Richardina taina, Anker, 2025

Richardina taina sp. nov.

( Figs. 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 , 4D View FIGURE 4 )

Type material. Holotype: female, pocl 3.35 mm, cl 3.95 mm, USNM 1550694, Caribbean Sea, Puerto Rico, NW of Isla Desecheo, Pichincho Fish Tail, 18°31’04”N / 67°50’10”W ( 18.5179 / -67.8361), Expedition Océano Profundo 2018, R / V Okeanos Explorer, ROV Deep Discoverer, Cruise EX 1811, Dive D16, depth: 497.57 m, together with ascidian ( Ascidia sp. ) and small demosponge ( Raspailiidae ), coll. D. Wagner, 16 November 2018.

Description. Body slightly compressed, subcylindrical. Rostrum short, about 0.2 of carapace length, reaching half-length of first article of antennular peduncle, slender, slightly descending in lateral view, broadly triangular in dorsal view, without supraorbital eave; each lateral margin with small tooth and minute, barely noticeable serrations subdistally; dorsal rostral carina feebly demarcated, unarmed, except for small subdistal tooth; ventral rostral carina unarmed; lateral carinae of rostrum poorly demarcated but distinct, unarmed ( Fig. 1A, G–I View FIGURE 1 ). Carapace without pronounced median ridge; orbital margin gently concave; inferior orbital angle rounded, slightly protruding; antennal spine stout, sharp, distinctly up-turned; anterolateral and pterygostomial margins continuous, with six small, acute or subacute, widely spaced teeth, ventral-most tooth marking pterygostomial angle; supra-orbital area with three sharp spines organised in oblique row and increasing in size posteriorly; hepatic spine absent; cervical groove deep; adjacent cervical cincture armed on each side with four prominent, anteriorly directed, sharp spines increasing in size ventrally; carapace surface posterior and ventral to cervical groove without spines ( Fig. 1A, G, I, J View FIGURE 1 ).

Thoracic sternum moderately broad; third sternite armed with pair of small subacute processes, at some distance from each other, located between and slightly posterior to coxae of third maxillipeds; fourth sternite with pair of similar, however stronger and more medially jointed, subacute processes; fifth sternite with pair of medially largely fused, distally blunt lobes, with deep median depression; sixth sternite with medially largely fused, somewhat elongate, distally blunt processes, with deep median depression; seventh sternite with medially almost fully fused plates, with W-shaped anterior margin; eighth sternite with medially fully fused plates, subrectangular, with pronounced anterolateral lobes ( Figs. 1B View FIGURE 1 , 3A View FIGURE 3 ).

Pleon smooth, not sculptured; fifth sternite with blunt lobe, other sternites unarmed; first pleonite short, depressed anteriorly, produced into blunt lobe ventrally; second to fourth pleonites with pleura rounded ventrally; fifth pleonite with each pleuron bearing two very small teeth on posteroventral margin; sixth pleonite 1.3 times as long as fifth (measured along mid-dorsal line), unarmed, with blunt lobe on posterior margin ( Fig. 1D, E View FIGURE 1 ).

Telson somewhat lanceolate, relatively broad, slightly constricted near base, gently tapering distally, about 2.5 times as long as maximal width; each lateral margin armed with prominent sharp tooth at about 0.6 of telson length; posterior margin broadly rounded; dorsal surface with two low, broad, longitudinal ridges separated by shallow median groove, each ridge armed with four spines increasing in size posteriorly, posterior-most spines very prominent, slightly surpassing distolateral margin but not reaching distal-most end of posterior margin; anterior portion of dorsal surface with two additional pairs of smaller spines, one pair of larger submarginal spines and one pair of smaller submedian spines at anterior border of shallow median groove ( Fig. 1F View FIGURE 1 ).

Eyes well developed; cornea hemispherical, narrower than eyestalk, with dark pigmentation in preserved specimen; eyestalk stout, unarmed (without spinules or denticles) ( Fig. 1G, I View FIGURE 1 ). Epistome with two small, anteriorly pointing processes (visible in Fig. 1K View FIGURE 1 , near antennal base).

Antennular peduncle overreaching mid-length of antennal scaphocerite; first article longest, unarmed; stylocerite very small, blunt; second and third articles unarmed, third smallest; both flagella well developed, dorsolateral one thicker and with some grouped setae (asthetascs?) starting from 8 th subdivision ( Fig. 1G, I View FIGURE 1 ). Antenna with basicerite stout, squarish, ventrolateral and distolateral margins each armed with one small tooth, dorsal margin unarmed; scaphocerite well developed, with broad blade, about 2.3 times as long as greatest width; lateral margin with two spaced teeth below distolateral tooth, latter subtriangular, not exceeding distal margin of blade; carpocerite very short, barely reaching one third of scaphocerite length, unarmed; flagellum long, moderately robust ( Fig. 1G, I, K View FIGURE 1 ).

Mouthparts as illustrated ( Fig. 1L–Q View FIGURE 1 ), similar to those of other species of Richardina (cf. Saito & Komatsu 2009; Komai 2011). Third maxilliped moderately elongate, rather stout, setose, especially on ventral and mesial surfaces and along margins of most articles; coxa with finger-shaped epipod; basis very short, much wider than long; ischium longest of all articles, 2.7 times as long as greatest width, with small tooth at 0.4 length of ventral margin, merus 0.7 length of ischium, narrower than ischium, unarmed, with slight rugosity on proximal half of ventral margin; carpus slightly longer than half length of merus, with slight rugosity on ventral margin; propodus longer than dactylus, latter tapering distally, both with slight rugosity on ventral margin (small bumps indicating insertions of setae); exopod well developed but not reaching distal margin of ischium ( Fig. 1R View FIGURE 1 ).

First pereiopod moderately slender, much shorter than other pereiopods, with all articles unarmed; ischium 0.7 length of merus; merus 0.8 length of carpus; carpus slightly widening distally, about 1.5 times as long as chela; carpo-propodal grooming apparatus moderately developed; palm subcylindrical; fingers subequal in length, about 0.6 length of chela, with unarmed cutting edges ( Fig. 2A, B View FIGURE 2 ). Second pereiopod slender, considerably longer than first pereopod, with all articles unarmed; ischium about half as long as merus; merus 0.7 length of carpus; carpus slightly widening distally, about 1.8 times as long as chela; palm subcylindrical; fingers subequal in length (pollex slightly shorter), 0.4 length of chela, with unarmed cutting edges.

Third pereiopods subequal in size and similar in shape between right and left, slightly dissimilar in armature (especially on ischium and merus), moderately elongate, much stouter than first and second pereiopods ( Fig. 2D, E, H View FIGURE 2 ; see also Fig. 4D View FIGURE 4 ). Coxa and basis without specific features, unarmed ( Fig. 1B View FIGURE 1 ). Ischium 0.8 length of merus (measured along dorsal margin), with oblique distal margin, armed with two (left P3) or three (right P3) small, widely spaced spines on distal half of dorsal margin, distal-most tooth slightly larger, distodorsal angle sharply protruding, more or less spine-like ( Fig. 2D, E, H View FIGURE 2 ). Merus noticeably shorter than carpus; dorsal margin with row of five or six small, acute or subacute spines extending to 0.6 meral length; ventral margin with two (left P3) to six (right P3) sharp spines; ventromesial surface of right P3 with three additional spines decreasing in size distally ( Fig. 2D, E, H View FIGURE 2 ). Carpus widening distally, 3.7 times as long as greatest width, much shorter than palm of chela; dorsal margin unarmed; ventromesial and ventral surface with spines of various sizes, mostly small, however, some enlarged; distomesial margin with deep cleft between two large, rounded lobes, ventral one with minute blunt tooth ( Fig. 2D–H View FIGURE 2 ). Chela moderately swollen, longer than ischium and merus combined, compressed; palm about 2.5 times as long as greatest height (width), subrectangular with smooth angles in cross-section; dorsal margin unarmed; distomesial surface and distoventral margin with field or row of mostly small spines; fingers 0.6 length of palm, subequal in length, smooth, moderately setose (more so distally), with fingertips strongly crossing, pollex slightly excavated mesially; cutting edge of pollex armed with large, protruding tooth proximally, its distal margin truncate or truncate with small point, remaining edge unarmed; cutting edge of dactylus with broad subtriangular tooth fitting into hiatus of opposed margin of pollex ( Fig. 2D–F, H View FIGURE 2 ).

Fourth and fifth pereiopods (for P4 vs. P5 assignment, see above) similar in shape and length, moderately long; ischium about 0.6 length of merus; merus about 0.8–0.9 length of carpus, entire; carpus about 1.7 times as long as propodus, subtly subdivided into two subarticles (suture barely distinct), unarmed; propodus not subdivided, ventral margin with more or less regular row of seven slender spiniform setae, including distal one proximal to propododactylar articulation; dactylus simple, subconical, somewhat compressed laterally, about 0.3 length of propodus length, stout, no more than three times as long as proximal width, strongly curved ( Fig. 2I–M View FIGURE 2 ).

First pleopod uniramous, shorter than other pleopods ( Fig. 2N View FIGURE 2 ). Second to fifth pleopods biramous, without appendices internae; protopod with blunt tooth on mesial margin ( Fig. 2O View FIGURE 2 ). Uropod with lateral lobe of protopod bluntly protruding; exopod moderately broad, not overreaching distal margin of telson or that of endopod; lateral margin slightly convex, serrated, i.e., with row of five subacute or acute teeth, posterior-most tooth by far largest and forming distolateral angle protruding slightly beyond distal margin; distal margin somewhat truncate, broadly rounded; dorsal surface with two almost parallel-running, longitudinal, low ridges, without teeth; endopod more slender than exopod, narrowly ellipsoidal, tapering distally; lateral margin unarmed; distal margin rounded; dorsal surface with single, longitudinal, low ridge, without teeth. ( Fig. 1S View FIGURE 1 ).

Gill/exopod formula as given in Table 1 below (see also Figs. 1C View FIGURE 1 , 3B View FIGURE 3 ).

Colour in life. Translucent white with pinkish tinge, possibly due to presence of reddish chromatophores on carapace, pleon and third pereiopods; eye corneas reflecting white light (based on low-quality photograph in Fig. 4D View FIGURE 4 ).

Etymology. The name of the new species derives from Taíno, the historic indigenous people who populated most of the larger Caribbean islands, including Puerto Rico and Hispaniola ( Arrom & Alegria 1998); used as a noun in apposition.

Distribution. Tropical western Atlantic: presently known only from the type locality situated about halfway between the islands of Puerto Rico and Hispaniola, north-west of Isla Desecheo and north of Isla de Mona, at the limit between the Caribbean Sea and Atlantic Ocean, although still part of the Caribbean plate as defined by Nerlich et al. (2014).

Ecology. The holotype of R. taina sp. nov. was collected by ROV at a depth of 497.57 m, together with two other organisms, more precisely a tunicate, Ascidia sp. ( Ascidiidae, Enterogona , Ascidiacea) (USNM 1550652) ( Fig. 4A, B View FIGURE 4 ), and a small unidentified demosponge, Raspailiidae (Poecilosclerida, Demospongiae) (USNM 1550695), which was apparently attached to the base of the tunicate ( Fig. 4C View FIGURE 4 , not visible in Fig. 4A View FIGURE 4 ). At this stage, it cannot be ascertained whether the new species (i) is associated with the ascidian, (ii) is associated with the demosponge, or (iii) is free-living, which is the least likely scenario given that some elements of its morphology (e.g., reduction of the dorsal rostral armature) is convergent to that of many hexactinellid-associated spongicolid shrimps. The association of R. taina sp. nov. with a demosponge is perhaps most likely in view of associations of two other species of Richardina with glass sponges (Hexactinellida) ( Saito & Komatsu 2009). However, a host switch resulting in a very unusual association of this peculiar stenopodid shrimp with a tunicate cannot be completely discarded.

Remarks. Richardina taina sp. nov. is tentatively assigned to Richardina primarily because of the simple dactyli on the fourth and fifth pereiopods, combined with the carapacial armature reduced essentially to the dorsal cincture of spines on the posterior margin of the cervical groove and few postorbital spines, and a relatively smooth pleon. The dentate anterolateral margin of the carapace (above the pterygostomial angle) and the general shape and armature of the third pereiopod are additional features of the new species that are shared by most other species of Richardina .

Among the seven presently known species of Richardina , R. taina sp. nov. stands out as the only species, in which the dorsal rostral carina is largely devoid of teeth (except for a small subdistal tooth) and the pleurobranchs appear to be absent (cf. Figs. 1A, C, I View FIGURE 1 ; Lo Bianco 1903; Macpherson 1978; Goy 1982, 2015; Saito & Komatsu 2009; Komai 2011). In addition, the fourth and fifth pereiopods of R. taina sp. nov., including their dactyli, are much stouter compared to those of the remaining six species (cf. Fig. 2I, J, L, M View FIGURE 2 ; Macpherson 1978; Goy 1982, 2015; Saito & Komatsu 2009; Komai 2011). The third pereiopod carpus with a deep cleft on the distomesial margin appears to be another autapomorphic feature of R. taina sp. nov. (cf. Fig. 2E, G View FIGURE 2 ; Macpherson 1978; Saito & Komatsu 2009; Komai 2011), although both third pereiopods are missing in the holotype of R. crosnieri ( Goy 2015) . The new species also has significantly fewer spines in the dorsal cincture along the posterior margin of the cervical groove, more precisely only four spines on each side compared to at least seven (and up to 16) spines on each side in the other species (cf. Fig. 1A, G, I View FIGURE 1 ; Macpherson 1978; Goy 1982, 2015; Saito & Komatsu 2009; Komai 2011). Furthermore, the eyestalks of R. taina sp. nov. are devoid of spinules or denticles, which are typically present in all other congeners, whereas the posterior-most dorsal spines of the telson are much more prominent than in the other species (cf. Fig. 1G, I View FIGURE 1 ; Holthuis 1946; Macpherson 1978; Goy 1982, 2015; Saito & Komatsu 2009; Komai 2011). The unique combination of all these morphological features results in a very isolated position of R. taina sp. nov. within the genus (see also below).

The gill-exopod formula and the gill type represent important characters for the taxonomy of Stenopodidea , especially at intergeneric level ( Saito & Takeda 2003; Komai & Saito 2006; Saito & Komatsu 2009; Goy 2010a). Most genera have trichobranchiate gills (exception being Globospongicola , with simple gills) arranged as following: one pleurobranch on Mxp3 and P1–5; one arthrobranch on Mxp1 and Mxp2; two arthrobranchs on Mxp3 and P1–5; one podobranch on Mxp2; one epipod on Mxp3 and P1–4; and one exopod on Mxp1–3, with some deviation from this pattern (Goy 2010: table 1). The small size and fragility of the holotype and presently the only specimen known of R. taina sp. nov. precluded a detailed examination of the gill formula (especially the area above the anterior two pereiopods and maxillipeds) without inflicting too much physical damage to the carapace. Nonetheless, there seem to be no pleurobranchs present above the first to fifth pereiopods, while the two unequal trichobranchs on P1–4 and a single large trichobranch on P5 inserted near the base of each appendage are herein interpreted as arthrobranchs ( Table 1; Figs. 1C View FIGURE 1 , 3B View FIGURE 3 ). The number of arthrobranchs at the base of the third maxilliped is somewhat unclear, although there appears to be two of them. If this gill formula is confirmed, R. taina sp. nov. would be the first stenopodidean shrimp without pleurobranchs. An alternative interpretation is the presence of one pleurobranch on P1–5 (on the anterior portion of each thoracic segment) and one arthrobranch on P1–4 (on the posterior portion of each thoracic segment). However, the main problem with the latter interpretation is that following the gill diagram in Goy (2010: fig. 65.13.E), the two arthrobranchs are typically aligned, one being anterior and the other posterior (exactly as in the case of the two trichobranchs on P1–4 of R. taina sp. nov.), whereas the single pleurobranch is supposed to be inserted slightly more dorsally between the two arthrobranchs (i.e., in an area without a visible gilllike structure in R. taina sp. nov.).

The uropodal endopod of R. taina sp. nov. has a single, smooth, dorsal ridge, a feature shared at least by R. spinicincta ( Goy 1982: fig. 1I) and R. crosnieri ( Goy 2015: fig. 25C, also stated in the text on p. 333). The illustration of the tail fan of R. fredericii in Lo Bianco (1903: pl. 8, fig. 28) only shows two dorsal ridges on the exopod but no ridges on the endopod, whereas Macpherson (1978: fig. 2) did not illustrate any of the uropodal dorsal ridges. The remaining three species, viz. R. parvioculata , R. ohtsukai and R. rupicola , have two dorsal ridges on the uropodal endopod ( Saito & Komatsu 2009: figs. 2E, 7D; Komai 2011: fig. 2E). The presence of one or two dorsal ridges on the uropod could be revealed as a character of phylogenetic and perhaps genus-level importance.

Because of the new species’ significant deviation from the typical morphology of Richardina , the possibility of establishing a new genus for R. taina sp. nov. was considered. The main differentiating characters of such genus would be the reduced cincture of spines on the carapace (with only four teeth on each side), the reduction of most of the dorsal rostral teeth (with only a small subdistal tooth remaining) and absence of ventral rostral teeth, the absence of pleurobanchs (or, alternatively, one of the arthrobranchs), the unarmed eyestalks, the presence of a deep cleft on the carpus of the third pereiopod, and the more robust fourth and fifth pereiopods, with much stouter dactyli. However, at least some of these characters seem to represent secondary reductions due to a species’ presumed (but yet not confirmed) symbiotic lifestyle. Whether R. taina sp. nov. represents a morphologically derived species nested within the Atlantic clade of Richardina or is a completely unrelated lineage currently remains unknown. This issue should be addressed by a more comprehensive molecular analysis of several nuclear and mitochondrial genes of R. taina sp. nov. and several other phylogenetically relevant taxa ( Chen et al. 2016; Anker 2023).