Physical determinants of the distribution and abundance of the burrowing ocypodid crab Paratylodiplax blephariskios Stebbing in the St. Lucia and Mhlathuze estuaries, KwaZulu-Natal, South Africa

Abstract

The burrowing ocypodid crab, Paratylodiplax blephariskios, is endemic to the southeast coast of southern Africa where it forms an important component of the estuarine benthic fauna in muddy substrata. Two estuaries in KwaZulu-Natal, namely St. Lucia and the Mhlathuze, were chosen to investigate the influence of temperature, salinity, distance from the mouth and nature of the substratum (median phi value, sorting coefficient and organic content) on the distribution and abundance of the crab. Multiple and stepwise regression showed that particle size, sediment sorting coefficient and distance from the mouth were significantly related to crab abundance in both estuaries, while both temperature, salinity and the organic content of the substratum had little effect. At St. Lucia, distance from the mouth, rather than the nature of the substratum appeared to be the main determinant influencing crab distribution and abundance. However, at the Mhlathuze P. blephariskios was only recorded from muddy sites located further from the mouth. It was concluded that a combination of median particle size and sorting coefficient best explained crab distributions within the two estuaries. The absence of this crab species from muddy substrata in the lake compartments of the St. Lucia system suggests that its dispersion is tidal here, which leads to higher densities in the lower Narrows region near the mouth.


Keywords: Paratylodiplax blephariskios; Ocypodidae; Brachyura; abundance; distribution; physical determinants; temperature; substratum; St. Lucia estuary; Mhlathuze estuary; KwaZulu-Natal


(Afr J Aqua Sci: 2000 25: 23-32)