First record of a rare pelagic octopod, Ocythoe tuberculata (Cephalopoda: Ocythoidae), in Korea

ABSTRACT The family Ocythoidae is found throughout the temperate and subtropical waters of the world’s ocean and contains a single species, Ocythoe tuberculata. Recently, a single female specimen of the rare pelagic species O. tuberculata was collected off Uljin in the East Sea/Sea of Japan of Korea by squid jigging for the first time. The appearance of this species could be considered as evidence of the changes in species distribution of sea surface. However, further studies are required to investigate whether this phenomenon is temporary or continuous.


Introduction
The family Ocythoidae is a monotypic taxon, and Ocythoe tuberculata is the representative species of the family (Packard & Wurtz 1994). This species shows sexually dimorphism in size. The female is large, up to 350 mm in mantle length (ML), with cross points on the ventral side, whereas the male is smaller, not exceeding 30 mm in ML, with a developed hectocotylus in the sac (Roper & Sweeney 1975;Jereb et al. 2016). This species occurs from the sea surface to depths of about 200 m in subtropical and temperate waters. In North Pacific, large females with true swimbladder commonly caught in drift net in the upper 10 m of the water at night, which underpin the pelagic behavior (Packard & Wurtz 1994;Nesis 2003;Tutman et al. 2008).
This species is known to be a cosmopolitan, especially in the northern hemisphere (Roper & Sweeney 1975). Although this species is known to be distributed in the North Pacific (Jereb et al. 2016), this is the first record of the rare octopod in Korea. In this study, we describe the morphological characteristics of the species and suggest a Korean name for it.

Sampling and Preservation
A single specimen of Ocythoe tuberculata was captured off Uljin, in Korea, by squid jigging (Fig. 1). The specimen was stored in a freezer and afterwards in 10% formalin.
A small tissue sample was taken from the mantle of the specimen before fixation, for molecular analysis. The specimen and tissue sample were deposited in the East Sea Fisheries Research Institute (ESFRI) of the National Institute of Fisheries Science (NIFS) of Korea.

Morphological analysis
Measurements were made according to Jereb et al. (2016), the taxonomic system and terminology also followed Jereb et al. (2016). Body measurements were made with digital calipers and tapeline, and the internal organs were investigated by dissection.

Molecular analysis
The extraction of total DNA and amplification of cytochrome c oxidase subunit 1 gene (COI) of the mitochondrial DNA (mtDNA) were performed according to Kim et al. (2016). The sequence was aligned with BioEdit ver. 7 (Hall, 1999) and submitted to the DDBJ/EMBL/ GenBank nucleotide sequence databases under accession number KY947106.

Description
Female. All measurements are listed in Table 1. Body is muscular. Dorsal side of mantle is smooth without warts. There is a reticular sculpture of criss-crossed skin ridges with tubercles at cross points on the ventral side of the mantle. A pair of cephalic water pores is present. Arms are long, dorsal and ventral arms are longer than lateral arms (dorsal arms are broken at the present specimen). First arms are thickest. Suckers are biserial, and suckers of the dorsal arms are larger than those of other arms. There are 8 suckers around the mouth. Web and arm fringes absent. Funnel is very long, accounting for 54% of mantle length. Locking apparatus of funnel is well developed and strongly connected to the mantle. Color is blue and violet on dorsal side and   (Voss 1967), eastern Indian Ocean off Australia (Roper & Sweeney 1976), eastern South Pacific Ocean (Cardoso 1991), North Pacific Ocean (Jereb et al. 2016), Japan (Berry 1912), and the East Sea/Sea of Japan of Korea (Jereb et al. 2016;present study).

Remarks
In this study, a single octopod collected in the East Sea/ Sea of Japan of Korea was identified as a female of Ocythoe tuberculata by its muscular body tissue, absence of web between all arms, well-developed funnel locking apparatus, and the presence of tubercles on the ventral side of the mantle. In the original description by Rafinesque (1814), this species was characterized by the smooth dorsal side and the reticular sculptured ventral side of the mantle in the female, elongated arms, biserial suckers, and eight suckers around the mouth. The description of the present specimen is consistent with that of Rafinesque (1814). A molecular analysis supported our morphological results. The mtDNA COI sequence of the specimen was identical to O. tuberculata deposited in GenBank.
The occurrence of O. tuberculata is often explained related to sea currents and sea warming. In the Mediterranean Sea (type locality), this species is probably transported from the open waters of the Mediterranean Sea to the Aegean Sea by strong currents toward the south-east (Corsini & Lefkaditou 1994). A similar distribution trend associated with the sea currents was proposed for O. tuberculata in the Adriatic Sea (Tutman et al. 2008). This species enters the Adriatic, probably from the Mediterranean Sea, following a north-westward current toward the northern Adriatic Sea. Tutman et al. (2008)  suggested that sea warming affects the occurrence of O. tuberculata in the southern Adriatic Sea. In this area, there was considerable year-to-year variability in SST between 1950 and 2005, with a clear increase in SST in this period (+1.2°C). As reported by Lefkaditou and Kallianiotis (2006), most females of this species collect near the coast at depths of a few meters during spring and early summer. Salman and Akalin (2012) reported that the limits of this species' range are closely related to sea surface temperature (SST). Therefore, appearance of this species is considered as a result of mixing effect of sea currents and increased SST. On the other hand, Caballero-Alfonso et al. (2009) suggested that the appearance of this species is related with an anomalous sea warming and high abundance of jellyfish in the North-east Atlantic and populations could be displaced temporarily in higher latitudes when anomalous water warming occurs.  Strong currents, the Tsushima Warm Current, move toward the north-east in the East Sea/Sea of Japan from the East China Sea around Korea. (Isobe 1999). In addition, annual average of SST in the East Sea/Sea of Japan of Korea was clearly increased during 41years from 1969 to 2008 and detected increase of +1.39°C (Seong et al. 2010). In this way, the occurrence of O. tuberculata in the East Sea/Sea of Japan could be considered as evidence of the changes in species distribution of sea surface. However, a more detailed observation is required to investigate whether this phenomenon is temporary or permanent.

Disclosure statement
The authors declare that they have no potential conflicts of interest.

Funding
This work was supported by a grant from the National Institute of Fisheries Science of Korea (R2017030).