Atlantasellus dominicanus Jaume, 2001

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Suggested Common Name: Biramous Pillslater
Etymology: after the Dominican Republic, where this species occurs. Common name refers to the biramous pereopod 5, a trait unique to this species among all other isopods
Taxonomic History: Atlantasellus dominicanus Jaume, 2001
Size Range: to 2.14mm
Description: (modified from Jaume, 2001) Body oblong, about 3x longer than wide, semicylindrical, conglobulating, pigmentless, dorsum with numerous smooth setae, both body and limbs covered in setulose scutellated scales. Eyes absent. Head frontal margin weakly convex at antennal placements with weakly triangular rostrum; ventral rostral process projected forward, triangular, separating antennae 1 bases; posterolateral corner produced into a long sickle-shaped posteriorly directed process with a blunt tip. Antenna 1 short, 5-segmented, about half as long as head, segments 1-2 expanded, segment 1 cup-shaped, segment 2 wider than long with convex distal margin, segments 3-5 contributing to sealing body during conglobulation, shorter than segments 1-2, inserted postero-subdistally on segment 2, segment 5 with 1 shorter proximal and 1 longer distal aesthetasc. Antenna 2 7-segmented, slender, uniramous, with smooth microscopically trifid setae, segment 3 shortened, curved laterally, segment 5 with several modified brush-like setae. Labrum rounded with transverse apical groove making it appear grooved laterally. Labium bilobed, medial lobe setulose. Mandible body stout, broad; left incisor and left lacinia similar, multicuspid; right incisor tip nearly entire, shallowly 3-cuspid; left lacinia somewhat narrower than incisor, unmovable, right lacinia slender, movable, distal margin oblique, finely serrate with 2 subdistal tricuspid teeth; left spine row with 1 club-shaped 4-cuspidate stout spine somewhat resembling a reduced lacinia and 2 similar distally serrate proximally expanded slender spine, with wide gap between spines and molar, right spine row with 2 unequal multicuspid spines; molar soft, lappet-like, with 5 apical setae, the middle 3 serrate and the lateral 2 smooth; palp segment 1 with 0-1 trifid setae, segment 2 elongate, about 4.3x longer than wide, with 1-2 pectinate robust subdistal, segment 3 sickle-shaped, short, about a third shorter than segment 2, with 4-5 anterodistal pectinate setae. Maxilla 1 rami with transverse rows of setules; exopod with 3 unimarginal dentate stout spines and 6 apical bipinnate stout spines; endopod digitiform with apical geniculate long-pinnate seta. Maxilla 2 protopod anteromedially with hyaline flamelike spinules; exopod 2-lobed, lateral endite 2 terminal pinnate setae, medial endite with 3 terminal pinnate setae; endopod with 4 pinnate setae, 1 sparsely tooth seta and 2 unimarginal toothed setae. Maxilliped coxae not confirmed; epipod slightly longer than basis; basis rectangular, somewhat longer than wide, with several clusters of anterior hyaline flamelike spinules, distal tip [fused endite?] subquadrate, distally crenulate with 1 smooth trifid medial setae, 7 heterogeneously ornamented distal spines, a row of distal simple setules, and 2 distomedial hooks; palp 5-segmented, segment 1 with 1 trifid smooth medial seta, segment 2 with 2 trifid smooth medial setae, segment 3-4 trifid smooth medial setae, segment 5 with 4 trifid smooth setae, 1 smooth stout apical setae and 1 subdistal bipinnate stout seta. Oostegites present on pereopods 2-4, large, elliptical, broadly overlapping, membranous. Pereonites 1-7 subrectangular, considerably wider than long. Pereonites 1-5 coxae/coxal plates ventrolaterally directed. Pereonite 1 as long as pereonite 7, shorter than pereonite 2; coxal plate wider than long, anterior margin deeply excavate, posterior margin even-rounded, lateral margin straight, laterodorsally depressed to accommodate head posterolateral process, with 6 smooth setae. Pereopod 1 strongly subchelate; dactyl with 2 distoventral toothed spines and 8 trifid smooth setae; propus roughly triangular, palm proximal angle at about 3/5ths the way up the segment, with 2 unequal proximal spines, proximalmost short, faintly toothed, distal one long, stout, with 4 strong proximal teeth and distolateral pectinate frill, palmar margin oblique, concave, with 2 medial spines similar to distoproximal spine along with pair of smooth trifid setae, posterior margin with 2 smooth trifid setae along with short, stout distal hirsute spine; carpus, merus and ischium shortened; merus with rounded lateral lobe. Pereopods 2-7 subsimilar, ambulatory, segments long-cylindrical; dactyl much shorter than propus, biseptate, with 1 short smooth slender posterior seta, 2-clawed; propus with rounded platelike distal spine. Pereopods 2-4 carpus with subdistal short shabby spine; coxae articulating dorsally, with 3 smooth setae. Pereonites 2-3 subequal in length, shorter than pereonites 4-5. Pereopod 2 coxa longer than wide, anterior margin excavate, posterior margin even-rounded. Pereopod 3-4 coxae subsimilar, longer than wide, anteriorly shallow-excavate. Pereonites 4-5 subequal in length, longer than other pereonites. Pereonites 5-7 coxal plates with 3 smooth setae. Pereonite 5 coxal plate ovoid, posteromedially strongly excavate to accommodate pereopod 5 exopod. Pereopod 5 exopod present, completely fused to base of basis, unsegmented, digitiform, with 4 apical setae. Pereonites 6-7 coxal plates directed dorsolaterally, longer than wide, anterior and posterior margins subparallel, posterolateral corner slightly produced. Pereonite 6 shorter than pereonites 4-5, longer than pereonite 7. Pereonite 7 coxal plate narrower than pereonite 6 coxal plate. Pleonites 1-2 slightly shorter than pereonite 7; epimera elongate, round-tipped (appearing posteriorly-pointing acute dorsally). ♂ pleopod 2 unknown. Pleopod 3 operculate, not fused, uniramous, nonsclerotized, lacking sculpture other than scales and smooth setae; protopod short, fused to exopod; endite truncate, laterally even-rounded, medially straight, widest at about a third from the base, with 3 proximolateral, 1 distolateral, 3 apical and 1 medial smooth setae. Pleopod 4 completely concealed under pleopod 3, biramous; protopod trapezoidal, expanded distally about as long as wide; exopod shorter than endopod, with suture, lateral margin setulose, set laterally on protopod, basis with proximomedial scale patch, endite short, subquadrate, with 2 short brushlike setae distally; endopod subrectangular, about as long as endopod and 2x longer than wide, medial margin with microspinules, with 3 short brushlike distal setae. Pleopod 5 somewhat reduced, consisting of a simple ovate plate. Telson as long as wide, ellipsoid, evenly convex dorsally and posteriorly; tip with shallow notches at uropod insertions, posterodistal corners of notches set about 0.27x the telson width apart, central lobe folded inward, rounded, with pair of tiny setules, notch corners not projecting past uropods. Uropods minute, bulge-like nonarticulating, completely incorporated into telson.
Geographic Range: Known from 2 fairly inland cenotes in or near PN Jaragua, Dominican Republic. At least one of the cenotes was freshwater. Co-occurs with stygomysids (Stygiomysis aemete Wagner, 1992, subterranean amphipods (Bahadzia jaraguensis, Jaume & Wagner, 1998 and Ottenwalderia kymbalion, Jaume & Wagner, 1998), Copeopods (Cyclopidae) subterranean decapods (Typhatya sp.) and thermosbaenaceans (Tethysbaena sp.).
Notes: The biramous pereopod 5 is unique among all known isopods. The fused cylindrical lobe that is presumed to be the exopod appears to be in the wrong location for an oostegite (the most likely other potential option) and is present even in mancae. Since all other isopods have uniramous pereopods, including two fairly large groups considered more basal to (Phreatoicidea) or sister to (Asellota) Microcerberidea, its likely that the expression of this feature is potentially a case of long deactivated genes have suddenly becoming active again. The rudimentary morphology of the exopod likely being due to random mutations over what is likely 400 million years, when the line leading to Isopoda split from other Peracarideans with biramous thoracopods.

Sources

Jaume, D. (2001). A new atlantasellid isopod (Asellota: Aselloidea) from the flooded coastal karst of the
Dominican Republic (Hispaniola): evidence for an exopod on a thoracic limb and biogeographical
implications. Journal of Zoology, 255(2), 221-233.

Published: Feb 15, 2023