Tigriopus brevicornis

McAllen, Rob, 2001, Hanging on in there - position maintenance by the high-shore rockpool harpacticoid copepod Tigriopus brevicornis, Journal of Natural History 35 (12), pp. 1821-1829 : 1827

The results of the scanning electron microscopy, video analysis and ¯ow-through chamber experimentation, indicate that the micro-morphology of T. brevicornis has major role in position maintenance on the high-shore rockpool substratum. This comparative study of the high rocky shore-dwelling T. brevicornis and the subtidal sand dweller H. X exus has shown how morphology varies with habitat. The sharp, stout, forward-facing claws (with micro-serrations) on the ®rst swimming legs of T. brevicornis enable them to cling on to the highly irregular microscopic topography sandstone. By using their antennules and antennae in combination with the ®rst swimming legs, they are able to anchor down the prosome. Lewis et al. (1998) suggested that the ®rst pair of swimming legs in T. californicus was used for movement, holding on to surfaces and for removing and grasping benthic pennate diatoms. They suggested that the ®rst pair of swimming legs were not used primarily for swimming. The present study tends to agree with these ®ndings. The large number of micro-serrations present on the spines of the second, third and fourth pairs of appendages allows the swimming legs to provide resistance to the water during the backstroke and so facilitate swimming. However, this highly serrated nature also permits the animal to counter wave action by placing parts of these appendages within the many pits seen in the sandstone micrographs as additional body anchorage (®gures 2D, 4B). In combination with this, T. brevicornis is able to downturn its caudal furcae and use the setules and hooks on the principal terminal seta to anchor on to the sandstone (®gure 3A). It has also been observed that T. brevicornis is able to extend its second, third and fourth pairs of appendages out from the body when on the substratum, which provides extra bracing ability addition to the gripping action (R. McAllen and T. KuÈnneman, personal observations). This increases its resistance to dislodgement by wave action. In comparison with T. brevicornis , H. X exus has a quite diOEerent detailed morphology. The ®rst pair of swimming legs does not terminate in sharp claws, but instead has heavily ornamented teeth that form a brush border. This is probably advantageou s to the sand-dwelling copepod, enabling it to move sand grains out of the way while moving. The absence of hooks on the principal terminal seta of the caudal furcae of H. X exus is unsurprising. The sand habitat is highly unstable and trying to remain stationary during a high tide would appear futile. In addition, such spines on the caudal furcae may possibly hinder progress. The animal can freely burrow into the sand and hence there is no need to anchor its body. Tigriopus brevicornis cannot burrow when on solid sandstone and hence needs its serrated morphological features to avoid being washed out during high tides. The ability of T. brevicornis to burrow into any loose sandstone or sediment that may be present when rockpools dry out has recently been suggested by McAllen 1999), who observed the presence of many T. brevicornis individuals under loose sediment in completely dehydrated rockpools. The present study con®rms these preliminary observations. The genus Tigriopus is extremely euryhaline withstanding salinities of 4±200 psu in its high-shore rockpool environment (Fraser, 1936; Ranade, 1957; Vittor, 1971; McAllen et al., 1998). However, members of the genus are not truly desiccation-resistan t (Dybdahl, 1994; Davenport and MacAlister, 1997). Hence the need for Tigriopus to ®nd any available moisture to prevent the lethal eOEects of desiccation. The present study shows how the copepod primarily uses its antennules and antennae to move loose sediment and then employs its swimming legs to enable the whole body to be buried within the sediment. This is highly advantageous to brevicornis when rockpool water evaporates and the pool dries up. The sandstone