Tigriopus brevicornis

This species of harpacticoid copepod is found in high shore splash pools on coastlines ranging from Portugal in the south to Iceland and Nova Scotia in the north. Sometimes it can be found below the high tide mark however, like in Sweden, where it was found at depths of 10 metres in the subtidal.^[4] These splash pools occur several metres above the high tide mark, isolated from the main coastal water as a microenvironment that can vary dramatically in chemical factors such as salinity, temperature and oxygen levels over relatively short temporal scales. T. brevicornis has the ability to survive these variable environmental conditions (factors that limit predators such as fish to lower pools in the intertidal zone) and as a result is known as a euryhaline osmoconformer. Temperatures in supratidal splash pools tend to track air temperatures more closely than ocean temperatures as they are often extremely shallow (only a few cm deep to a few meters deep). Salinity also changes as the pools evaporate or fill up (from 0-150 PSU) as the pools receive freshwater inputs from rain (especially so in the West of Ireland) and saltwater from wave action during spring tides or storm surges.^[5] The orange pigment Astaxanthin is synthesised by the organism as an aid against UVAR and UVBR radiation from the sun, as rock pools can be quite exposed to desiccation. The copepod's diet of phytoplankton who are rich in Highly Unsaturated Fatty Acids (HUFAs) allow them to synthesise this protective protein, granting them tolerance to radiation year-round.^[6]

Within the pools, these relatively small (~1 mm long adult) organisms can thrive as generalist benthic foragers, feeding primarily on biofilms of phytoplankton and other microbes on the rock pool bed. They also feed on pelagic phytoplankton present in the water and on epiphytic biofilms covering the dominant rock pool alga; Enteromorpha intestinalis. Aside from a feeding platform, T. brevicornis take advantage of the algae's hollow nature and are known to dwell within the actual thallus of the green macroalga. Especially during times of desiccation, the thallus provides a moist refugium for the copepod when rock pools completely dry out. Several hundred individuals can be found in a single strand of the hollow seaweed, where they can survive weeks longer compared to natural desiccation. Even when Enteromorpha spp. aren't present in splash pools and conditions are getting too dry, the copepod can burrow down where the loose sandstone still holds moisture. These behavioural adaptations may be a key explanation in how this species can live and thrive in such an environment as extreme and as variable as the supratidal.^[7]

Part of the Harpacticoid copepods of the genus Tigriopus Norman 1868, T. brevicornis belong to the subclass Copepoda which is of high ecological importance. Copepoda is the second largest Crustacean taxa and approximately 12,000 species of copepods have been described. They are one of the dominant taxa in aquatic zooplankton communities representing 70% of the ocean’s biomass and thus the principal link between the phytoplankton and higher trophic levels. Harpacticoids also play an important role in the marine meiobenthic food web, especially as food for juvenile fish.^[8]

Supratidal splash pools are often so high on the shoreline that they can become isolated from the coastal water and neighbouring pools for long periods of time. From this, scientists have speculated whether these neighbouring pools actually have different populations of copepods, and found that some pools only metres apart may never share genes through cross breeding.^[9] The upper shore rock pool mosaic with interspersed copepod communities therefore forms a metapopulation. This is a rare occurrence in a marine system, where dispersal ability is usually less impaired.^[10][11] This extremely restricted dispersal has perplexed researchers, as phylogeographical studies have shown that despite this, distant relatives of T. brevicornis were capable of occasional long distance transport, colonising large geographic areas within the last 12 to 15 thousand years. A 1 mm long copepod somehow managed to traverse the Atlantic Ocean in the past - over 4000 kilometres.^[4]

T. brevicornis undergoes several stages of development, with 12 distinctive post-embryonic developmental stages; 6 naupliar (copepod baby) stages, 5 copepodid (teenage) stages and an adult stage.^[8] Animals are sexually dimorphic and males are usually slightly smaller than females, possessing enlarged antennules. These antennules are used to perform a characteristic mate-guarding behaviour to secure a potential mate, where they grasp females before the actual mating event. Mating pairs will stay attached for several hours, and sperm is stored in a special organ known as a spermatheca. Females are highly fecund and usually bear multiple broods of eggs developed sequentially after a single mating with brood sizes of 30–50 nauplii. Fertilised T. brevicornis females are easily identifiable by the presence of a large dark egg sac attached to their hind segment. Other Tigriopus spp. females have varying numbers of egg sacs attached posteriorly (see image below).^[12]