Perisesarma samawati, Gillikin & Schubart, 2004

Gillikin, David Paul & Schubart, Christoph D., 2004, Ecology and systematics of mangrove crabs of the genus Perisesarma (Crustacea: Brachyura: Sesarmidae) from East Africa, Zoological Journal of the Linnean Society 141 (3), pp. 435-445 : 437-441

PERISESARMA SAMAWATI SP. NOV.

Description

A medium-sized crab, largest male encountered measuring 28.61 mm cw. Body form relatively vaulted (bh/cw = 0.52 ± 0.01, N = 11). Carapace broader than long with greatest width between exorbital angles (cl/cw = 0.81 ± 0.01, N = 11). Carapace regions clearly delimited, especially mesogastrical region ( Fig. 1A ). Carapace covered with sparse transverse rows of granules and tufts of small setae, especially in frontal region and forming striae on branchial regions. Front more than half of carapace width (iw/cw = 0.60 ± 0.01, N = 11), sharply deflexed, with broad median indentation. Frontal borders smooth and parallel. Interorbital region subdivided into four prominent frontal lobes covered with short rows of transverse granules, median lobes broader than lateral ones. Exorbital angle triangular, anterolateral margin anterior to notch straight and relatively short (el/cl = 0.13 ± 0.01, N = 10). Single epibranchial tooth pointed and slightly elevated, no indication of a second epibranchial tooth. Posterior lateral margins slightly concave and sloping ventrally, fringed by six rows of setae on pterygostomial region. Pterygostome and branchiostegite with dense cover of geniculate setae. Posterior border of orbit slightly perpendicular to front running posteriolaterally. Eyes pigmented, cornea large and wider than eyestalk. Suborbital ridge granular and setose. Epistome granular and glabrous except for Verwey’s groove, which is delimited by two rows of setae. Ventral border of epistome lined by row of tubercles (endostomial crista). Third maxillipeds granular, merus concave and broad (3mxl/3mxw = 0.73 ± 0.02, N = 10). Exopod with well developed setose flagellum.

Chelipeds equal and sexually dimorphic. Male chelae larger and stronger (pah/cw = 0.36 ± 0.04, N = 8) than female chelae (pah/cw = 0.26 ± 0.01, N = 4). Inner row of merus smooth with two longitudinal rows of setae, ventral row more prominent and continuous with longer setae, dorsal one interrupted with setal cover extending to dorsal border. Ventral face smooth, triangular with transverse row of tubercles distally; outer face covered with granules directed distally. All three borders serrate, prominent subdistal tooth on ventral border less serrate, smaller tooth on dorsal border. Carpus dorsally trapezoidal with smooth inner angle, inner border lined with pointed tubercles. Outer face of carpus granular, granules forming transverse groups; inner face with longitudinal row of minute granules and distal tuft of long setae ( Fig. 1C ). Upper surface of palm in males with 2 transverse pectinated crests. Proximal crest composed of 8– 14 teeth (most larger males 11–14), distal crest with 12–18 teeth, teeth of both crests similar in size. Both pectinated crests preceded and followed by several blunt tubercles and on inner side pronounced hump. In some cases, an additional proximal accessory crest comprising four teeth visible. In females, pectinated crests are less conspicuous and with shorter teeth; proximal crest in most cases replaced by row of tubercles. Upper margin of palm with strong blunt tubercles. Outer surface of palm bulbous and granular, granules forming indistinct median longitudinal line and ventral longitudinal groups ( Fig. 1B ); inner surface with smaller granules. Palm glabrous except for tuft of short setae in front of first pectinated crest. Distal third of surface of both fingers almost smooth. Cutting edge less than half the propodus length measured ventrally (0.37 ± 0.02, N = 10) with irregular number of triangular teeth. Ventral border of chela slightly concave at base of fixed finger, granular. Dorsal surface of dactylus bearing 7–9 blunt tubercles; first tubercle smaller, elongate and positioned more towards outer side, followed by 6–8 more prominent and round tubercles, becoming progressively less distinct and almost indiscernible towards tip ( Fig. 1B ). Dactylar tubercles are less distinct in females. Row of sharp, distinct tubercles (5–12) on proximal two thirds of inner edge of dorsal surface and scattered granules on outer edge of dorsal surface. Fingers with tips chitinous; adult males with variable gape width.

Third and fourth pairs of pereiopods longest (fourth tends to be slightly longer) (prp4/cw = 1.49 ± 0.09, N = 9). Meri of walking legs relatively broad, roughly twice as long as wide (4merl/4merw = 2.06 ± 0.13, N = 9); dactylus medium-sized (4dacl/prp4 = 0.17 ± 0.01, N = 9). Walking legs mostly glabrous, scattered long setae on dorsal and ventral borders, forming dense tufts at the dorsal propodus of all walking legs. Males with mat of setae on ventral propodus of pereiopod-2 and fringe of setae on ventral border of pereiopod-3. Dorsal face of meri granulate. Dorsal border of meri with row of small tubercles and sharp subdistal tooth.

Male abdomen with telson slightly shorter than width at base (tell/telw = 0.88 ± 0.05, N = 8), moderately pointed and longer than sixth abdominal segment (tell/6abl = 1.16 ± 0.08, N = 8); penultimate segment roughly half as long as wide (6abl/6abh = 0.52 ± 0.02, N = 8) ( Fig. 1D ). Second segment with median length slightly longer than lateral length. Abdomen of females fringed with long setae, evenly rounded, broadest at segment-5. In adult female specimens, abdomen touches coxae of walking legs. Taking this as a reference of adulthood, the smallest adult female encountered measured 21.79 cw (KBIN: I.G. 29968) and the largest juvenile female measured 12.92 cw (SMF 29335). Thoracic sternites in both sexes smooth and almost glabrous; episternites fringed with small setae on outer border. First male gonopods relatively slender, almost straight; apical process bent by approximately 45∞, with corneous tip ( Fig. 1E ). Few and scattered short setae along most of the gonopod, apical end covered by longer setae, almost completely covering corneous tip ( Fig. 1E ). Female gonopores prominent, located near the bottom of sternal cavity.

The coloration in life is characterized by a dark carapace with many aqua blue spots, black ocular peduncles mottled with sky blue blotches, corneas grey– black ( Fig. 2). Front and epistome dark with many sky blue spots. Pterygostomial region violet–blue, becoming mottled with sky blue spots on branchiostegal region ( Fig. 2); suborbital region sky blue with blotches of violet–blue. Ischium and merus of maxilliped sky blue with black blotches. Merus of cheliped brown to dark red with few sky blue spots on outer face and larger spots on inner face; carpus dark red, occasionally with sky blue spots; palm and dactylus bright red. Ambulatory legs with meri mottled sky blue and dark; carpus, propodus and dactylus with fewer or no sky blue spots. Colours fade quickly after preservation.

Etymology: The name of this new species, Perisesarma samawati , is derived from the Swahili word ‘Samawati’, which means ‘the sky, sky colour, azure’ (Oxford Swahili–English dictionary). It refers to the sky blue coloration of the front and of the dots on the carapace and walking legs ( Fig. 2), expressed in the language of the people from where this crab was discovered. Samawati is used as a noun in apposition.

Ecological observations

Perisesarma samawati sp. nov. is a semi-terrestrial (adults terrestrial; cf. Schubart, Cuesta et al., 2000) crab inhabiting the intertidal mangrove forests domi- nated by Rhizophora mucronata Lam. The crab occurs sympatrically with P. guttatum , but was not found to be distributed as extensively as P. guttatum . In the eastern riverine Mida Creek mangroves they occur across the full width of the muddy R. mucronata -dominated zones. No specimens were found in the more sandy sediments of the Ceriops tagal- dominated forests where P. guttatum was found to be common. In Gazi Bay (120 km south of Mida Creek), only a single female was found in a riverine forest type. In the fringe mangroves of Gazi Bay, no individual of P. samawati sp. nov. was ever found after hundreds of hours of field work, indicating that they are rare in this forest or do not inhabit fringe mangrove forests. Of the 23 plots investigated in Mida Creek, P. guttatum was found to occur in 95.7% of the plots (66.7% of 15 plots in Gazi), whereas P. samawati sp. nov. was found in 56.5% of the plots (0% in Gazi). Perisesarma samawati sp. nov. were always found sheltering in crevices under roots from which they did

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not readily venture far and quickly retreated to when approached. Perisesarma samawati sp. nov. was never recorded climbing on roots or trees and was active during daylight hours.

At the nine plots where both species of Perisesarma were found together, P. guttatum would re-emerge first after a disturbance (after 2.06 ± 1.86 min, N = 9), then P. samawati sp. nov. would re-emerge (4.56 ± 1.42 min, N = 9, Wilcoxon matched pairs, P <0.012). However, P. samawati sp. nov. was not as shy as Neosarmatium smithi (H. Milne Edwards, 1853) (9.89 ± 5.40 min, N = 9, Wilcoxon matched pairs vs. P. samawati sp. nov., P <0.03) which also inhabits the same areas. Distinct behavioural differences were noticed, in both males and females, when both species of Perisesarma were placed in an artificial situation and stimulated. All P. guttatum except for one female vigorously ran (83.3% ran, N = 6), whereas all P. samawati sp. nov. except for one male retracted all appendages and remained still (16.7% ran, N = 6).

When one leaf fragment was placed on the forest floor, P. guttatum was the first to reach it 100% of the time (N = 6). Although they would usually consume the leaves where they found them, they would also carry them off to other areas. In the case of abundant green leaves, P. samawati sp. nov. were observed to carry off fresh green leaf fragments. They were always observed to manipulate the leaf fragments at the edge of the refuge from which they came.

Molecular phylogeny

A phylogenetic analysis of 576 base pairs (70 variable and 21 parsimony-informative) revealed that the four included species of Indo-Pacific Perisesarma cluster together with high bootstrap support. The topologies vary according to the method used ( Fig. 3 ). The minimum evolution distance tree suggests a sister species relationship of the similarly coloured P. eumolpe and P. samawati sp. nov., while in maximum parsimony P. eumolpe forms an outgroup to the other species of Perisesarma . In both cases, however, P. guttatum is the sister species of the East Asian species P. bidens , thereby indicating that the two East African species are not sister species and thus probably colonized East African mangroves independently.

Figures

Figure 1. Perisesarma samawati sp. nov., male paratype (SMF 29334). A, dorsal view of carapace. B, outer face of chela. C, dorsal view of carpus of cheliped. D, male abdomen and sternum. E, first gonopods, one of them denuded. Scale bars: A, D = 10 mm; B, C = 5 mm; E = 1 mm.

Figure 3. Phylogenetic relationships among four Indo-Pacific species of the genus Perisesarma based on 575 base pairs of mtDNA coding for the 16S rRNA. Minimum evolution (neighbour joining, Kimura 2-parameter distances) (upper) and maximum parsimony (lower) trees with confidence values established after 2000 bootstrap replicates.