Introduction
The life cycles of pelagic cephalopods and their seasonal distribution are generally not well understood. For many species, data on the early-life stages are scarce or missing, and the identification of paralarvae is difficult. Lists of the Southern Ocean squid fauna have been published by Rodhouse (Reference Rodhouse1989) and Collins & Rodhouse (Reference Collins and Rodhouse2006). Data on the early stages of Galiteuthis glacialis (Chun, 1906) have been reported for the eastern sector of the Antarctic (Rodhouse & Clarke Reference Rodhouse and Clarke1986) but distribution data are mainly from summer captures with only a few from winter, mostly from the Weddell Sea.
Because of its oceanographic features, the Lazarev Sea is considered a gateway to the Weddell gyre and a source of the Weddell Sea fauna (Beckmann et al. Reference Beckmann, Hellmer and Timmermann1999). Many surveys have been conducted to study krill in the Lazarev Sea and other adjacent seas in the Southern Ocean. Paralarval and juvenile cephalopods are a regular bycatch taken with krill. The present paper gives an overview of the young cephalopods collected during the summer of 2005 and the winter of 2006 in the Lazarev Sea.
Methods and results
The material was collected during two LAKRIS (Lazarev Sea Krill Study) cruises with RV Polarstern: ANT XXIII-2 (summer, 19 November 2005–12 January 2006), and ANT XXIII-6 (winter, 17 June–21 August 2006), as a bycatch of the standard krill sampling gears: a RMT 8 net (Baker et al. Reference Baker, Clarke and Harris1973), and a RMT 8 Multinet (rectangular midwater trawls with an 8 m2 opening and a mesh size 1 mm at the cod end). The depth range was 0–4682 m. All the sampled stations (55 winter, 73 summer) are situated close to the Greenwich meridian (9°W to 6°E) (Fig. 1).
Fig. 1 Map of the study area with selected stations in the Lazarev Sea. Dots (●) represent winter stations, triangles (▴) the summer stations.
The cephalopods were removed immediately from the nets and fixed in 70% ethanol. Identification and morphometric measurements were conducted at the Zoological Museum Hamburg (ZMH). Measurements and counts followed Roper & Voss (Reference Roper and Voss1983). Abbreviations: ML, dorsal mantle length. The map of the sampling area was created using Ocean Data View (http://www.awi-bremerhaven.de/GEO/ODV, accessed 26 March 2009).
A total of 458 squids were captured: 194 during summer and 264 during winter. Seven taxa were distinguished during summer: Galiteuthis glacialis (Chun, 1906), Mesonychoteuthis hamiltoni Robson, 1925, Alluroteuthis antarcticus Odhner, 1923, Neoteuthidae sp., Onychoteuthidae sp., Cranchiidae sp. and one unidentified squid; only the former four taxa were taken during winter. The presence of tubercles on the nuchal region and the funnel cartilage are diagnostic features that appear in very young Cranchiidae, enabling identification of G. glacialis and M. hamiltoni of ML 4 mm and upwards (McSweeny Reference McSweeny1970, Reference McSweeny1978). The most abundant species were G. glacialis (63 (summer) and 253 (winter) specimens), M. hamiltoni (76 and 6) and A. antarcticus (37 and 3). The abundance varied strongly between seasons (Table I).
Table I Total number of individuals of squid identified from the Lazarev Sea in summer (129 trawls) and winter (64 trawls).
No difference was found on comparing the occurrence of G. glacialis in summer and winter. The average body size is smaller in winter (16.7 mm) than in summer (21.7 mm). Galiteuthis glacialis and M. hamiltoni were both quite abundant in summer and co-occurred across the whole sampled area. However, most of the young individuals of G. glacialis were of much larger body size (mean 21.7 mm ML) than those of M. hamiltoni (mean 10.3 mm ML). The smallest individuals of G. glacialis (ML 2–6 mm) were captured in summer as well as in winter. The smallest M. hamiltoni (ML 3–5 mm) and A. antarcticus (ML 5–7 mm) individuals were found in summer. The largest M. hamiltoni (ML 93 mm) was captured at 500 m in winter.
Discussion
This is the first study to compare the winter and summer composition of the paralarvae and juvenile squids from the Southern Ocean. Nearly all species occurred in higher numbers in summer, except for Galiteuthis glacialis, which were most abundant in winter. This could indicate a distributional pattern that helps to avoid competitive pressure between species. The presence of juvenile and young G. glacialis under the ice and near the surface has not been reported previously and demonstrates a wider distribution of this species in the water column (as assumed previously Nesis et al. Reference Nesis, Nigmatullin and Nikitina1998); it is not restricted to the bathypelagic region, at least in the winter.
The smallest paralarva of G. glacilis (ML 3 mm) is about the same size as the ripe egg, 3.0 mm in diameter (Laptikhovsky & Arkhipkin Reference Laptikhovsky and Arkhipkin2003). The smallest M. hamiltoni was 4 mm ML; no data are available for the size of the ripe egg of this species. The largest juvenile was found at depths around 500 m, suggesting that there is a migration to deeper water early in development.
The relative tentacle length proposed by Rodhouse & Clarke (Reference Rodhouse and Clarke1985) was estimated and there were great differences between the measured and the calculated ML of the specimens, leading to the conclusion that the proposed relation is not suitable for the present collection of squid.
Some specimens of adult squids of the species Kondakovia longimana Filippova, 1972 and Slosarczykovia circumantarctica Lipinski, 2001 were captured under the ice using a surface-and-under-ice trawl (Hauke Flores, personal communication 2010). The absence of Psychroteuthis glacialis Thiele, 1920 and Brachioteuthis sp. is noted. These species are present in other regions of the Antarctic, such as Prydz Bay (Filippova & Pakhomov Reference Filippova and Pakhomov1994, Hauke Flores, personal communication 2009), and there is no apparent explanation for their absence from samples during the present study.
Acknowledgements
The author thanks Volker Siegel, Ute Mühlenhardt-Siegel, Laura Würzberg, the crew of the Polarstern, the participants of the Lakris expeditions, and to Ian Gleadall for helpful comments.
