Rhopalaemon, Ashelby & Grave, 2010

Ashelby, Christopher W. & Grave, Sammy De, 2010, A new genus of palaemonid shrimp (Crustacea: Decapoda: Palaemonidae) to accommodate Leander belindae Kemp, 1925, with a redescription of the species *, Zootaxa 2372 (1), pp. 369-378 : 370-371

Rhopalaemon gen. nov.

Type species. Leander belindae Kemp, 1925 .

Diagnosis. Body subcylindrical, slightly compressed laterally. Rostrum laterally compressed, shorter than carapace, stout, without basal crest, with strong lateral ridge, with dorsal and ventral unarticulated teeth, single row of plumose setae present between the teeth. Carapace glabrous with antennal and branchiostegal teeth, branchiostegal groove arising dorsally to branchiostegal tooth. Branchiostegal tooth sub-marginal. Eyes pigmented, banded, with ocellus. Fourth thoracic sternite with well developed median process; eighth thoracic sternite of males with well developed process, that of females with small blunt tubercle. Well developed preanal tooth present. Mandible with palp, third maxilliped lacking subdistal spine on antepenultimate segment, other mouthparts as for Palaemon . Pereiopod 2 stout, being only slightly longer than ambulatory pereiopods. Pereiopods 3–5 with propodus distally expanded bearing 3–5 strong spines distally; distal most spines paired; dactyli simple, strongly curved. Grooming brush on pereiopod 5 reduced to a single poorly defined row hidden amongst the stout spines. Pleopod 1 of male without marginal appendix, with inner medial spines. Pleopods 2–5 with appendix interna in both sexes, pleopod 2 of male also with appendix masculina. Upper flagellum of antennula biramous, with rami fused for one quarter of their length. Antennular peduncle comprised of three articles, with well developed stylocerite, ventromedial tooth and anterodistal tooth present; statocyst present. Béc ocellaire well developed with strong median process. Fifth abdominal pleuron rounded posteroventrally. Telson with 2 pairs of dorsal spines situated dorsomedially; acute posteromedian process, 2 pairs of spines on posterior margin, outer pair much shorter than inner pair; 1 pair of very stout plumose setae, longer than inner pair of spines. Mobile medial spine of exopod of uropod slightly exceeding fixed tooth.

Gill formula as follows: 5 pleurobranchs (P1–5), 2 arthrobranchs (1 reduced, both Mxp3), 1 podobranch (Mxp2), 3 epipods (Mxp1–3), 3 exopods (Mxp1–3).

Derivation of name. The name is a combination of the Latin rhopalon meaning club like and the generic name Palaemon in reference to the club-like shape of the propodi of the ambulatory pereiopods; gender masculine.

Remarks. The subfamily Palaemoninae currently contains 18 genera, with a further two described elsewhere in this issue (Wowor & Ng this issue). Many of the generic diagnoses within this subfamily depend heavily on the presence and absence of a “hepatic” versus “branchiostegal” tooth. However, as pointed out by Walker & Poore (2003) these two teeth are in fact homologous structures, with the “hepatic” tooth merely being a branchiostegal tooth which has migrated further up the carapace during ontogeny. Notwithstanding this, the actual position of the tooth can continue to be used in differentiating genera.

The branchiostegal position of the tooth in the new genus, clearly differentiates it from Brachycarpus Bate , Macrobrachium Bate , Neopalaemon Hobbs and Pseudopalaemon Sollaud (in which the tooth is in a hepatic position), as well as from Troglocubanus Holthuis , Troglomexicanus Villalobos, Alvarez & Iliffe , Troglindicus Sankolli & Shenoy , Leptocarpus Holthuis and Cryphiops Dana (all these genera lack a branchiostegal tooth). Rhopalaemon gen. nov. can be differentiated from both Exopalaemon and Nematopalaemon , as it lacks the dorsal, rostral crest, characteristic of both these genera. The presence of a distinct branchiostegal groove further separates the new genus from Leandrites Holthuis , Creaseria Holthuis , Urocaridella Borradaile and Leander Desmarest , in which such a groove is lacking.

Rhopalaemon gen. nov., appears closely related to a rather heterogeneous assemblage comprised of Coutierella Sollaud , Palaemonetes Heller and Palaemon Weber. The relationships between these genera (and their constituent species) are poorly understood at present. Coutierella differs considerably from the other genera in the structure of its mouthparts. The laciniae of the maxillula are broad, the lower one being twisted, whilst the basal endite of the maxilla bears a fringe of very long setae; these and other differences clearly indicate an adaptation to a specific food source (see Bruce 1989). In contrast, the mouthparts are rather conservative in Palaemon , Palaemonetes as well as Rhopalaemon gen. nov. Although Palaemonetes , as currently defined, lacks a mandibular palp (vs. present in Rhopalaemon gen. nov.), the validity of this character has been questioned ( Fujino & Miyake 1968; Chace 1972).

Rhopalaemon gen. nov. primarily differs from both Palaemon and Palaemonetes in the conspicuous development of the distal part of the propodi of the ambulatory pereiopods, these being ventrally expanded (vs. not-developed); furnished with a concentrated row of stout spines (vs. dispersed along the medial margin); as well as the very reduced grooming brush on the fifth pereiopod (vs. comprised of at least two, distinct rows of serrulate setae). The development of the distal part of the propodi is reminiscent of those encountered in the unrelated genera Rapipontonia Marin (Pontoniinae) , Chlorocurtis Kemp (Pandalidae) , as well as in some species of Thor Kingsley and Hippolyte Leach. Although the precise function of this structure remains unclear, Marin (2007) suggested a grasping function to hold onto hydroids for Rapipontonia . A similar function could perhaps be inferred in Rhopalaemon gen. nov., although many species of Palaemon also live in hydrodynamically active environments, and lack such a grasping structure.