Life Cycles

Stephos longipes, Paralabidocera antarctica and Drescheriella glacialis are the three most dominant copepods in Antarctic sea ice (Thomas et al. 2008). They all rely on ice algae rather than phytoplankton in the water as their food source (Tanimura et al. 1996). Despite all inhabiting the same habitat areas and eating the same foods, these three very similar metazoans have evolved very different life history strategies for survival in an extreme environment.

Stephos longipes is a calanoid copepod that has adapted its whole life cycle (of one year) to fit in with the seasonal cycle of ice cover in the Antarctic (Fig.8). Sea ice is most predominant in late winter/early spring, during this time S. longipes is rarely found in the water column. Adults and nauplii are found in high numbers in the sea ice, as the ice starts to melt, large numbers are found at the ice-water interface and in the rotting ice at the bottom of the ice pack. In summer there is minimal /no ice cover and the copepod can be found in the surface layers of the water column where the nauplii develop into adults. As autumn approaches new ice begins to form and S. longipes is most abundant in the mid water, as the sea ice accumulates the copepods are found at the ice-water interface and in the sea ice, these tend to be nauplii and juveniles, the adult copepods are found deeper in the water column where the migrate to overwinter (Schnack-Schiel et al. 1995).

Figure 8: Life Cycle of Stephos longipes in the Antarctic. Adapted from Tanimura et al. (1996)

Paralabidocera antarctica and Drescheriella glacialis are two Antarctic copepods that are very similar but exhibit very different ice-associated life history cycles (Fig.9)

P. antarctica is a calanoid copepod that spends most of its life at the ice-water interface. It is a slow growing copepod with a year long life-cycle; this begins when young nauplii (offspring from the previous generation) enter the sea ice around April. As the ice starts to grow they develop into the later naupliar stages until the ice thickens, at which point they remain in the naupliar stage at the bottom of the ice pack. When ice growth stops (during October) population development begins again and the nauplier rapidly develop into early stage copepodites. Through November and December the population gradually shifts its habitat from the sea ice to the water immediately below. As the underside of the ice begins to melt, maturation and mating takes place. It is thought that eggs are spawned during the late summer when the sea ice is at its thinnest and that adults may die immediately after spawning (Tanimura et al. 1996).

Figure 9: Life Cycles of Paralabidocera antarctica and Drescheriella glacialis in the Antarctic. Adapted from Tanimura et al. (1996).

D. Glacialis differs from P. antarctica as it is a fast growing harpacticoid copepod with two or three generation a year (it is an R-strategist). The entire life cycle is carried out in the sea ice and the main reproductive season is winter, but reproduction does occur throughout the year with no resting stages (Tanimura et al. 1996). This allows the copepod to quickly colonize sea ice patches that are unpredictable in formation (Swadling, 2001).

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