ONTHEHETEROTREME-THORACOTREMEDISTINCTION IN THE EUBRACHYURADE SAINTLAURENT,1980 (, BRACHYURA)

BY

RICHARD VON STERNBERG 1/ andNEIL CUMBERLIDGE 2/ 1/ Departmentof Invertebrate Zoology, NHB-163, Smithsonian Institution, W ashington, D.C. 20560,U.S.A. 2/ Departmentof Biology, Northern Michigan University, Marquette, MI 49855,U.S.A.

ABSTRACT

Eubrachyuransare currently divided into two subsections based on the anatomical position of theseminal duct termini. Heterotremes have the seminal ducts passing through the pereiopod 5 coxae,emerging externally in the form of “ penes”to contact the basal segments of the gonopods. Thoracotrematouscrabs, on theother hand, have the “ maleopenings” located on sternite8 andthe ejaculatoryducts never pass through the pereiopod 5 coxae.Some brachyurologists maintain in the literaturethat the heterotreme-thoracotreme distinction re ects the extremes of two evolutionary grades.According to this speculation, “ advanced”thoracotrematous probably arose from intermediategoneplacid forms which, in turn, originated out of the Heterotremata in response to selectionpressures for improved locomotion. The cladistic hypothesis of De Saint Laurent (1980), incontrast,positions Heterotremata and as sister groups. Indeed, the heterotreme- thoracotremedifference considered alone does support two discrete types as no truly intermediate seminalduct condition bridges the subsections. Nor is there any evidence to suggest that the heterotrematousstate is theanatomical precursor of sternal openings for the seminal ducts. The fact thatbasal members of thePinnotheroidea De Haan are thoracotremes whereas more derived forms havean heterotrematous morphology, refutes De Saint Laurent’ s hypothesisof a deepphylogenetic cleavagebetween the two subsections. In addition to thisinformation, the 850 speciesof freshwater crabs(many of whichare semi-terrestrial) share an arrayof somatic apomorphies C with thoracotremes andyet belong to the Heterotremata. And the cladistic hypothesis of De Saint Laurent (1980) is also falsiŽed byprevious parsimony analyses which place a monophyletictaxon Thoracotremata within aparaphyleticgroup Heterotremata. It is therefore possible that Heterotremata is synonomouswith ,with Thoracotremata remaining as a distincteubrachyuran subclade. One trenchant conclusionis that the functional explanation for the heterotreme-thoracotr emedistinction lacks empiricalsupport.

RÉSUMÉ

LesEubrachyura sont couramment divisé s endeux sous-sections, sur la base de la position anatomiquede l’ extré mité des canaux sé minaux. Les hé té rotrè mes ont un canalsé minal qui traverse lacoxadu pé ré iopode 5, débouchant à l’extérieur sous la forme d’ unpé nis au contact de l’ article basaldu gonopode.Les crabes thoracotrè mes, d’ autrepart, ont les oriŽ ces sexuels mâ les situé s surle

c KoninklijkeBrill NV ,Leiden,2001 Crustaceana 74(4):321-338 ° 322 R.VON STERNBERG& N.CUMBERLIDGE sternite8, etle canalé jaculateurne passejamais par la coxa du pé ré iopode 5. Certainscarcinologistes maintiennentque la distinction hé té rotrè me-thoracotrè me re è teles extrê mes de deux grades d’évolution.Selon cette opinion, les crabes thoracotrè mes “ avancés” seraient probablement issus deformes goné placiennes intermé diaires qui auraient leur origine dans les Heterotremata en réponse aux pressions de sé lection pour amé liorer la locomotion dans des environnements semi- terrestres.L ’hypothèse cladistique proposé e parDe Saint-Laurent (1980), en revanche, considè re lesHeterotremata et les Thoracotremata comme groupes-frè res. En effet, la différence hé té rotrè me- thoracotrème considé ré e seulesupporte deux formes discrè tes, é tantdonné qu’ aucune condition intermédiaire du canal sé minal ne relie les deux sous-sections. De mê me, aucune é videncene suggère que l’ é tathé té rotrè me est le pré curseur anatomique de l’ ouverture sternale du canal séminal. Le fait que les membres basaux des Pinnotheroidea De Haan sont thoracotrè mes, alors quedes formes plus dé rivé es ont une morphologie hé té rotrè me, ré fute l’ hypothè se de De Saint- Laurentd’ un clivage phylogé né tique profond entre les deux sous-sections. En outre, les plus de 850espè ces de crabes d’ eaudouce (dont beaucoup sont semi-terrestres) partagent un ensemble d’apomorphiessomatiques avec les thoracotrè mes et, cependant, appartiennent aux Heterotremata. Deplus, l’ hypothè se cladistiquede DeSaint-Laurent (1980) est é galementfalsiŽ é eparles analyses deparcimonie anté rieures qui placent un taxon monophylé tique Thoracotremata à l’intérieur d’ un groupeparaphylé tique Heterotremata. Il est alors possible que Heterotremata soit synonyme de Eubrachyura,les Thoracotremata restant un sous- eubrachyourien distinct. Une conclusion netteest que l’ explication fonctionnelle pour la distinctionhé té rotrè me-thoracotrè me manque d’ un supportempirique.

INTRODUCTION

TheclassiŽ cation ofbrachyuran crabs was substantially revised in aseries of detailed worksthat focusedprimarily onthe positionof the female andmale sexual openings,thoracic sternal patterns, aspects ofthe endophragmalsystem, male Žrst andsecond pleopods, and male abdomenlocking mechanisms (Guinot,1978, 1979).Guinot’ s studies led tothe proposalof alogical (if perhapsnot phylogenetic) system ofbrachyuran relationships. Threenew sections ofthe Brachyurawere proposed(Podotremata, Heterotremata, andThoracotremata) with partitions based in part onthe anatomical location ofthe female andmale sexual apertures (see Guinot,1978, 1979). The section Podotrematacontains all brachyuranfamilies in whichthe oriŽces ofthe oviductsare located onthe coxaeof the third pereiopods,and where the spermathecal openingsare located onthe borderof thoracic sternites 7and8, with the spermathecae separated fromthe ovaries and oviducts(Hartnoll, 1968). All podotremeshave the male sexual apertures on the coxaeof the Žfth pereiopods.The sexual character states diagnostic forthe Podotremataare consideredgeneralized conditions for the Brachyura.The sections Heterotremata andThoracotremata share aset oftrenchant apomorphies, including oviductsthat openinto the spermathecal chambers,spermathecal chambersthat are connectedto sternal vulvaevia “vaginae”, andvulvae that are positionedon sternite 6.Heterotremes andthoracotremes differ in that heterotremes havemale