Ecology of the Antarctic octopus Pareledone from the Scotia Sea
The octopodid species, Pareledone turqueti and Pareledone polymorpha are caught in research and commercial trawls on the shelf around South Georgia. Specimens from a survey in Cumberland Bay in 1987 and groundfish surveys in 1989, 1991 and 1994 were examined. Although superficially similar in appearance the species are discernible by differing hectocotylus and beak morphology. The hectocotylus is larger in P. polymorpha and is long and pointed with deep transverse grooves. P. turqueti has a short, blunt hectocotylus with a smooth inner surface. In P. turqueti the lower beak is large with a curved blunt hood, and in P. polymorpha the beak has a significantly smaller hood that is flat and pointed (P<0.001), with a slightly upturned rostral tip in small specimens. The size and shape of the radula also differs between species; it is broader with blunt teeth with a multicuspid rachidian tooth in P. turqueti, while in P. polymorpha, the radula is narrow with sharp teeth and a unicuspid rachidian tooth. There are also interspecific differences in the relative size of the posterior salivary glands and in the number of suckers on the arms, with P. polymorpha having larger salivary glands (P<0.001), and a higher number of suckers per arm (P < 0.001) than P. turqueti. These differences suggest that these sympatric species occupy discrete trophic niches. Crustacean and polychaete prey are important components in the diet of both species. Shelled bivalve and gastropod prey, present in 62 and 32 % of P. turqueti specimens, were absent from all P. polymorpha. The fecundity of both P. turqueti and P. polymorpha is low, with a mean of 52 developing eggs in mature P. turqueti and a significantly higher mean of 69 in P. polymorpha (P < 0.001). Mature eggs are large in both species, about 16 mm along the major axis, and have short stalks, suggesting that they are laid separately. In maturing females there is a bimodal distribution of egg length, possibly as a result of competition for space and nutrients within the ovary, which limits the maximum number of developing eggs as they increase in size. Several spent females were identified, indicating that they had laid the majority of their eggs. The poor condition of these females, suggests that Pareledone spp. are semelparous, with spawning occurring shortly before death. Spermatophores in both species are large, up to 70 mm long. Although P. turqueti and P. polymorpha have overlapping geographic ranges, a significant difference is observed between the mean depth distribution of the species. During the 1994 survey P. polymorpha had a mean depth of 211 m and P. turqueti, a mean depth of 157 m (P < 0.001).