Stephos longipes – an ice copepod
Fig. 1: Stephos longipes, female. On the right the caudal end (Abdomen) with the forked tail base (Furca). Photo: Jan Michels, Alfred Wegener Institute
Stephos longipes is a small neritic calanoid copepod that is native to waters surrounding the Antarctic continent. Copepods are crustaceans that range from less than one to twelve millimetres in size and occur in the ocean from the shallow shelf areas to the deepest depths. In most marine regions they represent both the most frequent and the most diverse species of zooplankton. In their development to becoming an adult animal they go through eleven juvenile stages (six naupliar and five copepodid stages). Most copepod species primarily feed on phytoplankton and small unicellular zooplankters. They, in turn, serve as food for many predators, e.g. fish, and thus represent a major link between the primary producers (phytoplankton) and the higher levels of the food webs.
The copepod Stephos longipes (Fig. 1) has a circum-Antarctic distribution. Its lifecycle is closely connected to sea ice, which is mostly annual in Antarctica and thus undergoes substantial seasonal fluctuations. It melts in the summer and forms again in the autumn.
For a long time sea ice was regarded as hostile to life. In the last two decades, however, the great significance of sea ice for the biology of the polar seas has become evident (D. N. Thomas, G. S. Dieckmann (Hrsg.): Sea ice. Wiley-Blackwell. Oxford 2010).
The brine channels that form when seawater freezes represents a habitat for a large number of organisms, including diatoms, ciliates and turbellarians, and S. longipes as well as other small copepod species (which, however, belong to the family of Harpacticoidae and not to the Calanoidae, like S. longipes).
The hollow spaces in the ice filled with brine are extreme habitats. In winter the temperatures in the channels of the upper ice layers can drop to –15°C and the salt concentration may rise to over 100 PSU (PSU = practical salinity units, corresponding to per mil scale). The low temperatures and high salt concentrations require special adaptation on the part of sea ice organisms so it is not surprising that the species diversity of the animals living in the sea ice is considerably lower than in the water column or on the bottom. However, the system of hollow spaces within the ice floes also offers significant advantages over life in the water column. First of all, the light supply within the ice, though very low, is still higher and more stable than in the water column below it. This means substantial algal biomasses can form in the ice. Animals thus find food in the ice as well as protection in the channels and pores against larger predators that cannot enter the small brine channels.
Fig. 2: Schematic diagram of lifecycle of S. longipes
In winter and spring, a period with high ice cover and very low phytoplankton concentrations in the water column, the small naupliar stages of S. longipes predominantly dwell in the sea ice. Males and females, by contrast, primarily occur in low numbers in the water layer directly underneath the ice. After the ice melts in summer, S. longipes occurs in greater numbers in the upper water layers where it feeds and grows. In autumn, when the ice forms anew, S. longipes has two distribution focal points: the sea ice is home to a young population with nauplii whereas older copepodid stages live in the water column – though not on the surface as in summer, but in deeper water layers down to immediately above the seafloor (Fig. 2).
The lifecycle of S. longipes is therefore characterised by a seasonal change between ice cover and open water. Climate changes, such as the warming of the atmosphere and thus the decline in sea ice, will have far-reaching impacts on the sea ice dwellers and consequently on the lifecycle of S. longipes as well. (S. B. Schnack-Schiel et al., Progress in Oceanography 36, 45, 1995).
References
Gradinger, R. (2002). Sea ice microorganisms. In: Encyclopedia of Environmental Microbiology (Ed. G.E. Bitten), pp. 2833-2844. Wiley & Sons.
Schnack-Schiel, S .B., Thomas, D., Dieckmann, G. S., Eiken, H., Gradinger, R., Spindler, M., Weissenberger, J., Mizdalski, E. & Beyer, K. (1995). Life cycle strategy of the Antarctic calanoid copepod Stephos longipes. Progress in Oceanography 36: 45-75.
Thomas, D.N., Dieckmann, G.S. (Hrsg). Sea Ice, 2nd edition. Wiley-Blackwell, Oxford (2010).