rare accidents: these probably convey only a few individuals at a time from one suitable locality to another and thus expose populations to genetic drift (founder effects). As, within provinces, there is little or no evidence for this, it seems most probable that gene frequencies are stabilized by selection, and as long as an allele is not lost altogether, the interplay of selective forces will tend to bring gene frequencies back to normality. Whereas it thus appears probable that the polymorphism of Clithon is kept in being by selection, there is no reason to suppose that any particular pattern is better adapted, over the life cycle as a whole, than any other. The whole array of variability may thus be adaptively neutral.

On a Tornaria found in British Seas

Before 1888 no specimen of Tornaria, the well-known pelagic larva of Balanoglossus, had been taken off the English Coast. But on August 9th of the past year Mr. Weldon, using the surface net near the Eddystone Lighthouse, took several young specimens of Tornaria, and subsequently the same gentleman, during a cruise of a week's duration towards the end of August, captured many larger and more mature specimens of the same larva. Other specimens were taken by us up to September 21st, and in the month of August, Mr. Rupert Vallentin found several specimens in the vicinity of Falmouth.The specimens taken on these occasions form the subject of the present memoir, and their anatomy is detailed at some length, both because of the interest attached to a form bitherto unknown to England, and because it is in some respects of morphological importance.The specimens taken by us came from the offing, and were not taken within four miles of theshore; Mr. Vallentin's Tornaria were found close to the shore at Falmouth. As he was able to bring his back alive and preserve them at leisure in his laboratory, they are better preserved than those taken by us, since we were obliged to preserve our catch in a somewhatrough manner out at sea. An examination of Mr. Vallentin's specimens leaves me no doubt that the larva is Tornaria Kröohnii, aspecies found in the Mediterranean. The different forms of Tornariahave not been with certainty referred to their adult forms, but T. Kröohnii must belong to one of the Mediterranean species of Balanoglossus, that is to say, to B. Kowalevsltii, B. minutus, or B. claviger, species which have not hitherto been recorded from the English Channel, unless we suppose that the larva of B. salmoneus v. sarniensis, which occurs in the Channel Islands andat Roscoff, is identical with Tornaria Kröohnii.