The diamondback squid Thysanoteuthis rhombus Troschel, 1857 is a large oceanic squid, which reach a maximum mantle length of 1 m and 30 kg of total weight (Jereb & Roper, 2010). It is considered a cosmopolitan species distributed worldwide in tropical and temperate open waters. The rhomboidal fins are large and extend all the length of the mantle, giving to this species a characteristic diamond shape. T. rhombus is known in the Mediterranean Sea since the XIX century (Troschel, 1857); however, its oceanic lifestyle hinders its detection. Although it is a common species in Mediterranean waters (Biagi & Bello, 2009), records are scattered in the scientific literature.

During the night of the 29^th August 2017 a specimen of T. rhombus was caught in the fishing grounds off the Catalan coast in the northwestern Mediterranean Sea (40.75527° N, 1.34805° E) at seafloor depth of 180 m, by the purse seine fleet from the Port of Tarragona. It was an immature subadult (Fig. 1A), measuring 93 mm mantle length and 33 g of total weight. Its sex was not possible to visually assess due the lack of detectable gonads at a macroscopic level. In Table 1, the dorsal mantle length, the ventral mantle length, the maximum mantle width, the fin length, the fin width, the head length, the head width, the length of the right arms I-IV and the right tentacle are provided according with the guidelines of Roper & Voss (1983). The characteristic well-developed anal photophore of juveniles and early subadults was present (Fig. 1B). The stomach content weighted 0.94 g and was entirely formed by fish fragments, including 3 vertebrae and 5 bones (0.37 g). Unfortunately, no otolith was recovered and the fish remain unidentified.

Some previous records of T. rhombus are known for the Iberian waters. The first one consisted in a single large individual fished in Mahón (Minorca) in 1978 (Morales, 1981), followed by the record of an egg mass by Guerra & Rocha (1997). Guerra et al. (2002) reported four additional egg masses in Mediterranean waters and other four in the Canary Islands. Escánez Pérez et al. (2012) reported several egg masses in the Canary Islands. Three large specimens were also reported in the Cantabrian Sea (Guerra et al., 2012). In the last work, it is also mentioned the capture of two individuals of both sexes in a tuna net in Ceuta. Templado and Luque observed two additional individuals in the Almeria coast (Guerra, 1992). For a comprehensive list of Catalonian and Iberian cephalopods, see Sánchez (2009) and Guerra (1992), respectively. Zaragoza et al. (2015) recorded 56 T. rhombus paralarvae sampled in 2004 and 2005. The presence of paralarvae at least during two consecutive years near the Ibiza Channel (Zaragoza et al., 2015), as well as the egg mass at the South of the Balearic Islands (Guerra & Rocha, 1997) and the newly reported subadult from Tarragona suggest that T. rhombus might be a common resident of the Balearic Sea. Besides those Iberian records, the species have been cited in other areas of the Mediterranean Sea (Issel, 1920; Sanzo, 1929; Biagi, 1982; Vardala-Theodorou et al., 1991; Jereb & Ragonese, 1994; Ezzeddine-Najai, 1996; Giordano et al., 1998; Bello, 1999; Salman et al., 2003; Marčic et al., 2009; Thessalou-Legaki et al., 2012; Yahel et al., 2017). Its oceanic lifestyle together with the absence of any target jigging fishery for oceanic squids in the area (Jereb & Roper, 2010) might complicate further encounters with this large oceanic squid. Despite its size and palatability, only a targeted fishery occurs in Japanese waters (Arkhipkin et al., 2015), but small catches are also reported in Jamaica, the Canary Islands and India (Aiken et al., 2007; Escánez Pérez et al., 2012; Sajikumar et al., 2020).

Material examined. Unsexed individual. 29/08/2017, at night. 40.75527° N, 1.34805° E. Fished by a purse seine vessel at surface waters (seafloor at 180 m of depth). M. Hernández leg. Voucher accession number ICMC000069, Biological Reference Collections of the Institut de Ciències del Mar (CBR-ICM, Barcelona, Spain), formalin fixed.