oa Annals of the Natal Museum - On the ecology and habitat adaptations of two intertidal spiders, Desis formidabilis (O.P. Cambridge) and Amaurobioides africanus Hewitt, at "The Island" (Kommetjie, Cape Peninsula), with notes on the occurrence of two other spiders

Volume 20, Issue 1
  • ISSN : 0304-0798



Four species of spiders, namely Desis formidabilis (Cambridge), 1890 (Amaurobiidae: Desinae); Amaurobioides africanus Hewitt, 1917 (Miturgidae: Amaurobioidinae); Muizenbergia abraham; Hewitt, 1915 (Hahniidae: Hahniinae) and Erigonopsis littoralis Hewitt, 1915(Erigonidae), were found permanently inhabiting the intertidal zone of the rocky shores of "The Island," Kommetjie on the western coast of the Cape Peninsula, South Africa. The intertidal zone of the "The Island" was surveyed and a map drawn to show the distribution of the four species. The habitats are compared. D. formidabilis was found to inhabit areas of the rocky shore situated between Low Water Neap and High Water Neap Tides, and A. africanus areas between High Water Neaps and High Water Springs. Both species were found in very large numbers. M. abrahami and E. littoralis were both sparcely distributed in areas inhabited by D. formidabilis. Physical factors, behavioural adaptations and biotic factors were investigated in order to discover whether any of these are responsible for the sharp delimitation between the territories of D. formidabilis and A. africanus. Wave action was found to strongly determine the location of nest sites in both species but to be of no importance in their interspecific distribution. Selective choice of nest material, depending on the nature of mollusc shells used and the amount of exposure to wave action, is demonstrated for both. Both species are shown through measurements to make use of an equally effective physical gill and to be able to sustain prolonged immersion. Anatomical and histological investigations of the body surface, booklungs and spiracles have failed to prove the existence of a plastron. They are shown to be able to withstand an extreme condition of desiccation, and there is an indication that D. formidabilis is more sus­ceptible to temperatures above 20°C. The relative humidity of their micro-habitat is shown not to vary to any significant degree. Both spiders are nocturnal and have adapted their periods of activity to tidal rhythms. The eggs of D. formidabilis were found to be parasitised by a Scelionid wasp belonging to a new genus and species, namely Echthrodesis lamorali Masner, 1968. A new species of Mesostigmatid mite belonging to the genus Veigeiia (Veigeiidae) was found living in and around the nests of D. formidabilis and A. africanus. In D. formidabilis and A. africanus intraspecific competition for nesting sites only becomes a problem when prospective sites are scarce. Density of populations is governed by availability of nesting sites only and not by territorial behaviour. Interspecific competition for nesting sites is very marked on territorial fringes. Experimental evidence has shown that D. formidabilis usually outfights A. africanus, apparently due to the fact that D. formidabilis has larger chelicerae, and this appears to be one of the factors controlling the distribution of A. africanus. Part of the diet of D. formidabilis and A. africanus together with an apparent prey selectivity characteristic of each spider are descriptionbed. The latter is offered as a possible answer to the distribution of these two spiders.

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