Capeorchestia capensis

Capeorchestia capensis

Capeorchestia capensis is a widely distributed sand-hopper along the South African coasts (Fig 13), encompassing at least three major biogeographic regions (Branch et al. 2008, Baldanzi et al. 2013). The historical spatial distribution of C. capensis ranged from Alexander Bay (Northern Cape, South Africa) to Durban (Qwa- Zulu Natal, South Africa) (Branch et al. 2008). Recently, a fine scale study (Baldanzi et al. 2013) slightly modified the extent of the spatial distribution of C. capensis (about 2500 km of coastline), which now ranges from Port Nolloth (Northern Cape, South Africa) to Port St Johns (Eastern Cape, South Africa). The same study reported differences in sizes between males and females, with the latter being smaller than the former. The abundance of C. capensis was found to be site-specific, subject to influence of the morphodynamic state of the shore and the substrate temperature (Baldanzi et al. 2013).

Among talitrids, the ecological category of sand-hoppers is usually found in the inter- and supratidal zone of a shore, burrowing into moist sand during the day, avoiding the stresses of heat and desiccation (Williams 1995, Morritt 1998) and emerging at night, when the air temperature is cooler and the risk of predation is reduced (Marsden 1991a, b; Poulin & Latham 2002). Capeorchestia capensis fits well under such category as it burrows during the day and migrates across-shore during the night (Miur 1954, Van Senus 1988, Baldanzi pers obs). Nonetheless, C. capensis also shows higher densities towards the dune base or even into the dune slacks rather than the supralittoral (Van Senus & McLachlan 1988). Hesp & McLachlan (2000) found populations of C. capensis living among and utilizing nabkha, small, discrete dune hummocks formed by two plant species, Arctotheca populifolia and Gazania rigens in the Alexander dune field (a 40 km wide coastal system located in the South Coast of the county). In Gansbaai, (Western Cape, South Africa) C. capensis mainly burrows underneath kelp, but it also found within terrestrial plants which have colonised areas close to the beach slope (Baldanzi, pers. obs).

The reproductive ecology of C. capensis is known to be similar to other species of talitrids (Van Senus 1988). Ovigerous females of C. capensis can be found throughout the year, however a study of two populations from the Cape region and the east coast, respectively, found two major peaks of abundance in summer and winter (Van Senus 1988). Egg developmental time, size and number of eggs vary between eastern and western populations (Van Senus 1988), according to the size of the animals (Muir 1954, Van Senus 1988) and the substrate temperature (Baldanzi et al. 2015a). Temperature is negatively related with the developmental time of eggs and juveniles within the brood. Laboratory experiments reported that low incubation temperatures are associated with longer developmental time for eggs (Baldanzi, unpublished data). Also, incubation temperature has an effect on the maternal investment of C. capensis, inducing females to produce larger, but less dense eggs at lower temperature (Baldanzi et al., 2015a).

The ecological physiology of C. capensis has been investigated in relation to temperature. Populations of Capeorchestia capensis showed intraspecific differences in metabolic responses to increasing and decreasing temperature, and this was likely related to changes in variability and predictability of environmental temperatures between the western and eastern limits of its distribution (Baldanzi et al. 2015b). Such physiological plasticity among populations has important evolutionary implications and could be a key factor allowing C. capensis to exhibit a widespread distribution along South African coasts.

The feeding ecology of C. capensis is poorly understood. Although C. capensis is mainly found underneath macroalgae of different origin (both marine and riverine) its feeding habit is not clear and needs further investigation. A study conducted on the diet of C. capensis by Porri et al. (2011) using stable isotope analysis, found no trophic link between sand-hoppers and the detritus underneath which animals were found. The authors suggested an opportunistic feeding behaviour for C. capensis. During samplings in 2010, 2012 and 2014, however, individuals of C. capensis have been observed crawling and actively feeding on kelp stranded on the shore of Betty’s Bay, in the Western Cape of South Africa (SB pers. obs). This observation, confirms the high degree of uncertainty about the feeding ecology of this species, keeping open the question regarding the relationship between C. capensis and the wrack-beds that they burrow underneath: source of food, shelter or both?