On a Remarkable New Genus and Species of Alpheid Shrimps (Malacostraca: Decapoda: Caridea) from the Tropical Western Atlantic †

: Synalpheopsis gen. nov. is established for a remarkable new alpheid species, Synalpheopsis laureae sp. nov., presently known only from the male holotype collected at 111–162 m east of La D é sirade, Guadeloupe, Lesser Antilles. Synalpheopsis gen. nov. peculiarly combines features of two genera, Alpheopsis Couti è re, 1897 and Synalpheus Spence Bate, 1888; however, it is presumably closer to the former genus. The new genus is characterised by the moderately developed orbital hoods, well-developed rostrum and orbital teeth, sixth pleonite without articulated ﬂap, tip of the third maxilliped with crown of spiniform setae, chelipeds with two strong teeth on distolateral margin and lacking snapping mechanism on ﬁnger cutting edges, and gill formula without mastigobranchs and setobranchs.

During the KARUBENTHOS 2 expedition in June 2015 (see Acknowledgements), a single specimen of a peculiar alpheid shrimp was dredged at a depth of 111-162 m east of the small island La Désirade, part of the Guadeloupe Archipelago, Lesser Antilles.The body of the specimen was largely intact, except for the detached chelipeds.Only one (right) cheliped was found in the vial together with the body; however, the same size of the right and left coxae led to the assumption that the chelipeds of this alpheid were probably equal or at least subequal in size.The unique and remarkable combination of morphological features of this specimen leaves no doubt that it belongs to a completely new lineage within the Alpheidae.Therefore, a new alpheid genus is established to accommodate the below described new species.Unfortunately, no colour photo of the specimen was taken after its collection, as "species of small size, i.e., with a total length of less than 1-2 cm, were usually not photographed during the expedition because of lack of time" (Poupin & Corbari 2016 [36]).
The holotype of the new species is deposited in the collections of the Muséum National d'Histoire Naturelle, Paris, France (MNHN).The carapace length (cl) and total length (tl) were measured from the tip of the rostrum to the posterior margin of the carapace and the telson, respectively.Abbreviations used in the text include A1-antennule; Mxpmaxilliped; P-pereiopod.

Taxonomy
Family Alpheidae Rafinesque, 1815 Genus Synalpheopsis gen.nov.Diagnosis.Body not compressed, moderately slender.Carapace without armature on dorsal or lateral surfaces.Rostrum well developed, with acute tip; dorsal carina present; ventral carina unarmed.Orbital hoods moderately swollen; orbital teeth well developed, sharp.Pterygostomial angle bluntly protruding.First to fifth pleonites distoventrally rounded or bluntly angular; sixth pleonite without articulated plate.Telson moderately slender, with two pairs of spiniform setae on dorsal surface; posterior margin rounded, with two pairs of spiniform setae; anal tubercles absent.Eyes well developed, concealed dorsally and laterally.Antennular peduncle moderately slender; stylocerite well developed, overreaching distal margin of first article of peduncle, distally acute, with small blunt process near dorsomesial margin; ventromesial carina with sharp tooth; lateral flagellum with accessory ramus largely fused to main ramus.Antenna with basicerite moderately stout, armed with sharp tooth; scaphocerite with well-developed blade and distolateral tooth; carpocerite slender, reaching beyond scaphocerite.Mouthparts typical for family.Mandible with two-articulated palp; molar and incisor processes well developed.Third maxilliped with well-developed exopod; coxa with small, acutely produced lateral plate; antepenultimate article with distodorsal spiniform seta; tip of ultimate article with crown of stout spiniform setae.First pereiopods (chelipeds) probably equal or subequal in size, of unknown degree of asymmetry, carried extended with dactylus in dorsolateral position; ischium with dorsal and ventral margins distally armed with spiniform setae; merus with dorsal and ventromesial margins armed with spiniform setae, dorsal margin ending in strong sharp tooth, ventrolateral margin distally armed with two strong sharp teeth; carpus short, cup-shaped, without comb-like rows of setae on mesial surface; chela not particularly enlarged or swollen, palm somewhat crenulated, with spiniform seta on ventral margin and large, sharp, distodorsal tooth reaching beyond base of dactylus; fingers subequal to palm, cutting edges armed with low, blunt teeth, without snapping mechanism; adhesive discs absent.Second pereiopod moderately slender; coxa with spiniform seta ventrally; ischium unarmed; carpus with five subdivisions; chela not especially modified, with tufts of simple setae.Third, fourth, and fifth pereiopods slender; merus and ischium unarmed; propodus with slender spiniform setae, grooming brush of fifth pereiopod well developed; dactylus moderately slender, biunguiculate.Second male pleopod with appendix masculina exceeding appendix interna and reaching beyond endopod.Uropodal exopod and endopod not modified; diaeresis sinuous, with small lateral tooth adjacent to stout distolateral spiniform seta.Gill-exopod formula is presented in Table 1.
Etymology.The name of the new alpheid genus is a combination of two other generic names, Synalpheus and Alpheopsis, referring to a series of morphological features shared with both genera (see below); gender: feminine.
Distribution.Presently known only from the tropical western Atlantic.
Remarks.Synalpheopsis gen.nov.uniquely combines morphological features of Alpheopsis with those of Synalpheus.All features shared between Synalpheopsis gen.nov.and each of these two genera are not exclusive synapomorphic characters, i.e., most of them are variously present in other alpheid genera.The four most important features of the new genus shared with many species of Alpheopsis are: (i) the general shape of the cheliped(s), with the presence of spiniform setae on the ischium and merus, and simple teeth on the finger cutting edges; (ii) the ventromesial carina of the first article of the antennular peduncle armed with a sharp tooth; (iii) the second pereiopod chela with tufts of simple setae; and (iv) the male second pleopod with a well-developed appendix masculina (e.g., Banner 1953 [37]; Anker et al. 2005 [38]; Anker 2015a [39], 2017 [18]).The presence of these features in Synalpheopsis gen.nov.immediately separates the new genus from Synalpheus, which is characterised by (i) the presence of a powerful snapping mechanism (plunger + fossa) on the major chela; (ii) the ventromesial carina of the first article of the antennular peduncle being unarmed; (iii) the second pereiopod chela bearing short brushes of thickened, stiff setae; and (iv) the second pleopod of both sexes lacking appendix masculina (e.g., Coutière 1899 [40], 1905 [41]; Dardeau 1984 [4]; Ríos & Duffy 2007 [6]).On the other hand, Synalpheopsis gen.nov. shares with Synalpheus four important characters: (i) the well-developed, moderately swollen orbital hoods; (ii) the absence of a subtriangular articulated plate on the sixth pleonite; (iii) the absence of mastigobranches on the coxae of the third maxilliped and all pereiopods (+ absence of setobranches on all pereiopods); and (iv) the tip of the third maxilliped armed with a crown of stout spiniform setae (present in most species of Synalpheus) (e.g., Coutière 1899 [40]; Dardeau 1984 [4]; Ríos & Duffy 2007 [6]).Using these four features, Synalpheopsis gen.nov.can be separated from Alpheopsis, in which (i) the orbital hoods are not noticeably swollen; (ii) the sixth pleonite has a subtriangular articulate plate; (iii) the mastigobranches are present on the coxae of the third maxilliped to the third or fourth pereiopods (with setobranches present accordingly on the coxae of the first to fourth or fifth pereiopods); and (iv) the tip of the third maxilliped ends in a corneous point, sometimes armed with one or two small, subdistal, or distal spiniform setae (Anker et al. 2005 [38]; Anker 2015a [39], 2017 [18]).In addition, in both Synalpheopsis gen.nov.and Synalpheus, the ischium of the third and fourth pereiopods is unarmed, whereas in most species of Alpheopsis, it is armed with one or two spiniform seta(e) (Banner 1953 [37]; Dardeau 1984 [4]; Anker 2001 [42]; Anker et al. 2005 [38], 2012 [8]; Anker 2015a [39], 2017 [18]).The presence of well-developed, sharp orbital teeth in the new genus is shared with some species of Alpheopsis (A. trispinosa (Stimpson, 1860) group) and many species of Synalpheus (e.g., Anker et al. 2005 [38], 2012 [8]).
Synalpheopsis gen.nov.also has two autapomorphic features, which are unique within the Alpheidae.The distal armature of the ventrolateral margin of the cheliped merus, which consists of two very stout and sharp teeth, is not present in this form in any other alpheid shrimp.The presence of a minute blunt process near the dorsomesial margin of the antennular stylocerite seems to be present only in Synalpheopsis gen.nov., although due to its small size, this process may have been overlooked in other alpheid genera.Other morphological features, not exclusive to the new genus, but adding to its distinctiveness, are: (i) the lateral antennular flagellum with the accessory ramus largely fused to the main ramus, i.e., non-individualised; (ii) the presence of a stout sharp tooth on the distal margin of the palm, reaching well beyond and mesially overhanging the base of the dactylus (similar to that of many species of the Alpheus macrocheles (Hailstone, 1835) group); (iii) the greatly elongated appendix masculina, reaching far beyond the distal margin of the endopod (uncommonly seen in some other alpheid genera, e.g., in some species of Athanas and Potamalpheops, as well as in Yagerocaris Kensley, 1988 and Crosnierocaris Anker, 2022; see Kensley 1988 [43]; Yeo & Ng 1997 [44]; Anker 2022 [45]); and (iv) the endopod of the first male pleopod armed with slender spiniform setae.
Pleon (Figure 1D) with first to fifth pleura rounded distoventrally, fifth slightly more angular; sixth pleonite not elongated, without articulated plate.Telson (Figure 1E) relatively broad, subrectangular, gently tapering distally, about twice as long as proximal width, with two pairs of stout spiniform setae situated approximately at 0.45 (anterior) and 0.7 (posterior) length of telson, at some distance from lateral margins; posterior margin broadly rounded, with two pairs of spiniform setae, one at each posterolateral angle, mesial more than four times as long as lateral.
Eyes (Figure 1A,B) completely concealed in dorsal view and largely concealed in lateral view, with well-developed, normally pigmented corneas; anteromesial margin bluntly protruding, with small portion visible in lateral view.
Antennule (Figure 1A,B,F,G) relatively slender; stylocerite slender, not swollen laterally, with acute tip, latter reaching well beyond distal margin of first article, dorsomesial margin with small process near dorsomesial margin, ventromesial carina with slender, sharp, strongly anteriorly directed tooth; second article 2.3 times as long as wide; lateral flagellum with accessory ramus largely fused to main ramus, not individualised, with several groups of aesthetascs, first on fifth flagellar subdivision.
Mouthparts typical for family.Mandible (Figure 2A) with well-developed, biarticulated palp; incisor process slightly expanded, with eight blunt or subacute teeth; molar process broad, with semi-circular rows of microscopic setae and minute teeth.Maxillule (Figure 2B,C) with typical distal and proximal endites; palp distally bilobed, dorsal lobe with one fine seta, ventral lobe with one much stouter, spinule-like seta.Maxilla (Figure 2D) with typical distal and proximal endites, distal endite with deep cleft; palp not subdivided, of normal size; scaphognathite narrow.First maxilliped (Figure 2E) with distal endite densely furnished with setae on distal and distomesial surface, proximal endite without setae; palp not subdivided, with some plumose setae; exopod long, with poorly developed caridean lobe.Second maxilliped (Figure 2F) with typical endopod composed of five articles; exopod very long; epipod rounded, without podobranch.Third maxilliped (Figure 2F-J) moderately slender, pediform; coxa with distally acute lateral plate, without mastigobranch; antepenultimate article about five times as long as greatest width; penultimate article short, slightly widening distally, almost cup-shaped, with stout spiniform seta on distodorsal margin; ultimate article about 0.7 times as long as antepenultimate article, gently distally, setose, tip armed with crown of six stout spiniform setae; exopod well developed, overreaching distal margin of penultimate article.
First pereiopods (chelipeds) possibly equal or subequal in size (based on similar size of coxae), of unknown degree of asymmetry, carried extended forward or slightly bent ventrally, not completely folding.Right cheliped (Figure 3A-D) neither particularly robust nor slender.Coxa (Figure 3D) with blunt ventral process.Basis (Figure 3D) short, without notable features.Ischium (Figure 3A,B) short, widening distally; dorsal margin with one long spiniform seta distally; ventromesial margin with smaller spiniform seta subdistally.Merus (Figure 3A-C) trigonal in cross-section, distally widening, almost three times as long as distal width; dorsal margin ending in stout, sharp tooth distally, and with three (one apparently broken) spiniform setae of variable length in proximal half; ventromesial margin ending bluntly distally, with row of six spiniform setae of variable length; distolateral margin with two large, sharp teeth.Carpus (Figure 3A-C) short, cup-shaped, unarmed; mesial surface without rows of setae.Chela (Figure 3A-C) moderately enlarged, not particularly swollen or elongated.Palm (Figure 3A-C) slightly compressed, about twice as long as high (wide); dorsal surface slightly crenulated; dorsomesial margin ending in stout, sharp, distal tooth, latter reaching well beyond and mesially overhanging base of dactylus; ventral margin slightly crenulated proximally, with two spiniform setae, first situated in proximal quarter of palm, and second, more mesially located, near mid-length of palm; remaining surface of palm smooth, without grooves, notches, or tubercles.Fingers (Figure 3A-C) almost equal to palm in length, slightly gaping when closed, with crossing fingertips; pollex slightly shorter than dactylus, with gently curved tip, cutting edge with eight low teeth of variable size and width, proximal-and distal-most weakest, teeth near mid-length strongest; dactylus slightly flattened, with strongly curved tip, cutting edge with nine teeth similar to those of pollex, distal-most weakest.Left cheliped unknown.
Second pereiopod (Figure 4A) relatively slender; coxa with small spiniform seta on ventral surface; ischium almost six times as long as wide; merus 1.1 times as long as ischium, six times as long as wide; carpus slender, as long as ischium and merus combined, with five subdivisions, proximal longest, ratio of carpal subdivisions approximately equal to 3.0:1.5:1.0:1.0:1.5;chela simple, with fingers distinctly longer than palm.
Third to fifth pereiopods moderately slender.Third pereiopod (Figure 4B-D) with ischium almost four times as long as wide, ventrolateral surface unarmed; merus about 1.5 times as long as ischium, about 5.3 times as long as wide; carpus not noticeably slenderer than merus, about 0.6 times as long as merus, with stiff seta on distoventral margin; propodus significantly longer than merus, with seven spiniform setae on ventral margin and one pair of spiniform setae distally, near propodo-dactylar articulation; dactylus 0.3 length of propodus, moderately stout, not particularly elongated, gently curving, biunguiculate, with accessory unguis small, located at about 0.7 length of dactylus, slightly deviating from main axis of main unguis.Fourth pereiopod (Figure 4E) generally similar to third pereiopod, although noticeably slenderer; propodus with five spiniform setae on ventral margin in addition to one pair of spiniform setae flanking propodo-dactylar articulation.Fifth pereiopod (Figure 4F-I) slenderer than third and fourth pereiopods; ischium less than three times as long as wide; merus almost double length of ischium, six times as long as wide; carpus 0.7 times as long as merus; propodus much longer than merus, with seven appressed spiniform setae on ventromesial margin, including one distal seta near propodo-dactylar articulation, and well-developed grooming brush, consisting of 10 rows of microserrulate setae; dactylus biunguiculate, similar to those of third and fourth pereiopods.
Gill-exopod formula as given for genus.Remarks.See under genus.