The first record of Tremoctopus violaceus sensu stricto Delle Chiaje,1830 in southwestern Gulf of Mexico gives a hint of the taxonomic status of Tremoctopus gracilis

Abstract

Knowledge on species taxonomic identity is essential to understand biological and biogeographical processes and for studies on biodiversity. Species the genus Tremoctopus have been confused in the past and are inconsistently identified. To clarify of the taxonomic diagnosis Tremoctopus violaceus Delle Chiaje, 1830, an evaluation of morphological and meristic characters, as well as morphometric indices and genetic analyses, was undertaken. The analyzed octopod was an opportunistically collected mature female of 640 mm in total length, with a mantle length of 135 mm and a total weight of 1.02 kg. Evidence of autotomy as a defensive mechanism for protecting the egg mass is presented. The 16S haplotype sequenced from this specimen represents the first one publicly available for this species from the Gulf of Mexico. The genetic divergence between this haplotype and those reported from the Pacific Ocean is representative of interspecific variation in other taxa, which suggests that “T. violaceus” in the Pacific Ocean (KY649286, MN435565, and AJ252767) should be addressed as T. gracilis instead. Genetic evidence to separate T. violaceus and T. gracilis is presented. The studied specimen from the Gulf of Mexico represents the westernmost known occurrence of T. violaceus and the first record from the southwestern Gulf of Mexico.

First Integrative Morphological and Genetic Characterization of Tremoctopus violaceussensu stricto in the Mediterranean Sea

“…Our three sequences clustered together in clade B with an individual of T. violaceus from the Gulf of Mexico were well separated from the other three sequences available in GenBank (accession numbers: MN435565, KY649286, and AJ252767), grouped in clade A. As already pointed out by [ 7 ], the sequences in clade A were likely misidentified and corresponded to T. gracilis . The estimate of the divergence between the groups resulted in a 6.4% distance, while very slight differences were found among the sequences within the clades (0.6% divergence in clade A and 0% in clade B; Table 2 ).…”

“…The absence of an entire web in all the females analyzed confirmed the significance of the autotomy as a defensive tool by means of the detachment of segments [ 38 ]. The chromatic pattern of the web has been described with large round spots in T. violaceus [ 1 , 7 , 9 ] and with series of patches that form clusters arranged transversely with respect to the longitudinal axis of the arm in T. gracilis [ 9 , 17 ]. In our samples, the chromatic pattern, even if observed only in the remains of pieces of webs, seemed to match the second case and to be similar to the webs figured by [ 17 , 39 ] and photographed in the coastal water of Ponza Island (in [ 9 ]).…”

“…They are characterized by marked sexual size dimorphism, with small dwarf males and large females, some of which reach 2 m in length [ 3 , 4 ]. Four species are recognized as valid within the genus Tremoctopus , i.e., Tremoctopus gelatus Thomas, 1977, which is meso-bathypelagic with circumtropical and temperate distributions, Tremoctopus gracilis Souleyet, 1852 that occurs in the Pacific and Indian oceans, Tremoctopus robsoni Kirk, 1884 described from waters off New Zealand, and Tremoctopus violaceus originally portrayed in the Mediterranean waters by Delle Chiaje (1830) and distributed also in the Atlantic Ocean including the Gulf of Mexico and the Caribbean Sea [ 5 , 6 , 7 ]. Apart from T. gelatus , of which the males and females are easily recognized being characterized by their gelatinous tissues, separating the other three species is not an easy task.…”

“…The difficulty in separating these taxa, as well as the paucity of molecular data, could cause several errors in identifications, especially involving T. violaceus and T. gracilis , as exemplified by the records of these two species outside their known geographical limits [ 7 ]. For instance, in the Mediterranean Sea, in the past, T. violaceus was the only member of the genus known to live in the area, while nowadays it is reported that two species of the genus Tremoctopus occur in the Mediterranean Sea, i.e., T. violaceus as the native species and T. gracilis as an alien species probably arriving in the Mediterranean due to human-mediated transfer or with Lessepsian migrations [ 8 , 10 , 11 ].…”

“…It cannot be excluded that other T. gracilis are misidentified with T. violaceus in the past, since T. violaceus is the only member of the genus known to live in the Mediterranean [ 10 ]. All these Mediterranean records of T. violaceus and T. gracilis refer to female specimens identified only by morphological characters that have been reported to be not always solid enough to avoid misidentifications [ 7 ]. Indeed, no genetic analyses have been conducted to date on the genus Tremoctopus in the Mediterranean Sea, and no sequences are available yet in the public repositories from this area.…”