Dotilla fraternalis, Mitra & Trivedi & Mendoza, 2020

Mitra, Santanu, Trivedi, Jigneshkumar N. & Mendoza, Jose Christopher E., 2020, On the two ‘ forms’ of Dotilla intermedia De Man, 1888 (Crustacea: Brachyura Dotillidae) from the Bay of Bengal, India, with description of a new species, Zootaxa 4809 (3), pp. 521-534 : 526-532

publication ID	https://doi.org/ 10.11646/zootaxa.4809.3.6
publication LSID	lsid:zoobank.org:pub:46DA13F2-1B01-4E07-B134-BCBEFAB44A73
DOI	https://doi.org/10.5281/zenodo.4324275
persistent identifier	https://treatment.plazi.org/id/03E58796-FFCD-FFF1-30E1-F8A2FBC792A9
treatment provided by	Plazi
scientific name	Dotilla fraternalis
status	sp. nov.

Treatment

Dotilla fraternalis View in CoL n. sp.

( Figs. 6 View FIGURE 6 , 7A, 7D View FIGURE 7 , 8A, 8E View FIGURE 8 )

Dotilla intermedia View in CoL (“low form”), Kemp 1919: 331 (in part), figs. 10b, c, f; Dev Roy & Bhadra 2005: 512. Not Dotilla intermedia De Man, 1888 View in CoL .

Dotilla intermedia, Bairagi 1995: 278 View in CoL . Not Dotilla intermedia De Man, 1888 View in CoL .

Material examined. Holotype, male, CW 7.9 mm, CL 5.9 mm ( ZSI-C7125 /2); Digha Beach, West Bengal, India, coll. S. Mitra, 17 July 2017.

Paratypes: 18 males, CW 6.4–8.0 mm, CL 5.1–6.1 mm, 7 females, CW 6.2–7.9 mm, CL 5.0– 5.9 mm (ZSI- C7126/2), collection data same as holotype .

Description. Carapace ( Fig. 6A View FIGURE 6 ) subtrapezoidal, width 1.2–1.3 times length, anterolateral margins convergent towards anterior. Dorsal surface areolate, strongly sculptured, with several sulci/grooves; deep median groove running from front to posterior margin. Gastric region with four small but distinct lobules more or less centrally located, anterior to these and within median groove is a smaller, fainter 5 th lobule. Several grooves present. One oblique groove runs from mesogastric region towards external orbital angle; this groove connects with longer oblique groove at mesogastric region, which runs from there towards posterolateral angle; mesially, a third, shorter groove originates from cardiac region and also runs towards posterolateral angle. Lateral to these a near-vertical groove runs parallel to anterolateral margin of carapace, from just beneath anterolateral angle toward posterolateral margin, this groove does not bifurcate at its anterior end. Transverse groove present close to posterior margin, interrupted medially by areola of intestinal region; two parallel grooves run from this posterior groove to edge of gastric region, marking lateral edges of cardio-intestinal region. Side (branchial) wall of carapace with one longitudinal and two, much shorter, transverse grooves.

Front relatively narrow ( Fig. 6B View FIGURE 6 ), deflexed ventrally, appearing bilobate from dorsal view but margin entire and triangular in shape in frontal view. Supraorbital margins, granulate, continuing smoothly from base of front toward external orbital angle, which terminates in a small, acute, laterally directed tooth; this tooth separated from anterolateral margin of carapace by wide V-shaped indentation. Anterolateral margins subparallel, convergent anteriorly, slightly convex, much longer than posterolateral margins; posterolateral margins short, deeply concave; posterior margin slightly sinuous, central region slightly concave.

Eyes well developed ( Fig. 6B View FIGURE 6 ), peduncles long, corneas bulging. Antennule with large basal article occupying most of antennular fossa, distal articles much smaller. Antenna small, tip of flagellum not reaching tip of eyes. Epistome well developed, central region broadly triangular. Immediately subjacent edges of pterygostome perpendicular to posterior border of epistome. Second maxilliped with penultimate article of endopod much expanded, Spoon tipped setae ( Fig. 7D View FIGURE 7 ) on merus of second maxilliped six-tined at its distal end with bifurcate peduncles. Third maxillipeds large, leaving a slight median gap when closed together; merus larger than ischium, with inverted V-shaped sulcus; palp flattened; small, thin exopod concealed beneath endopod.

Thoracic sternum ( Fig. 6D View FIGURE 6 ) without tympana; immediately beneath anterior margin of sternite 4 two large triangular teeth present, positioned on either side of sternite 4 and anterior to tip of sternoabdominal cavity. Sternal tubercle of sternopleonal locking mechanism located on anterior part of sternite 5 just next to suture with sternite 4.

Chelipeds (P1) ( Fig. 6A View FIGURE 6 ) symmetrical, homochelous. Merus with oval tympanum on dorsal surface. Carpus ovate, inflated, external surface finely granulate. Palm of chela ( Fig. 6C View FIGURE 6 ) subrectangular, inflated, superior and inferior margins convex, external surfaces finely granulate; a distinct ridge of small granules positioned on palm external surface next to inferior margin, continuing into and running along most of the length of fixed finger. Fingers relatively straight, distinctly longer than palm, cutting margins with small teeth but never with pronounced lobes; dactylus with granulate ridge on external surface.

Ambulatory legs ( Fig. 6A View FIGURE 6 ) (P2–P5) relatively stout, moderately long. Meri with oval tympana on both flat surfaces. P5 dactylus slender, with row of long stiff setae on dorsal margin; approximately twice the length of propodus.

Male pleon ( Figs. 6D View FIGURE 6 , 7A View FIGURE 7 ) with six freely articulated somites (pleomeres) and telson. Anterior margin of 4 th pleomere, slightly concave, overlapping 5 th pleomere with band of stiff setae on extending along the whole width of anterior margin. Telson with broadly rounded tip, slightly convex lateral margins. Male and female pleons similarly narrow and triangular, with the female pleon only slightly wider.

G1 ( Fig. 8A View FIGURE 8 ) slender, sinuous, somewhat S-shaped, tapering towards distal tip; distal tip mostly without setae, row of stiff setae arranged sparsely along lateral and mesial margins on distal two-thirds. G2 ( Fig. 8E View FIGURE 8 ) stout with rounded apical lobe.

Variation: Two large triangular teeth present on the anterior margin of sternite 4 are present in the larger adult males (CW at least 7.0 mm) but such structures are not observed in smaller males (CW <7.0 mm).

Etymology. The specific epithet, fraternalis (Latin, adj., pertaining to a brother) alludes to the initial conflation of this species with its congener, D. intermedia . Used as an adjective.

Remarks. Dotilla fraternalis n. sp. basically corresponds to the “low form” of D. intermedia sensu Kemp (1919) , who provided the following distinguishing morphological characters: “In the ‘low’ male, the first abdominal sternum [= thoracic sternite 4] bears anteriorly a pair of large outstanding triangular teeth; these are in advance of the limit of the abdominal trough [= sternopleonal cavity] and are thus placed considerably further forward than the ridges in the ‘high’ male. The dactylus of the chela bears a low rounded lobe near the base of its prehensile edge (further back than in the ‘high’ male) and there is no lobe or large tooth on the fixed finger. The copulatory appendage [= G1] is more slender, strongly sinuous, and terminates in a fine point which is turned inwards and does not bear conspicuous setae.” ( Kemp 1919: 331, figs. 10b, c, f).The present material agrees with this description (see Figs. 6C, D View FIGURE 6 , 8A View FIGURE 8 ).

Aside from these differences, D. fraternalis n. sp. can further be distinguished from D. intermedia s. str. by: (1) the abruptly perpendicular orientation of the pterygostome margin immediately subjacent to the posterior margin of the epistome ( Fig. 6B View FIGURE 6 ) (versus gradually oblique in D. intermedia ; Figs. 1B View FIGURE 1 , 2B View FIGURE 2 , 3B View FIGURE 3 ); (2) the male telson has a relatively broader tip and slightly convex lateral margins ( Figs. 6D View FIGURE 6 , 7A View FIGURE 7 ) (versus tip of telson narrower and lateral margins slightly concave in D. intermedia ; Figs. 1D View FIGURE 1 , 2D View FIGURE 2 , 7B View FIGURE 7 ; cf. Kemp 1919: fig. 10e); and (3) the spoon-tipped setae on the merus of the second maxilliped are six-tined at their distal end, and are with bifurcate peduncles ( Fig. 7D View FIGURE 7 ) (versus spoon-tipped setae on merus of second maxilliped five-tined at distal end, with simple peduncle in D. intermedia , Fig. 7E View FIGURE 7 ).

The statement by Kemp (1919) that his “high” and “low” forms are conspecific is not supported by the present study. He used as basis for this pronouncement merely the similarity in the sculpturing of their dorsal carapace and their co-occurrence in the same general habitat. He considered the stark differences in the G1 morphology as an ultimately physiological difference between breeding (high form) and non-breeding (low form) males. There is, as discussed earlier in the remarks for both species, ample morphological evidence that suggests that the ‘low’ and ‘high’ forms are distinct species. Foremost among these are the differences in the armature of the male sternite 4, and the drastic differences in the morphology of the G1, of which the latter is rather suggestive of reproductive isolation, and which in turn is usually a reliable character for the discrimination of species ( Guinot 1979; Ng et al. 2008; Davie et al. 2015a, b). Further studies on the genetics and behavior of these species should confirm their distinctness.

Dev Roy & Bhadra (2005) reported “high” and “low” forms of Dotilla intermedia from the coast of Andhra Pradesh, they distinguished those two ‘forms’ well, but did not formally recognise the “low” form as a separate species.

Earlier Bairagi (1995) recorded D. intermedia from Sagar Island, West Bengal.While he did not use the morphological distinctions of Kemp, he noted that all the specimens possess a pair of large triangular teeth positioned anteriorly on the thoracic sternum, which clearly indicate that the crabs are more properly identified as D. fraternalis n. sp.

According to the observation of Kemp (1919), the ‘low’ forms should be small in size (carapace length of males rarely exceed 4.5 mm), but current observations show that males of D. fraternalis n. sp. can be larger, reaching 6.1 mm in carapace length.

Dotilla fraternalis n. sp. also has some morphological similarities with D. blanfordi Alcock, 1900 , as both those species have 4 lobules on the gastric region of the carapace, a mid-dorsal groove extending from the front to the posterior margin, and a transverse posterior groove present on the posterior region of the carapace, and there is no tooth below the orbital angle. The new species differs from D. blanfordi in many morphological characters: (1) there is a single oblique groove running along the side of the cardiac region towards posterolateral angle of the carapace in D. blanfordi ( Alcock 1900: fig. 3; Kemp 1919: fig. 9e) (versus two oblique grooves running along the side of cardiac region towards posterolateral angle of the carapace in D. fraternalis n. sp.; Fig. 6A View FIGURE 6 ); (2) the lower surface of the palm is not carinate (J. Trivedi, pers. obs.) (versus lower surface of palm carinate in D. fraternalis n. sp.; Fig. 6C View FIGURE 6 ); and (3) the spoon-tipped setae on the merus of the second maxilliped are four-tined at their distal end, and are with a simple peduncle ( Vogel 1984: fig. 4) (versus spoon-tipped setae on merus of second maxilliped six-tined at distal end, with bifurcate peduncles in D. fraternalis n. sp.; Fig. 7D View FIGURE 7 ).

Dotilla fraternalis n. sp. is presently known from Digha Beach, West Bengal (type locality), and Chandipur, Odisha. It occurs sympatrically with D. intermedia on the littoral zone at Digha (West Bengal, India). However, while D. intermedia prefers the upper to middle littoral zone with a sandier substrate, D. fraternalis n. sp. tends to occupy the upper littoral zone, where the substrate is mostly silty (S. Mitra, pers. obs.).