Stygobromus caecilius Holsinger, sp. n.
Creators: name: Holsinger, John R.
name: Ansell, Lynnette Meador
name: Shafer, Justin
Publisher: name: Zenodo
Year: 2011
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Amphipoda
Crangonyctidae
Stygobromus
Stygobromus caecilius

description: Stygobromus caecilius Holsinger, sp. n.
Figures 2, 3, 4
Material examined. Holotype specimen 2.0 – 2.5 mm (partly dissected) and paratype ca. 2.0 mm (completely dissected), Belvedere Seepage Woods, 500 m NNE of Belvedere, Cecil County, Maryland, collected by Daniel Boward and, Daniel Feller, 1 Sept. 2009. The holotype (dissected) is deposited in the National Museum of Natural History, Smithsonian Institution (USNM 1154084); paratype (fragments) in research collection of J. R. Holsinger at Old Dominion University.
Diagnosis. Tiny, presumably rare species of troglomorphic facies distinguished by low number of spines and/ or setae on most parts of body; absence of spines on palmar margins of gnathopod propodi except at defining angles; bifurcate lateral sternal gills on pleonite 1; reduced number of articles in pleopod flagellae; absence of peduncular process on uropod 1; and narrow, rectangular-shaped telson which is more than 2X longer than broad. Largest specimen (male or female), ca. 2.5 mm.
Description. Antenna 1 (Fig. 2 a) 0.35 – 0.40% length of body, approximately 1.3X longer than antenna 2; primary flagellum with 6 articles, lacking aesthetacs; accessory flagellum little longer than first flagellar article. Antenna 2 (Fig. 2 b): flagellum with 3 articles. Mandibles (Figs. 2 c, d) differing in a few minor ways: spine row of right with 4 lightly plumose spines, that of left with 2 such spines; lacinia mobilis of left reduced to single spine; that of right reduced to single forked spine; incisors 4-dentate; molar of left obsolete, that of right cone-shaped and bearing single seta; article 2 of right palp bearing single, short seta distally and article 3 with 3 E setae; article 3 of left palp with single, short apical seta. Lower lip (Fig. 2 e): inner lobes vestigial. Maxilla 1 (Fig. 2 f): inner plate with 2 apical plumose setae; outer plate with 6 apical spines (3 notched); palp with 2 apical setae. Maxilla 2 lost in preparation. Maxilliped (Fig. 2 g): inner plate with 3 apical spines; outer plate with 4 apical spines; palp article 2 broader and longer than article 1, inner margin with 3 relatively long setae; palp articles 3 and 4 much shorter, with fewer setae; article 4 (dactyl) with prominent terminal nail and 3 or 4 setae.
Gnathopod 1 (Fig. 3 a): propodus approximately 30% larger than propodus of gnathopod 2, palm nearly straight and finely serrate, approximately equal in length to posterior margin, bearing only 2 spine teeth and 2 setae on opposite sides; dactyl nail approximately 20% length of dactyl; posterior margin with single longish seta; carpus about 75% length of propodus, bearing 3 long setae on ventral margin; coxa subrounded, broader than deep, with 2 setae on rounded margin. Gnathopod 2 (Fig. 3 b): propodus smaller than that of gnathopod 1, palm straight, finely serrate, approximately equal in length to posterior margin, bearing only 2 very small, distally-bifid spines near defining angle corner and several longish setae nearby; posterior margin with single, longish seta; dactyl nail approximately 40% length of dactyl. Posterior margin of carpus of gnathopod 2 strongly lobed, with 3 or 4 longish setae but lacking rasellate setae. Coxa of gnathopod 2 broadly rounded, bearing 2 long setae.
Pereopods 3 and 4 (Figs. 4 a,b) subequal, approximately equal in length to pereopod 5; coxae subquadrate, approximately equal in size, anterior margins convex, posterior margins straight, ventral margins with 2 short setae each. Pereopods 5–7 (Figs. 4 c,d,e): 6 slightly longer than 5 but shorter than 7; basis of 5 approximately subquadrate with prominent distoposterior lobe, those of 6 and 7 longer than broad, with smaller distoposterior lobes, posterior margins generally straight; dactyls relatively long, approximately 50% length of corresponding propodi. Coxal gill on pereopod 5 relatively large and subovate (Fig. 4 c); not apparent on pereopods 6 and 7. Paired median sternal gills (processes) present on pereonites 6 and 7 (Figs.4 d, e); 1 pair bifurcate sternal gills on pereonite 1 (Fig.4 f).
Pleonal plates 1,2,3 (Fig. 4 g): posterior margins nearly straight, each with single seta; posterior corners distinct in plates 2 and 3, rounded in plate 1; ventral margins lacking setae or spines. Pleopods (Fig. 4 h) biramous, articles bearing long plumose setae. Uropod 1 (Fig. 4 i): inner ramus slightly longer than outer ramus, about 75% length of peduncle, armed with 6 spines of unequal length, outer ramus with 4 apical spines; peduncle with 3 spines. Uropod 2 (Fig.4 j): inner ramus longer than outer ramus, subequal in length to peduncle, armed with 5 apical spines; outer ramus with 3 apical spines; peduncle with 2 spines. Uropod 3 (Fig. 4 k): ramus approximately 80% length of peduncle, armed with 2 apical spines. Telson (Fig. 4 l) narrow, more than 2X longer than broad, bearing 2 lateral setae and armed with 4 relatively long apical spines.
Type locality. Belvedere Seepage Woods, 500 m NNE of Belvedere, Cecil County, Maryland (Fig.1).
Etymology. The epithet caecilius is a Latin form of Cecil and is based on the occurrence of this species in Cecil County, Maryland.
Distribution and ecology. This tiny, rare species is known to date from groundwater seeps apparently developed in relatively recent Coastal Plain sediments. However, these sediments are in contact with and probably overlay to some extent much older metamorphic rocks of Paleozoic age. This habitat may be similar to the one occupied by Stygobromus sextarius Holsinger (2009) approximately 120 km to the southwest in and around Washington, D.C. Although a precise source for the water feeding the local groundwater seeps cannot be determined, it appears to be overflow and/or percolation from underlying sediments.
Remarks. It may be that the specimens at hand are not sexually mature, although they do not appear to be juveniles of any known species of Stygobromus. However, it should be noted that Feller (in litt.) collected 27 specimens of the common, widespread stygobiotic amphipod Stygobromus tenuis s. lat. (see Holsinger 1978) from this locality on 15 April 2010. However, these specimens are approximately 4.0 mm in length and are obvious juveniles of the relatively common, wide spread Stygobromus tenuis s. lat. They are clearly not larger specimens of the new species described above. Despite the possible immaturity of the specimens of the new species described here, it seems best to document its occurrence. Hopefully, future collecting will result in obtaining fully sexually mature specimens.
description: Published as part of Holsinger, John R., Ansell, Lynnette Meador & Shafer, Justin, 2011, Four new species of the subterranean amphipod genus Stygobromus (Amphipoda: Crangonyctidae) from shallow groundwater habitats on the Coastal Plain and eastern margin of the Piedmont in Maryland and Virginia, USA, pp. 1-21 in Zootaxa 2972 on pages 2-6, DOI: 10.5281/zenodo.205655