Aristeus alcocki Ramadan, 1938

Yang, Chien-Hui, 2017, Photophore counts in the deep-sea commercial shrimp Aristeus alcocki Ramadan, 1938 (Crustacea: Decapoda: Aristeidae), with a revised key to the Indo-West Pacific species of the genus, Zootaxa 4329 (4), pp. 392-400 : 393-396

Aristeus alcocki Ramadan, 1938

( Figs. 1 , 2A, C , 3 )

Aristeus alcocki Ramadan, 1938: 40 , figs. 1, 2a, 3a (type locality: Gulf of Aden); George 1967: 340; 1979: 26, fig. 2b; Crosnier 1978: 61, fig. 25j; Holthuis 1980: 9; Susellan 1989: 54, fig. 1; Pérez Farfante & Kensley 1997: 40; De Grave & Fransen 2011: 206; Chakraborty et al. 2015: 1719, figs. 4–5.

Aristeus semidentatus — Wood-Mason & Alcock 1891: 280; Alcock 1901: 31; Alcock & McArdle 1901: pl. 49-fig. 3; Kemp & Sewell 1912: 17; George 1967: 339; 1979: 27, fig. 2c; Holthuis 1980: 10 (p.p.). (non Bate, 1881)

Material examined. Sakthikulangara fishing harbor, Kollam district , Kerala, 20 March 2017, 2 females cl 38.3 and 41.4 mm (DABFUK) ; 6 females cl 29.7–52.6 mm (DABFUK), 10 females cl 31.2–52.8 mm (DABFUK 20–29), 2 females cl 31.8 and 32.9 mm (DABFUK), 2 females cl 30.5 and 36.3 mm (DABFUK).

Diagnosis. Integument glabrous. Rostrum usually bearing only 3 dorsal teeth at base, very long and far overreaching scaphocerite in females but short and failed to reach distal margin of scaphocerite in males. Carapace with lower end of cervical carina located at level slightly lower than antennal spine and far from branchiostegal carina ( Fig. 2C ); distance between lower end of cervical carina and branchiostegal carina similar to that between antennal and branchiostegal spines. Pleurobranchs of thoracic somites IV to VII (corresponding to pereiopods I to IV) tiny, greatly reduced to simple papillae and without pinnules ( Fig. 2A ). Pereiopod I ( Fig. 3A ) with subdistal movable spine on lateral surface of merus, chela 1.14–1.46 times as long as carpus, fingers 1.51–1.83 times longer than palm; carpus with 14–24 photophores, propodus with 4–7 photophores, dactylus without photophores. Pereiopod II ( Fig. 3B ) also with sudistal movable spine on lateral surface of merus, chela 0.93–1.11 times as long as carpus, fingers 1.27–1.62 times longer than palm; carpus with 9–15 photophores, propodus with 3–6 photophores, dactylus lacking photophore. Pereiopod III ( Fig. 3C ) without subdistal movable spine on merus, chela 0.70–0.88 time as long as carpus, fingers 1.09–1.38 times longer than palm; carpus bearing 6–13 photophores, propodus with 3–6 photophores and dactylus without photophore. Pereiopod IV ( Fig. 3D ) with carpus 0.99–1.10 times as long as merus; carpus with 18–30 photophores, propodus with 19–30 photophores while dactylus without photophore. Pereiopod V ( Fig. 3E ) with carpus 1.08–1.28 times longer than merus, carpus with 14–32 photophores, propodus with 25–44 photophores and dactylus with 9–21 photophores. Abdominal somite III without posteromedian spine.

Coloration ( Fig. 1 ). Carapace and cephalic appendages generally reddish, eyes black brown. Abdomen pinkish red and with posterior border of each somite more reddish. Pereiopods pinkish red, photophores purplish red with iridescence. Pleopods and tail fan generally reddish.

Distribution. Gulf of Aden, Arabian Sea, India and Bay of Bengal, at depths of 250–1087 m.

Remarks. Although both Aristeus semidentatus Bate, 1881 and A. alcocki have often been reported from India (e.g., George 1967, 1979), Suseelan (1989) argued that only A. alcocki is found in Indian waters (also see Ramadan 1938). Nevertheless, recent shrimp checklists from India still included A. semidentatus ( Radhakrishnan et al. 2012; Samuel et al. 2016). The abundant material of Aristeus observed in Sakthikulangara fishing harbor during this work contained only A. alcocki . Whether A. semidentatus actually occurs in Indian water still needs to be confirmed. As mentioned above, both the relative lengths of the different segments of the pereiopods and number of photophores have been used to separate the species of Aristeus . These characters, however, are known to have large variations, including geographical variations ( Crosnier 1978, 1985, 1994a, b; Komai 1993). The availability of many fresh material of A. alcocki enables the assessment of the taxonomic values of these characters at least for distinguishing the six species in the Indo-West Pacific. Table 1 summaries the known phophophore counts and ratios of the different segments at the pereiopods in these species. Aristeus antennatus ( Risso, 1816) has much more numerous photophores than the other species of the genus, while A. mabahissae Ramadan, 1938 has the smallest number of photophores. On the other hand, A. pallidicauda Komai, 1993 is unique in the carpi of both pereiopods I and II lacking photophores, thought it generally has more photophores on the posterior two pereiopods than most other species except A. antennatus . Aristeus virilis ( Bate, 1881) also differs from other species in the dactyli of pereiopods II and IV sometimes bearing photophores but generally has fewer photophores than most of the other species. Nevertheless, its photophore counts always overlap with those of other species. The photophore counts of the remaining two species A. semidentatus and A. alcocki largely overlap with each other though A. semidentatus generally has fewer photophores as compared to A. alcocki . At present there is a discrete difference in the number of photophores only at the propodus of pereiopod IV, and it is 15–18 in A. semidentatus and 19–30 in A. alcocki . As the difference is extremely slight, it is highly likely that this photophore count will also be merged when more is known on the variations in these two species.

The ratios of the various segments of the pereiopods have even larger overlapping amongst the Indo-West Pacific species, though there is a tendency of longer or shorter in different species. Usually the chela and fingers of the anterior three pereiopods are shorter in A. mabahissae and A. semidentatus . Moreover, the carpus of the pereiopod IV is also somewhat shorter for A. mabahissae . Other than the photophore counts and ratios of the segments in the pereiopods, A. virilis and A. alcocki possess some unique characters amongst the Indo-West Pacific species of the genus. Aristeus virilis is very distinctive in the pubescent body integument (versus completely naked) and the possession of a movable subdistal spine on the merus of the pereiopod III (versus lacking such a spine). In A. alcocki the pleurobranchs of thoracic somites IV to VII are greatly reduced to minute papillae without pinnules ( Fig. 2A , versus reduced but still obvious and feather-like with many distinct pinnules or gill filaments, Fig. 2B ). This difference in the gills, however, are difficult to observe in the field and without dissection. Nevertheless, it is found that the position of the lower end of the cervical carina relative to the branchiostegal carina as suggested by Crosnier (1978) is a very useful character to separate A. alcocki from the other Indo-West Pacific species of the genus. The distance between the lower end of the cervical carina and the branchiostegal carina is nearly as long as the length between the antennal and branchiostegal spines in A. alcocki ( Fig. 2C ). In all the other species, the distance between the lower end of the cervical carina and the branchiostegal carina is only about half the length between the antennal and branchiostegal spines ( Fig. 2D ).

The GenBank only has six COI sequences of species of Aristeus that can be used for usual DNA barcoding analysis ( Hebert et al. 2003, Hajibabaei et al. 2007). Amongst them four are from A. alcocki , one from A. antennatus and another from A. virilus . It should be noted that the sequence of A. virilus ( JQ928295 ) used in Chakraborty et al. (2015: fig. 6b) for constructing a COI parsimony tree is actually not the region commonly used for COI barcoding match and therefore excluded in the present analysis. The present work generated COI sequences (> 600 bp) for all the Indo-West Pacific species except A. antennatus . It reveals that there is a misidentification in the GenBank sequence for “ A. virilis ” ( KX381807 , from India), which has 99.7% sequence similarity with the present material of A. alcocki . With the consideration of this misidentification, the present molecular genetic analysis confirmed that the six currently recognized taxa of Aristeus in the Indo-West Pacific are all distinct species with more than 6.9% sequence divergence (between A. mabahissae and A. semidentatus ) in the barcoding gene COI (see Darling 2011, Vergamini et al. 2011, Yang et al. 2016). On the other hand, the highest intraspecific sequence divergence noticed is 3.8% in A. alcocki amongst the Indian material. Thus, the following revised key is constructed for the Indo-West Pacific species of Aristeus , with the consideration of long preserved material might have the photophores difficult to recognize.

Figures

FIGURE 1. Aristeus alcocki Ramadan, 1938, Sakthikulangara fishing harbor, southwestern India, female cl 41.4 mm (DABFUK), atypical in having only two instead of three dorsal rostral teeth.

FIGURE 2. A, C, Aristeus alcocki Ramadan, 1938, Sakthikulangara fishing harbor, southwestern India, A, female cl 41.4 mm (DABFUK), C, female cl 52.6 mm (DABFUK )); B, D, A. virilis (Bate, 1881), Dasi fishing port, northeastern Taiwan, female cl 38.8 mm (NTOU M 02048). A, B, branchial chamber of thoracic segment VI and VII (corresponding to pereiopods III and IV), a = arthobranch, e = epipods, p = pleurobranch, P = pereiopod; C, D, pterygostomian region of carapace. Scales: A, B = 2 mm, C, D = 5 mm.

FIGURE 3. Aristeus alcocki Ramadan, 1938, Sakthikulangara fishing harbor, southwestern India, female cl 52.6 mm (DABFUK). A, left pereiopod I, chela to distal part of merus; B, left pereiopod II, chela to distal part of merus; C, left pereiopod III, chela to distal part of merus; D, left pereiopod IV, dactylus to distal part of merus; E, left pereiopod V, dactylus to distal part of merus. Dots representing photophores. Scale = 2 mm.