Introduction to Brachyura1)

CHAPTER 71-1 INTRODUCTION TO BRACHYURA1) BY FREDERICK R. SCHRAM AND PETER CASTRO Contents. – Kinds of crabs. Where are crabs in the scheme of things? What defines Brachyura? Where do brachyurans live? What do brachyurans do for a living? Appendix. Bibliography. KINDS OF CRABS The scientific identification of “crabs” goes back to the very beginning; Cancer pagurus Linnaeus, 1758 cuts to the heart of the foundations of binomial nomenclature. The name Cancer denoted “aquatic arthropods” in the 18th century, and Cancer pagurus still stands as the type species of the genus Cancer, a genus now of course restricted to a narrow assemblage of related species of cancrid brachyurans (Schram & Ng, 2012). The distinct status of brachyurans was formally recognized when Latreille (1802) erected that group. Further historical details can be gleaned from Chapters 71-16 and 71-18 in this volume. The crab is one of the iconic images for crustaceans along with shrimps and lobsters. It is probably one of the animals we can almost guarantee seeing when wading through tide pools at the seashore or for many of us going shopping for seafood. We often attach the word “crab” to creatures that are really not brachyurans such as “hermit crabs”, “mole crabs”, and “king crabs” (which are at least crustaceans), but also “horseshoe crabs” (which most definitely are not). Most lay members of the public probably, if coaxed, could come up with a basic description of “a crab” as a crustacean with: (1) a flattened body, (2) a carapace that is typically wider than long, (3) wide thoracic sternites, and (4) a pleon [abdomen] reduced in length and neatly flexed under the thorax. This is a rather basic, bare-bones characterization, which in the end is not particularly informative. As Scholtz (2014) points 1) Manuscript concluded March 2015. © Koninklijke Brill NV, Leiden, 2015 Crustacea 9C (71-1): 3-9 4 F. R. SCHRAM & P. CASTRO out, if we tag crabs as members of the clade Brachyura, then not all brachyurans are crab- like. Furthermore, not every crab-like crustacean is a brachyuran. Within the clade Brachyura, there are creatures that do not particularly conform to our four crab-like descriptors just given. This is especially true within the podotreme brachyurans. Members of Dromiidae do have a crab-like Gestalt, but families such as Homolodromiidae and Raninidae have carapaces longer than wide and pleonal segments, which, while usually reduced, often project directly posteriad. In fact, the crab-like form is only truly evident within the subclade Eubrachyura, and even then the tremendous degree of variation in body habitus sometimes can stretch belief (classic examples of which are illustrated in Thompson, 1917). On the other hand, the anomuran families Porcellanidae (porcelain crabs) and Lithodidae (king crabs) are almost “more Catholic than the Pope” in that the superficial crab shape of their bodies obscures their true affinities with squat lobsters and hermit crabs, respectively. Borradaile (1916) was amongst the first to recognize the convergences that can occur to a crab-like form through a process of “carcinization” when he took up consideration of the hermit crab, Porcellanopagurus, a rather “crabbie” hermit crab. However, the phenomenon has long attracted attention, most recently by McLaughlin & Lemaitre (1997), McLaughlin et al. (2004), and Scholtz (2014). Scholtz (2014) also drew the slipper lobsters into the carcinized fold when he invited comparisons to a form like Ibacus. But if we are to go to that extent, then one could also draw into the scheme of consideration creatures out of the fossil record, viz., amongst the Palaeozoic pygocephalomorphs certain members of Notocarididae (Brooks, 1962) evoke aspects of carcinization with their highly flexed pleons. Even more dramatic are the Palaeozoic/Mesozoic Cyclida that, while possibly phylogenetically allied to the copepods and thecostracan maxillopodans, have a most striking crab habitus (Schram et al., 1997; Dzik, 2008), the most impressive examples occurring amongst the Cretaceous cyclid genera †Maastrichtiocaris and †Alsasuacaris (Fraaije et al., 2003; Van Bakel et al., 2011). Fascinating as this all is, because in this volume of the Treatise we focus on decapods, we take up members of the clade Brachyura, which almost all authorities accept as a monophyletic group (Schram, 1986). WHERE DO CRABS FIT IN THE SCHEME OF THINGS? Determining the relationship of crabs to other reptant decapods must entertain multiple alternative hypotheses. The classic placement, found in just about all text-books, has brachyuran crabs at the pinnacle of decapod evolution, or at least in a clade alongside their purported sister group, the anomalans. This pairing, designated Meiura by Scholtz & Richter (1995), makes sense in many ways in that we then have a clade of big- clawed, rather highly derived decapods that, for want of any better expression, often do ‘strange things’ with the pleon, i.e., shrink it, reduce it, permanently flex it, or twist it. All the morphologically based cladistic analyses inevitably feature an expression of this.

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