BULLETIN OF MARINE SCIENCE, 71(1): 209–221, 2002 PELAGIC CEPHALOPODS OF THE ARABIAN SEA WITH AN EMPHASIS ON STHENOTEUTHIS OUALANIENSIS Michael V. Chesalin and German V. Zuyev ABSTRACT Taxonomic composition, vertical and horizontal distribution, and the biology of cepha- lopods were studied during the 30th cruise of the Soviet research vessel PROFESSOR VODYANITSKY in the Arabian Sea from February to April 1990. Small planktonic cephalo- pods were collected by midwater trawl down to 500 m depth, while large nektonic spe- cies were captured by jiggers and hand nets during night drift stations from the surface to 120 m depth. Nineteen species of 13 families were recorded. Mainly early life stages and small-sized planktonic oceanic species were present in trawl samples, with Sthenoteuthis oualaniensis, Abralia marisarabica, Abraliopsis lineata, Ancistrocheirus lesueurii, Onychoteuthis banksii, Chtenopteryx sicula, and Liocranchia reinhardti prevailing. The nektonic squids S. oualaniensis, O. banksii, Thysanoteuthis rhombus and pelagic octo- puses Tremoctopus violaceus and Argonauta argo were recorded on the surface. Among all cephalopods, S. oualaniensis absolutely dominated numbers and biomass in the epipelagial of the Arabian Sea. Juveniles (3–10 cm ML) and middle-sized specimens (10–30 cm ML) were observed at night, hunting on lantern fishes and flying fishes around the drifting vessel. The giant females (30–62 cm ML) were caught mainly close to the thermocline. The mean biomass of S. oualaniensis on studied area was estimated as 4.5 t km-2, and the most productive sites were identified. The Arabian Sea is one of the richest regions for cephalopods in the Indian Ocean (Aravindakshan and Sakthivel, 1973), but fishing activity in the open sea is still low, due to insufficient studies of the resource. Chun (1910) provided the most detailed study on Indian Ocean cephalopods. Belyaev (1962) pointed out the important role of cephalo- pods in the ecosystem of the Arabian Sea, based on the distribution of beaks in the sedi- ments. Filippova (1968) and Silas (1968) presented some data about taxonomy composi- tion and biology of cephalopods in the Indian Ocean and the Arabian Sea. Zuyev (1971) provided detailed descriptions of distribution and biology of ten neritic cephalopods of the suborder Myopsina and of one oceanic species (Sthenoteuthis oualaniensis) in the northwestern Indian Ocean, including the Arabian Sea. Aravindakshan and Sakthivel (1973) reported on the location of cephalopod nurseries in the Indian Ocean. Nesis (1974) de- scribed the cephalopod larvae from the western Arabian Sea. In addition, several publica- tions considered descriptions and biological features of several species (Nesis, 1970; Okutani, 1983; Tsuchiya, 1991; Tsuchiya et al., 1991). More recently, comprehensive studies on the fauna and distribution of the adult and early life stages of pelagic cephalo- pods in the Arabian Sea were published by Piatkowski and Welsch (1991) and Piatkowski et al. (1993). For many years, special attention has been focused on the study of the purpleback flying squid S. oualaniensis (Lesson, 1830). It has local commercial significance in the Arabian Sea and the Indian Ocean (Aravindakshan and Sakthivel, 1973; Nesis, 1977; Okutani and Tung, 1978; Silas et al., 1982; Roper et al., 1984; Raje and Savaria, 1987), but its resources are poorly exploited. The most comprehensive earlier information on S. oualaniensis from the tropical zone of the Indian Ocean was presented by Zuyev (1967, 1971), Zuyev and Nesis (1971), Zuyev et al. (1985). The most detailed information on S. 209 210 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002 oualaniensis in the Arabian Sea was published by Chesalin (1993) and Nesis (1993). However, the faunal composition of cephalopods of the Arabian Sea as well as many features of the distribution, biology, productivity, life cycle of S. oualaniensis, are still poorly known, so that any new information has important scientific significance. MATERIALS AND METHODS Cephalopods were sampled outside the 200 nautical mile (nmi) exclusive economic zones of Oman and Pakistan from February to April 1990 during the 30th cruise of RV PROFESSOR VODYANITSKY. The sampling was conducted on a grid of stations between 14o and 21o30'N, and 59o and 65o30'E (Fig. 1). The early stages and macroplanktonic adult cephalopods (generally <3 cm ML) were taken from macrozooplankton samples collected by an enlarged Isaacs Kidd midwater trawl with a small- cellular netting insert of 500 µm mesh size and a mouth opening of approximately 3 m2. Towing speed varied between 2.8 and 3.5 kts. The duration of tows was 30 min and the volume of water filtered ranged between 6680 and 7945 m3. Trawling was carried out obliquely from 100 m to the surface, as well as horizontal tows in the maximum of the sound-scattering layer (SSL) at night and in the maximum of the deep-scattering layer (DSL) in daytime. In total of 24 samples were ob- tained, i.e., 10 night samples from oblique hauls (0–100 m), eight night samples in the epipelagic SSL (20–120 m), and six daytime samples in the mesopelagic DSL (225–475 m). Samples were stored in 3–4% buffered formalin. Cephalopods were sorted from the samples in the laboratory, identified according to the guide of Nesis (1988), counted, and measured. The abun- dance of the various species and taxonomic groups was expressed as number per trawl (n tr−1). The horizontal distribution of some cephalopod paralarvae from the night hauls in the top 120 m are given as number per cubic meter (n m−3). During night three drift light stations were conducted to count and catch nektonic squids. The distance between stations varied from 5 to 30 nmi. In total, 146 drift light stations were performed Figure 1. Map showing the sampling grid for cephalopods on the 30th cruise of RV PROFESSOR VODYANITSKY, February to April 1990. Crosses, night drift stations; triangles, trawl samples. CHESALIN AND ZUYEV: PELAGIC CEPHALOPODS OF THE ARABIAN SEA 211 in the open Arabian Sea, with a total duration of 345 h. The micronektonic juveniles (generally 3– 10 cm ML) and adult nektonic squids (10–33 cm ML) were caught at the surface from the leeward side of the ship by two fishermen with jiggers, hand nets (diameter about 1 m). The large-sized S. oualaniensis (>30 cm ML) were caught on each such station from the windward side of the ship by two fishermen with manual reinforced jiggers with sinkers (and without bait) from depths down to 120 m. The biomass of young and middle-sized S. oualaniensis on the surface was estimated on the drift light stations using the method of visual observations of Zuyev et al. (1985). It is based on the behaviour of the squids to approach the surface and to be attracted by the lights of a stopped ship so that they are easily counted from shipboard. The biomass of squids was estimated by multiplying their number during 1 h of observation with the average weight of the different size-groups, and dividing by the speed of vessel drift and the width of the lighted zone. For estimation of the biom- ass (B, kg km−2) of large-sized S. oualaniensis at depth we used a new method, based on jig catches of one fisherman for 1 h, B = Y / (N . T . V . D), where Y is the total catch (kg); N is the number of fishermen; T is the duration of fishing effort (h); V is the drift speed of the ship (km h−1); and D is the width of fishing zone for one jig (0.005 km). RESULTS TAXONOMIC COMPOSITION.—Nineteen species were identified from our collection (Table 1). Three species were added in the list from Piatkowski and Welsch (1991) and eleven species were not recorded by these authors. Sixteen species belong to ten families of order Teuthida and three species are from three families of order Octopoda. Planktonic paralarvae of the benthic sublittoral Octopus defilippi evidently had been carried away by currents beyond the natural coastal biotope. The other cephalopod spe- cies are oceanic. Only Abralia marisarabica can be considered as endemic to the Arabian Sea. This species was described by Okutani (1983) from the northern Arabian Sea, but there is a questionable record in the region of Seychelles Islands (Nesis, 1985). According to zoo- geographical classification of Nesis (1985), 63% of the found species are cosmopolitan, 16% are Indo-West Pacific and 16% are Atlantic-Indo-West Pacific. Most species inhabit tropical and subtropical waters (74%) other species have tropical ranges. The bulk of the teuthofauna is made up by seven species: Sthenoteuthis oualaniensis, A. marisarabica, Abraliopsis lineata, Ancistrocheirus lesueurii, Onychoteuthis banksii, Chtenopteryx sicula and Liocranchia reinhardti (Table 2). The most abundant species were representatives from six families: Enoploteuthidae (51%), Ommastrephidae (28%), Onychoteuthidae (8%), Chtenopterygidae (8%), Ancistrocheiridae (3%) and Cranchiidae (2%). Species Enoploteuthis anaspis, Pterygioteuthis gemmata, Octopoteuthis sicula, Taningia danae and Taonius pavo each occurred in single specimens. In the trawl samples, cephalopods were represented mainly by paralarvae. Juvenile and adult specimens occurred rarely, except for small-sized adult enoploteuthids of the gen- era Abralia and Abraliopsis. Mantle length of cephalopods in the samples ranges from 2 to 45 mm. Paralarvae S. oualaniensis of 3–5 mm ML predominated in the majority trawl catches. The species composition in the observations and in the catches from the water surface was not diverse. Three species of nektonic oceanic squids, S.