Coxicerberus mexicanus (Pennak, 1958)

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Suggested Common Name: Unarmored Beach-Cerberus
Etymology: refering to the country this species was described from. Common name refers to the apparent lack of tergal lobes on pereonites 2-4.
Taxonomic History: Microcerberus mexicanus Pennak, 1958; Coxicerberus mexicanus Wägele, Voelz & Vaun McArthur, 1995
Size Range: to 0.925mm
Description: (modified from Pennak, 1958) Eyes absent. Antenna 1 5-segmented, segments 1-2 robust, segment 2 with 1 apical simple, 1 lateral simple and 1 lateral plumose setae, segment 4 shorter than others, segment 5 long-cylindrical with 4 apical setae and 1 apical aesthetasc set on a small tubercle. Antenna 2 2x longer than antenna 1; peduncle 5-segmented, segment 2 basally expanded with robust poorly sclerotized medial spine [=scale?], segment 3 small, bent, segment 4 with 5 simple and 1 plumose setae, segment 5 elongate with 4 simple and 1 plumose apical setae along with 1 elongate spatulate-plumose apical aesthetasc[?]; flagellum 5-segmented, segment 5 with 4 apical setae and 1 slender apical aesthetasc. Mouthparts small, compact. Pereonites 1-2 fused to head, pereonite 2 indicated by lateral sutures. Pereopod 1 dactyl long, curved, with 2 elongate acute toothed ventral processes pointing towards tip; propus large, palm with 2 digitiform finely-toothed proximal processes along with 3 flat acute asymmetrical toothed palmar spines, all curving distad. Pereopods 2-7 moderately long, subsimilar; dactyl 2-clawed, 1 claw slightly longer and slenderer than other. Pereonites 2-4 tergal lobes apparently absent. Pereonites 5-7 longer than preceding pereonites. Pleonite 1 shorter than pleonite 2. ♂ pleopod 2 elongate; protopod subrectangular, flattened; exopod small, lobe-like, with 1 apical seta; endopod [based off figs] lateral lobe short, stubby, functioning as “base” to apical lobe, apical lobe extremely elongate, margins roughly subparallel, curving slightly laterally to point directly posteriorly, bifid at tip with lateral (=proximoapical) lobe acuminate, longer and wider than truncate medial (=distoapical) lobe, lateral margin of apical lobe with row of small evenly-spaced minute teeth, lateral lamella small, on basal half of apical lobe. Pleopod 3 flat, entirely fused into one piece[?]. Pleopods 4-5 half as long as telson, biramous; rami bluntly-tapered, fused with protopod. Uropod protopod broad, slightly tapering, with 5 short medial setae, 2 apical setae and 1 lateral seta; exopod minute with 2 elongate setae; endopod long-conical with about 5 simple and 2 plumose apical setae.
Geographic Range: Only known from the type location at Acapulco (Guerrero). Reported in the supratidal zone about 1-5m above the high tide line in 6-12cm of sand, on a moderate-energy beach. The area where the isopods occured is regularly hit by waves at high tide. At th type locality this species co-occured with numerous harpacticoid copepods.
Notes: The fusion of the first two pereonites to the head and the apparent complete lack of tergal lobes on pereonites 2-4 is highly distinctive among members of Coxicerberus. Unfortunately, this species was relatively poorly described, and many important details, such as the mouthparts, remain undescribed. The ♂ pleopod 2 clearly place this species in the mirabilis group, indicating that this species is likely a highly derived member of that group. This species is in a dire need of a redescription, especially to determine the state of the tergites and coxal plates on pereonites 2-4.

Sources

Pennak, R. W. (1958). A new micro-isopod from a Mexican marine beach. Transactions of the American Microscopical Society, 77(3):298-303.

Wägele, J. W., Voelz, N. J., & Vaun McArthur, J. (1995). Older than the Atlantic Ocean: discovery of a fresh-water Microcerberus (Isopoda) in North America and erection of Coxicerberus, new genus. Journal of Crustacean Biology, 15(4):733-745.

Published: Feb 15, 2023