First genetic validation and diagnosis of the short-finned squid species of the genus Illex (Cephalopoda: Ommastrephidae)

Squids of the genus Illex are representative of the family Ommastrephidae and account for 65% of the world’s cephalopod captures. Illex is formed by four taxa distributed throughout the Atlantic Ocean (I. ar-gentinus, I. coindetii, I. illecebrosus and I. oxygonius), whose identification and phylogenetic relationships based on morphological characters remain controver-sial. Thirtyseven enzyme-coding loci were analysed in 230 individuals from seven populations of Illex and ten specimens of Todaropsis eblanae, which were used as the outgroup. Two to four enzyme loci (ALPDH*, IDHP-1*, MEP* and SOD*) were diagnostic among Illex species depending on the species-pair comparison. Individuals morphologically identified as I. oxygonius were also found genetically distinct, which proves the taxonomic validity of this species. No significant intra-specific genetic heterogeneity was detected within Illex argentinus, I. coindetii and I. illecebrosus (Mean GST= 0.011, 0.003, 0.017, respectively). I. illecebrosus and I. oxygonius were shown as sister species with a close relationship to I. argentinus, whereas I. coindetii formed a different lineage. P. Martınez (&) Æ A. Sanjuan Xenetica Evolutiva Molecular, Facultade de Ciencias-Bioloxıa, Universidade de Vigo, 36200 Spain E-mail: [email protected]. M. Perez-Losada Department of Integrative Biology, Brigham Young University, WIDB 401, Provo, Utah, 84602-5181, USA A. Guerra Ecobiomar, Instituto de Investigaciones Marinas (IIM), CSIC, Eduardo Cabello 6, 36208 Vigo, Spain Current address: P. Martınez Instituto Nacional de Toxicologıa, Luis Cabrera 9, 28002 Madrid, Spain Introduction Cephalopods have come to constitute one of the top invertebrate fisheries with catches of about 2.9 million tons per year (FAO 1998). Among them, the short-fin-ned squids of the genus Illex Steenstrup 1880 (Ommastrephidae, Illicinae) account for about 65% of the world’s commercial cephalopod catches (Caddy 1995). At present, the genus is considered to be constituted by four species: Illex argentinus (Castellanos 1960), I. coindetii (Verany 1839), I. illecebrosus (Le Sueur 1821) and I. oxygonius Roper, Lu and Mangold 1969, which are distributed throughout the Atlantic Ocean (Roper and Mangold 1998). I. argentinus inhabits the southwest Atlantic, I. coindetii is found along the eastern Atlantic and Mediterranean Sea, as well as in the Gulf of Mexico and Caribbean Sea, while I. illecebrosus lives along the northwestern Atlantic and I. oxygonius off the eastern USA, South to the Mexican Gulf (Lu 1973; Roper et al. 1998) (Fig. 1). Therefore, Illex coindetii, I. illecebrosus and I. oxygonius are sympatric in the northwestern Atlantic Ocean. The morphological similarity of these three species, their overlapping distributions and the disjunctive occurrence of I. coindetii on opposite sides of the Atlantic has led to problems of species identification, which in turn are an impediment for understanding Illex systematics (Roper and Mangold 1998; Roper et al. 1998). Previous morphometric and meristic analyses (e.g. Roper et al. 1998) have not been able to consistently identify all of the Illex species because such characters are influenced by sex, age, growth or environment so that variability within and among taxa is high and overlapping measurements may occur (Lu 1973; Roper and Mangold 1998). Recently, taxonomic identification on I. argentinus, I. coindetii and I. illecebrosus has been assessed based on body and beak structures through stepwise discrimination analysis, with an average of 83% correct designations (Martınez et al. 2002). However, important systematic questions such as the following are Fig. 1 Illex spp. Schematic distribution of Illex species and sampled areas (*) for Illex argentinus (IaF, Ia42), I. illecebrosus (IiN, IiWo), I. coindetii (IcW, IcM), I. oxygonius (IoF), and Todaropsis eblanae (TeW). Sample codes are as in Table 1 still unresolved within the genus Illex: (1) the existence of an I. coindetii complex of morphotypes (Roper and Mangold 1998); (2) the uncertain recognition of I. oxygonius as a valid species sensu stricto due to the difficulty of obtaining wild specimens and its morphological similarity to the other three Illex when mature (Laptik-hovsky and Nigmatullin 1993); (3) the existence of I. illecebrosus and I. coindetii morphotypes not attrib-utable to either one taxon or the other in some geo-graphical areas (Lu 1973); and (4) the possible occurrence of subspecies within I. argentinus (Carvalho et al. 1992; Thorpe et al. 1986, cited in Carvalho and Nigmatullin 1998). Molecular markers such as allozyme polymorphisms are an alternative to morphology and their application to population, taxonomic and phylogenetic studies has been largely proved (Whitmore 1990; Perez-Losada et al. 1999; Wiens 2000). Allozyme polymorphisms are con-servative characters for identifying sibling or morpho-logically indistinguishable taxa, and for providing an independent estimate of the species phylogeny (Ayala 1983; Wiens 2000). Cephalopods seem to contain lower levels of allozyme variability than other invertebrates (Sanjuan et al. 1996; Carvalho and Nigmatullin 1998; Perez-Losada et al. 1999), which may make the intra-specific analysis of their populations difficult (Nevo et al. 1984; Ward et al. 1992). However, complex population structures and even cryptic taxa have been inferred using allozyme polymorphisms (e.g. Brierley et al. 1993; Yeatman and Benzie 1994; Perez-Losada et al. 1999). At present, few genetic studies have been carried out within the genus Illex (Carvalho and Nigmatullin 1998; Jerez et al. 1998; Adcock et al. 1999, Martınez et al. 2005) and none of them look at the four species combined. Hence, Illex species identification and evolutionary relation-ships as well as their intraspecific population structure remain controversial. Therefore, the main aims of this study were to genetically characterize the four Illex taxa using allozyme polymorphisms, to validate the taxo-nomic status of the presumptive I. oxygonius, and to infer the phylogenetic relationships within the genus. Materials and methods