Decapod Crustaceans

Abstract

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 particularly important. It presents the paradox of brachyuran adults and anomuran larvae, so well known among carcinologists but, until now, totally unexplained. There is a possible alternative explanation for my other example in this group, the anomalous larva of Dorhynchus, so had I restricted my attention to decapod crustaceans I would have had only one example that seemed to demand a new hypothesis. 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 paradoxes no other explanation has been offered, and for others I regard the accepted explanations as weak or flawed.

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