Opaepupu, Anker & Grave, 2021

Anker, Arthur & Grave, Sammy De, 2021, Opaepupu, a new genus and species of bivalve-associated shrimp (Decapoda Caridea: Palaemonidae) from Hawai’i, Zootaxa 4903 (1), pp. 55-70 : 56-59

Opaepupu gen. nov.

Diagnosis. Small-sized palaemonid shrimp with dorsoventrally somewhat flattened body. Carapace smooth, glabrous, without pubescence or pilosity. Rostrum moderately developed, mid-dorsally carinate, otherwise unarmed, including ventrally; lateral carina proximally expanded. Supraorbital, hepatic and epigastric teeth absent. Orbit well developed, inferior orbital (= infraorbital) angle distinct. Antennal tooth well developed, acute. Fourth thoracic sternite unarmed. Pleon smooth, glabrous; brood chamber of female strongly expanded laterally; posteroventral angle of first to fifth pleura rounded, sixth pleuron quadrate, posteriorly unarmed. Telson with two pairs of submarginal cuspidate setae on dorsal surface and two pairs of short spiniform setae on posterior margin. Eyes with well-developed, globular cornea; accessory eye (nebenauge) absent. Antennule with first article ending in blunt distolateral lobe, not sharp tooth; stylocerite small, triangular; lateral flagellum with accessory ramus composed of single unit. Antennal scaphocerite with well-developed, broad blade; distolateral tooth vestigial. Epistome unarmed. Mandible without palp; incisor process with five distal teeth. Maxillule with broad laciniae; palp (endopod) distally bilobed, ventral lobe with small, acute, tooth-like, distoventral extension and long stiff seta. Maxilla with palp (endopod) moderately broad; endite with deep cleft; scaphocerite well developed. First maxilliped with endites broad; palp (endopod) absent; exopod with well-developed caridean lobe; epipod moderately developed, bilobed. Second maxilliped typical for family, with epipod moderately developed, subquadrate, without podobranch. Third maxilliped pediform, not operculate; coxal lateral plate poorly developed, rounded; basis and ischiomerus fused; arthrobranch absent. First pereiopods short, fingers of chela not spatulate, not dentate. Second pereiopods (chelipeds) moderately enlarged and robust, somewhat elongate, sexually dimorphic, slightly more robust in males, subequal in size, symmetrical in shape; cutting edges of chela fingers with simple robust teeth proximally, without fossa. Ambulatory pereiopods relatively slender; ischium, merus, carpus and propodus unarmed; dactylus elongate, compressed, biunguiculate; main unguis slender, acute; corpus with single acute distal tooth (= secondary unguis), latter stouter but shorter than main unguis. Male second pleopod with well-developed appendix masculina. Uropod with lateral lobe ending bluntly; exopod with reduced distolateral tooth; diaeresis feebly developed, unarmed. Gill/ exopod formula: 5 pleurobranchs (P1–5), 2 epipods (Mxp1–2), 0 arthrobranchs, 0 podobranchs, 3 exopods (Mxp1– 3).

Type species. Opaepupu huna sp. nov., by monotypy and original designation.

Other species included. None.

Distribution. Central Pacific: presently only known from the Hawaiian Islands.

Etymology. The generic name ( Opaepupu ) is derived from the Hawaiian word`ôpae, a general term for shrimp or prawn, and pűpű, a general term used for marine shells, referring to its association with a bivalve mollusk. Gender neuter.

Remarks. The phylogenetic position of Opaepupu gen. nov. is not immediately obvious and it cannot be excluded that the new genus is more closely related to a genus or a group of palaemonid genera with species not associated with bivalves (see below). However, for obvious reasons, the new genus is herein morphologically contrasted with all other genera containing bivalve-associated species. Among the above-mentioned palaemonid genera known to contain shrimps associated with bivalves, Opaepupu gen. nov. appears to be morphologically most similar to the Indo-West Pacific genera Platypontonia , Anchiopontonia , Pinnotherotonia , Bruceonia , Cainonia and the Atlantic-East Pacific genus Pontonia . The remaining Indo-West Pacific genera, namely Conchodytes , Anchistus , Paranchistus and Neoanchistus appear to be much more distant to the new genus (see below).

Opaepupu gen. nov. is easily distinguishable from Platypontonia , currently with two species, P. brevirostris ( Miers, 1884) and P. hyotis Hipeau-Jacquotte, 1971 ( Bruce 1968; Hipeau-Jacquotte 1971, 1974; Suzuki 1971). For instance, these two genera differ in the form of the rostrum, being distally undivided and having a distinct mid-dorsal carina in Opaepupu gen. nov. vs. distally bifurcate and dorsally non-carinate in Platypontonia ; the shape of the dactylus of the ambulatory pereiopods, which is elongate and biunguiculate in Opaepupu gen. nov. vs. stouter and not biunguiculate in Platypontonia ; the ventral lobe of the maxillular palp (endopod) with a long stiff seta dorsal to a small hook-like process in Opaepupu gen. nov. vs. with a hook-like process but without a seta, in Platypontonia ; and the first maxilliped with two distinct endites and lacking palp (endopod) in Opaepupu gen. nov. (but see below) vs. with the endites completely fused and with a small palp in Platypontonia (cf. Bruce 1968; Hipeau-Jacquotte 1971). Additional, minor, differentiating characters between these genera are in the size of the cuspidate setae of the telson, which are relatively small (especially in the female) in the new genus vs. large and stout in the two species of Platypontonia ; and in the first article of the antennular peduncle distolaterally unarmed (ending in a blunt lobe) vs. armed with a small distolateral tooth in Platypontonia ) (cf. Bruce 1968; Hipeau-Jacquotte 1971). In addition, P. brevirostris associates with the cockscomb oyster, Lopha cristagalli (Linnaeus, 1758) , whilst P. hyotis associates with large, heavily calcified oysters of the genus Hyotissa Stenzel, 1971 (as Pterostrea in Suzuki 1971); both hosts are morphologically and ecologically very different from the trapezid clam host of the type species of Opaepupu gen. nov. (see below).

Opaepupu gen. nov. can be easily separated from the monotypic Anchiopontonia by the shape of the dactylus of the ambulatory pereiopods, which is elongate and strongly biunguiculate in the new genus vs. very stout and with a unique distal armature in Anchiopontonia (cf. Bruce 1992). The two genera also differ in the configuration of the rostrum, which is proximally broad and dorsally carinate in Opaepupu gen. nov. vs. rather narrow and not carinate in Anchiopontonia ; the shape of the anterolateral margin of the carapace, with a strong antennal tooth and without an infraorbital lobe in Opaepupu gen. nov. vs. with a small antennal tooth and with a rounded, protruding infraorbital lobe in Anchiopontonia ; the development of the distolateral tooth of the scaphocerite, which is greatly reduced in the new genus vs. well developed in Anchiopontonia ; and the first maxilliped lacking a palp in Opaepupu gen. nov. vs. with a small palp in Anchiopontonia (cf. Bruce 1992). In addition, A. hurii ( Holthuis, 1981) lives exclusively in association with spiny oysters of the genus Spondylus Linnaeus, 1758 , which are morphologically and ecologically very different from the trapezid host of the type species of Opaepupu gen. nov. (see below).

Opaepupu gen. nov. also differs in many respects from the monotypic Pinnotherotonia , containing P. rumphiusi Marin & Paulay, 2010 , a rare and peculiar shrimp associated with the burrowing clam Periglypta crispata (Deshayes, 1854) . The new genus can be separated from Pinnotherotonia by the body and appendages being glabrous vs. densely pubescent in Pinnotherotonia ; the rostrum dorsally carinate and distally pointed vs. not carinate and distally broadly truncate in Pinnotherotonia ; the posterior margin of the telson armed with two pairs of small spiniform setae vs. fringed with simple setae in Pinnotherotonia ; the ventral lobe of the maxillular palp with a long stiff seta dorsal to a small hook-like process vs. with a small spinule and without a stiff seta in Pinnotherotonia ; the first maxilliped with two distinct endites and lacking a palp vs. with the endites completely fused and with a small palp in Pinnotherotonia ; the third maxilliped relatively slender, pediform vs. broad, suboperculiform in Pinnotherotonia ; the second pereiopods more slender, with proportionally longer fingers; the dactylus of the ambulatory pereiopods elongate and not covered by setae vs. much stouter and densely covered by numerous setae on both mesial and lateral surfaces in Pinnotherotonia (cf. Marin & Paulay 2010).

Some of the characters separating Opaepupu gen. nov. from Pinnotherotonia also apply to Bruceonia , presently containing only the type species, B. ardeae ( Bruce, 1981) , associated with the Pacific jewel-box, Chama pacifica Broderip, 1835 ( Fransen 2002) . These are the absence of pilosity on the body and appendages in the new genus vs. its presence on at least the carapace, all pereiopods and parts of the tail fan in Bruceonia ; the rostrum being distally pointed and dorsally carinate in the new genus vs. distally truncate and dorsally not carinate in Bruceonia ; the ventral lobe of the maxillular palp with a long stiff seta dorsal to a small hook-like process in Opaepupu gen. nov. vs. without such a seta in Bruceonia ; and the first maxilliped with two distinct endites and lacking a palp in Opaepupu gen. nov. vs. with the endites completely fused and with a reduced palp in Bruceonia (cf. Fransen 2002). In addition, Opaepupu gen. nov. can be separated from Bruceonia by the presence of a stout sharp antennal tooth and the infraorbital angle not produced anteriorly vs. with a greatly reduced antennal tooth and infraorbital angle anteriorly protruding in Bruceonia ; the posterior margin of telson with two pairs of spiniform setae vs. three in Bruceonia ; the distolateral tooth of the scaphocerite greatly reduced vs. well-developed, sharp in Bruceonia ; and the dactylus of the ambulatory pereiopods elongate, biunguiculate, with the secondary unguis almost parallel to the main unguis in Opaepupu gen. nov. vs. more robust, biunguiculate, with the secondary unguis being distinctly divergent from the axis of the main unguis, the latter also with micro-ornamentation in Bruceonia (cf. Bruce 1981; Fransen 2002).

Opaepupu gen. nov. can also be distinguished from the monotypic Cainonia , for instance, by the general shape and length of the rostrum; the development of the distolateral tooth of the antennal scaphocerite; several features of the mouthparts, including the same distinguishing features on the maxillule, maxilla and first maxilliped used in the comparisons above; and the general shape and armature of the dactylus of the ambulatory pereiopods (cf. Bruce 1980, 2005; Fransen 2002). The single species of this genus, C. medipacifica ( Edmondson, 1935) , is associated with spiny oysters, Spondylus spp. ( Fransen 2002, as Dactylonia medipacifica ), morphologically and ecologically very different from the trapezid clam host of the type species of Opaepupu gen. nov. (see below).

Pontonia , as redefined by Fransen (2002), presently contains 11 species ( Fransen 2002; Marin & Anker 2008), all of them distributed in the Atlantic and East Pacific. All but one species are associated with various bivalves, including spiny and pearl oysters and fan clams ( Spondylus , Pinna Linnaeus, 1758 , Pinctada R̂ding, 1798), the only exception being P. panamica Marin & Anker, 2008 , associated with ascidians. Using the diagnosis of Pontonia and species accounts in Fransen (2002), Opaepupu gen. nov. can be distinguished from Pontonia by the slender ambulatory pereiopods, each ending in an elongate biunguiculate dactylus, with the secondary unguis almost parallel to the main unguis vs. the stouter ambulatory pereiopods, each ending in a stout biunguiculate dactylus, with the secondary unguis distinctly divergent from the axis of the main unguis in Pontonia (cf. Fransen 2002). Additional distinguishing features between the two genera are the dorsally carinate rostrum in Opaepupu gen. nov. vs. not carinate in Pontonia ; the coxal endite of the maxillule not particularly enlarged vs. more or less expanded, sometimes hypertrophied, in Pontonia ; the ventral lobe of the maxillular palp with a long stiff seta dorsal to a small hook-like process vs. typically with a small hook-like process, but never with a stiff seta, in Pontonia ; and the first maxilliped without a palp and with the coxal and basial endites separate vs. with a well-developed palp and with the two endites completely fused in Pontonia (cf. Fransen 2002). Furthermore, in most species of Pontonia , the second pereiopods are unequal in size, more or less asymmetrical in shape and very robust, especially in males, contrasting with the subequal and more slender chelipeds in both sexes in the new genus.

Opaepupu gen. nov. clearly differs from Conchodytes , currently represented by 10 species, by a series of morphological characters, involving the rostrum, telson, antenna, maxillule, maxilla, first and third maxillipeds, first and second pereiopods, as well as ambulatory pereiopods, especially the strikingly different configurations of the dactylus (cf. Johnson 1967, as Chernocaris ; Bruce 1977, 1989, 2005; Marin 2011, as Lacertopontonia ; Fransen & Reijnen 2012). Remarkably, members of the genus Conchodytes have been reported from an array of bivalve hosts from eight bivalve families ( Fransen & Reijnen 2013 and references therein), indicating a relatively low specificity and tendency to host switching (see also Horká et al. 2016).

Finally, Opaepupu gen. nov. can be easily distinguished, using many morphological criteria, from the remaining three genera containing exclusively bivalve symbionts, viz. Anchistus (7 species), Neoanchistus (2 species) and Paranchistus (6 species) ( De Grave & Fransen 2011; Fransen & Reijnen 2012). The new genus differs from all of them by the posterior margin of the telson furnished with two pairs of spiniform setae inserted at the same level vs. with three pairs of spiniform setae, the most lateral pair located distinctly above the two other pairs, in Anchistus , Neoanchistus and Paranchistus ; the distolateral tooth of the scaphocerite greatly reduced vs. moderately developed to very strong in the other three genera; several features on the mouthparts (e.g., maxillule, first maxilliped, see above); the configuration of the dactyli of the ambulatory pereiopods, which are very different in the other three genera; and the lateral lobe of the uropodal protopod ending bluntly vs. ending in sharp tooth in the other three genera (cf. Kemp 1922; Holthuis 1952; Bruce 1975, 1978). The new genus differs specifically from Neoanchistus by the antennal tooth being strong and sharp in the new genus vs. reduced to a blunt lobe in Neoanchistus and the first pereiopod chela with the finger cutting edges simple, blade-like in the new genus vs. excavated and subspatulate in Neoanchistus (cf. Bruce 1975). It differs specifically from Paranchistus by the rostrum being dorsally non-dentate vs. distally toothed in Paranchistus and the carapace lacking an articulate hepatic tooth, as well as a protruding infraorbital lobe, both of which are present in Paranchistus (cf. Bruce 1978). The members of these three genera associate with a variety of bivalve hosts, both sessile and burrowing, including giant clams of the genus Tridacna Bruguière, 1797 (cf. Bruce 1975, 1978; Fransen 1995; Britayev & Marin 2011; Fransen & Reijnen 2012).

In view of numerous host switches reported in the Palaemonidae (e.g., Marin & Anker 2008; Horká et al. 2016) and no morphological feature indicating a potential close phylogenetic proximity of Opaepupu gen. nov. to any of the other bivalve-associated palaemonid genera, it seems quite possible that the new genus may be part of a different lineage otherwise containing taxa not (or not only) associated with bivalves. For instance, the general shape of the second pereiopods (chelipeds) is somewhat reminiscent of several coral-associated genera, such as Philarius Holthuis, 1952 or Harpilius Dana, 1852 . Of course, these might be only superficial resemblances, plesiomorphies or distant convergences. Although the status of Opaepupu gen. nov. as a new genus is not in doubt, a cladistic analysis of the entire family is needed to elucidate its phylogenetic position.