Thysanoteuthis rhombus Troschel, 1857

Rajkumar, M, Midun, M, Thirumalaiselvan, S, Rajkumar, R & Saravanan, R, 2022, A unique occurrence of large-sized diamondback squid Thysanoteuthis rhombus (Troschel, 1857) in the Gulf of Mannar, Indian Journal of Geo Marine Sciences 51 (7), pp. 650-653 : 650-652

Thysanoteuthis rhombus Troschel, 1857

The squid was caught in the hook and line of a 13.5-meter-long boat equipped with a 20-hp engine. The fishing operation was carried out at a depth of 20 m, 25 km off the coast of Pamban in the Gulf of Mannar. The squid, measuring 633 mm in mantle length and weighing 15.1 kg ( Fig. 2 ), was auctioned off at the landing centre for Rs. 2350/-.

This bright red squid had a cylindrical muscular mantle that gradually tapered to a blunt end posteriorly. The fins were long and extended all the way down the sides of the mantle. The fin was typically rhomboid in shape, with a broad middle and tapering towards the anterior and posterior ends. The head was narrower and the eyes were prominently placed. Cirrate trabeculate covered the long oral arms with protective membranes. The outer lateral arms were longer than the inner dorsal arms and had a crest-like muscular projection at each arm's base. Such structures were absent from the other intermediary arms. Each arm had two rows of suckers with 20-26 sharp teeth on the sucker rings. The tentacles were weak, and the clubs were slightly bulged in the middle, with four rows of suckers and sucker rings with 15 – 20 sharp teeth.

Table 1 shows the specimen's morphometric measurements. Large populations of this squid can be found in the waters of Japan 11, where it is commercially exploited by some local fishermen but appear to be underutilised in other parts of the world 12, 13. Diamondback squid biomass in the global oceans is estimated to be 1.5 – 2.5 million tonnes 14, and it is abundant in the productive zone of the open ocean 15, 16. T. rhombus geographical distribution within the Indian EEZ is currently unknown, but sporadic and rare occurrences of T. rhombus in fish landings have been reported ( Table 2 ).

Filioppova 2 found this species in the Indian Ocean at nine different locations, none of which are along the Indian coast. Barnard 17, Silas 18 and Silas et al. 19 noted in their cephalopod catalogue that an adult specimen of this species was stranded at the Cape of Good Hope. Landings of the same species have also been reported in Veerapandiyanpattinam (Gulf of Mannar) from a drift gill net 4 with a mantle length of 700 mm, 370 mm in Maharashtra from a trawler, 630 mm in Chennai from a drift gill net 8 and 1070 mm in Munambam from a trawler 9.

Sasikumar et al. 20 reported that the trawl landings mostly comprised of squids of 155 – 570 mm DML, whereas the larger squids of 570 – 670 mm DML were usually caught in drift-gillnets and hooks & lines (jigs). The most common size in the trawl fishery was 250 mm DML. Adult catches by trawlers and other vessels are incidental and the landing of the present specimen is typically a bycatch of myopsid squids.

Figures

Fig. 2 ― Diamondback squid Thysanoteuthis rhombus caught in hook and line operated in the Gulf of Mannar