Decapod Crustaceans

4 Decapod Crustaceans Differences between Brachyura (true crabs) and Anomura (including hermit crabs)-Adult Dromiodea are brachyurans but their zoea larvae are anomurans and resemble larvae of hermit crabs-Explanations involving misclassification and larval conservatism rejected-Suggestion that early dromioid, with no zoeal phase, acquired larvae from early hermit crab by hybridization-Adult Dorhynchus is a spider crab of family Inachidae but zoea larvae share characters of Inachidae and Homolidae-Comparable anomalies occur in some echinoderms and all are explicable as result of hybridization The main subject of this book is the postulated transfer of embryonic and larval forms from one phylogenetic line to another, and I start with the decapod crustaceans. This is a group that contains very few examples that can be attributed to this phenomenon, but the case of the Dromioidea is par ticularly important. It presents the paradox of brachyuran adults and anomuran larvae, so well known among carci nologists but, until now, totally unexplained. There is a pos sible alternative explanation for my other example in this group, the anomalous larva of Dorhynchus, so had I re stricted my attention to decapod crustaceans I would have had only one example that seemed to demand a new hy pothesis. When, however, I eventually came to consider groups in other phyla I found no shortage of paradoxes that could be explained as examples of the transfer of embryonic and larval forms from distinct lineages. For some of the para doxes no other explanation has been offered, and for others I regard the accepted explanations as weak or flawed. In the two examples known to me of crustacean larvae that seem inconsistent with their corresponding adults, the adults are both crab-like. The Crustacea were usually re garded as a class of animals until about 20 years ago (e.g., D. I. Williamson, LARVAE and EVOLUTION © Springer Science+Business Media Dordrecht 1992 Borradaile et al., 1935), but some more recent texts classify them as a subphylum (e.g., Barnes, 1980) or even a phylum (e.g., Barnes et al., 1988). The group covers a great variety of forms, but the only ones that concern us here are the Brachyura and the Anomura, both of which are groups of the order Decapoda. All the more familiar shrimps, lobsters, and crabs are classified in this order. The name refers to their 10 (five pairs) of legs, appendages that are typically used for walking, although some or all of them may be modified for other purposes. The name Brachyura means "short tails, " and this group comprises the true crabs, in which the ab domen (tail) is always quite small and fits into a groove un der the thorax. The head and thorax are fused together and are often very broad. The female always has a receptacle for storing spermatophores received from the male so that mat ing may preceed egg-laying. Females of some families of crabs mate only once, but egg-laying may take place at intervals over several years. The Anomura are the "intermediate tails," in which the abdomen is usually much less reduced and more readily extensible than in the Brachyura. Female anomurans lack means of storing sperm, so the eggs can be fertilized only if a male is present when they are laid. Within this group, the hermit crabs (families Paguridae and Diogenidae) are particularly relevant in the present context. The abdomen is quite large and without a hard shell, but the hermit crab usually protects it with the empty shell of a gastropod mol lusc. Most brachyurans and anomurans hatch as zoea larvae, that is larvae that swim with the outer branches (exopods) of some of the thoracic appendages, but the larvae of the two groups are quite distinct. The carapace of a typical brachyuran zoea (Fig. 3.2f) is almost spherical and usually bears a ventrally pointing rostrum, a dorsal spine, and a lat eral spine on either side, although in some cases one or more of these spines is missing. The telson is in the form of a two pronged fork. The carapace of a typical anomuran zoea (Fig. 3.2e) is more elongated, and the rostrum points forward; dorsal and lateral carapace spines are very rare but a posterior spine LARVA E AND EVOLUTION 28 on either side is very common. The telson is a flat plate, often triangular, on which the outermost process is a short spine and the one next to it a fine hair. For over a century carcinologists have been trying with little success to reconcile the adult and larval features of Dromia, the sponge crab (Fig. 4.3a,b) (see Williamson, 1988b). The name "sponge crab" refers to the habit of carrying a sponge over the back, holding it with the last two pairs of legs. This and related genera, which together make up the superfamily Dromioidea, look like early evolved members of the Brachyura. Their zoea larvae, on the other hand, bear little resemblance to those of the Brachyura (compare Figs. 4.3b and 3.2f) but show a very close resemblance to larvae of the Anomura (Fig. 3.2e), particularly to the larvae of her mit crabs (Paguridae and Diogenidae). Now there are a num ber of anomurans, including the stone crabs (Lithodidae) and the porcelain crabs (Porcellanidae), which, as adults, super ficially resemble true crabs. They have a broad carapace and a relatively small abdomen that bends forward under the thorax, just as in true crabs, but there any resemblance to true crabs ends. They show clear resemblances to the Ano mura rather than the Brachyura in the form of the larva, in the arrangement of the endophragmal skeleton (which forms attachments for muscles and supports some internal organs), and also in the fact that the female has no spermathecae to store spermatophores from the male. There is no argument over the Lithodidae or the Porcellanidae: they are universally accepted among biologists as anomurans, which, as adults, look superficially like crabs. They do show, however, that it is possible to look superficially like a true crab and yet not be one. Are the dromioids true brachyurans, or are they another example of anomurans that superficially resemble brachy urans, like the lithodids and porcellanids? This is the ques tion that has been puzzling carcinologists for so long, and it puzzled me for over 30 years . For most of that time my view was expressed by adapting Darwin's comment on barnacles, quoted in the previous chapter. In my version, I would say DECAPOD C R UST AC EANS FIG. 4.1. Three crabs with unusual larvae. Adults in dorsal view, larvae in lateral view; to different magnifications. (a, b) Adult Dromia and stage I zoea, with dor sal view of zoeal telson; (c, d) adult Dorhynchus and last zoeal stage; (e, f) adult Homola and last zoeal stage. (a redrawn from Monad, 1956; b redrawn from Gurney, 1942; c redrawn from Thomson, 1874; d original; e redrawn from Rice and Provenzano, 1970; f redrawn from Rice and von Levetzow, 1967.) "even the illustrious CaIman did not perceive that a drom ioid was, as it certainly is, an anomuran, but a glance at the larva shows this to be the case in an unmistakable manner." It must be admitted, however, that not a single student of adult dromioids has been swayed by this assertion, and I am now convinced that the cases of the barnacles and the drom ioids are not parallel. Detailed examination of an adult bar nacle shows it to be a crustacean, just as its larvae suggest, but detailed examination of an adult dromioid does not show it to be an anomuran, in spite of the form of its zoea larvae. The endophragmal skeleton of adult dromioids is essentially brachyuran, not anomuran, and the females of the group have spermathecae resembling those of early brachyurans but un known in anomurans. Recent descriptions of adult and larval dromioids merely confirm the situation: the adults really do belong to the Brachyura and the zoea larvae to the Anomura. There can be from one to six zoeal stages in different species, and the last zoeal stage will moult to produce a megalopa, a larva that swims with its abdominal appendages before eventually settling. The megalopa is the final larval phase, and it is suc ceeded by the first juvenile stage. Not all dromioids, how ever, pass through a zoeal phase in their development, for many hatch as a megalopa. The known megalopas of the Dromioidea all seem to have brachyuran rather than ano muran characters, but it must be admitted that the distinc tions between the two groups are much less clearly defined in megalopas than in zoeas, juveniles or adults. One sugges tion is that the Brachyura evolved from anomuran ancestors and the Dromioidea somehow managed to keep the ancestral form of zoea larva while the adults evolved brachyuran fea tures, but I reject this explanation. Apart from dromioid lar vae, all the other evidence, from both adults and larvae, sug gests that the Brachyura did not evolve from an anomuran ancestor, but the evolutionary lines that led to the Anomura and the Brachyura diverged separately from lobster-like ancestors (Guinot, 1978; Williamson, 1988b). The characters of adult Dromioidea, both extant and fossil, are quite con- LA R VA E AND EVOLUTION 32 sistent with evolution from this lobster-like ancestor. Most dromioid zoea larvae, however, show a mixture of the zoeal characters of the two families of hermit crabs, the Paguridae and the Diogenidae. In the only known exceptions, some fea tures of dromioid zoeas are not those of present-day hermit crabs but features that probably occurred in their ancestors.

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