Aspects of the Cryobiology of the Intertidal Harpacticoid Copepod Tigriopus brevicornis (O. F. Müller)

Cold-resistance studies of marine invertebrates have concentrated on intertidal sedentary organisms, which are often subjected to subzero air temperatures in winter. Mobile rock pool inhabitants have been rarely studied because such habitats are thought to buffer environmental variation. However, it is not uncommon for small upper-shore rock pools (∼2 by 1 cm) to become completely frozen. Such supralittoral habitats are subject to extreme physicochemical fluctuations especially in salinity (0 to 300‰) and temperature (-1 to +32°C) due to evaporation and dilution. The dominant invertebrate in such habitats is the harpacticoid copepod Tigriopus brevicornis. Aspects of the cryobiology of T. brevicornis were investigated using differential scanning calorimetry (DSC). Thermograms obtained from DSC allowed determinations of freeze-onset (supercooling point, SCP), melt-onset, and melt-peak (melting point, MP) temperatures, together with estimation of the proportion of water freezing in the samples. The effects of acclimation salinity, temperature, starvation, and reproductive state on these cryobiological parameters were investigated. Acclimation to increasing salinity depressed the SCP, with the highest salinity (70‰) producing the lowest SCP, melt-onset, and MP temperatures at -27.5, -15.2, and -9.5°C respectively. The highest acclimation temperature (20°C) produced the lowest SCP (-23.4°C). Starvation significantly increased the SCP, melt-onset, and MP temperatures in comparison to fed individuals acclimated to the same salinity. The presence of eggs or ovaries in individual copepods elevated the SCP compared to nongravid females and males. LT₅₀ studies showed that acclimation to high salinity improved the ability of T. brevicornis to survive in frozen seawater. Seventy parts per thousand acclimated individuals had an LT₅₀ of 64.9 h compared with just 1.4 h for 5‰ acclimated individuals in frozen seawater at -5°C. The study shows that the cold-resistance capabilities of T. brevicornis can be affected by several different factors, and the link between the osmoconforming nature of this species and its cold resistance is discussed.