Wednesday, 30 December 2020

MONGABAY: A New Species of the Hermit Crab Genus Cancellus


[Crustacea • 2020] Cancellus heatherae • A New Species of the Hermit Crab Genus Cancellus H. Milne Edwards, 1836 (Decapoda: Diogenidae) from A Mesophotic Deep Bank in the northwestern Gulf of Mexico

Cancellus heatherae  
Felder & Lemaitre, 2020

Recent sampling on mesophotic deep banks in the northwestern Gulf of Mexico has produced a previously undescribed hermit crab assignable to the genus Cancellus H. Milne Edwards, 1836. Members of the genus are most often found to occupy cavities of eroded coral, siliceous sponges, porous calcareous rock fragments, algal concretions, or worm tubes as shelters. The present specimen was found loose as by-catch in a dredged rhodolith sample taken for algal life history studies. In situ, it likely occupied a cavity within one of the collected calcareous rhodoliths or small sponges in the by-catch. While our description is based on a single female specimen, the holotype is fully mature and intact, and it was solidly frozen in seawater until its coloration could be photographically documented and tissues extracted for sequencing. In comparison to the three other known western Atlantic species, the frontal rim of the carapace shield in the new species is continuous between the blunt lateral teeth as in C. ornatus Benedict, 1901 and C. viridis Mayo, 1873, and thus distinct from the subdivided front found in C. spongicola Benedict, 1901. The rim itself is somewhat flattened as in C. ornatus rather than inflated as in C. viridis. However, each of the ocular scales bears a pair of spines at the tip, as in C. viridis. The lower palms of the chelipeds, while distinctly rugose, do not have a separated patch of stridulating ridges comparable to those reported for C. spongicola. The yellow-orange to deep-orange pigmentation of the color pattern differs from fresh coloration in both C. ornatus and C. viridis, but that of C. spongicola is unknown for other than preserved specimens. Description of the single available specimen is in this case justified by the low likelihood for timely acquiring of additional samples from the type locality or adjacent habitats, most of which are deep banks warranting protection under pending habitat management changes. Our diagnosis includes GenBank accession numbers for COI sequences to facilitate future molecular phylogenetic comparisons.

Keywords: Crustacea, new species, Anomura, Cancellus, mesophotic, Gulf of Mexico

FIGURE 3. Cancellus heatherae n. sp., female holotype, sl 4.0 mm (USNM 1618800 = ULLZ 18309), northwestern Gulf of Mexico: A, habitus, dorsal; B, habitus, right side; C, habitus, ventral; D, sixth tergite and telson, dorsal.
 Cancellus viridis Mayo, 1973, female, sl 2.2 mm (USNM 1544358 = ULLZ 8947), southeastern Gulf of Mexico: E, in carcinoecium, front; F, habitus, dorsal. 
Cancellus ornatus Benedict, 1901, female, sl 6.9 mm (USNM 1539291 = ULLZ 2089), northeastern Gulf of Mexico: G, front; H, habitus, dorsal.

Family Diogenidae 

Genus Cancellus H. Milne Edwards, 1836 

Cancellus heatherae n. sp.

Diagnosis. Cephalothoracic shield width not exceeding length; rostrum well developed, angular, produced anteriorly beyond blunt anterolateral projections; narrow flattened frontal rim continuous across front between anterolateral projections, crossing rostrum without disjuncture; dorsal surface with angular depression on either side of shield posterior to rim behind each ocular peduncle. Median calcified plate of cardiac region subrectangular, slightly longer than wide. Ocular acicles armed with 2 spinules distally. Antennal acicle extending anteriorly less than onehalf length of ocular peduncle. Cheliped and second pereopod distal segments adapted to form operculum when withdrawn into habitat opening, outer surfaces of carpus and propodus with concave opercular depression. Cheliped propodus (palm) with opercular depression limited mesially by crest of 7 or 8 marginally denticulate lobes. Second pereopod propodus with opercular depression limited laterally by crest of 5 marginally denticulate lobes. Pleon elongate, with lightly sclerotized transverse tergites dorsally, 4 minute biramous pleopods on left side, depressed longitudinal groove along left side; sixth pleonite greatest width exceeding three-fourths greatest length, anterior lobes to either side of median incision each armed by 7 or 8 strong spines. Uropods and telson symmetrical; telson subovoid, anteriorly with subtriangular median dorsal prominence, posterior margin entire, unarmed except for setae. Color pattern of orange to yellow-orange dominating most dorsal surfaces and superior surfaces of anterior appendages, darker blotches and spots of deeper orange to reddish brown. A diagnostic COI gene sequence available under GenBank accession number MT800937. 

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