Shell utilization and morphometries of the hermit crab Diogenes brevirostris Stimpson

Fecundity, shell utilization, and crab and associated shell morphometries were investigated for the hermit crab Diogenes brevirostris collected from three intertidal sites in the eastern Cape. The relationship between crab fresh mass and egg number was linear. D. brevirostris was found to occupy 33 gastropod shell species of which Bumupena predominated on the rocky shore (B. lagenaria, 35,9%; B. cincta, 17,5%; B. pubescens, 16,5%) while Bullia was more commonly used near sandy shores (S. armulata, 4%; B. digitalis, 2,7%; B. rhodostoma, 2,5%). Although Oxystele is common, it was hardly used by D. brevirostris (O. sinensis, 2,7%). Whereas shell type (Bumupena lagenaria, B. cincta, B. pubescens and Bullia rhodostoma) was found to have no significant influence on the crab length/mass relationships, significant differences were found between elevations but not slopes of the fresh mass/volume relationships (p < 0,05). B. rhodostoma was significantly different from all three species of Bumupena suggesting that small D. brevirostris occupy Bullia shells of a larger volume than a similar-sized crab occupying a Bumupena shell to compensate for the increased ‘dead space’ incurred with a high spire. Overall, there did not appear to be any selection towards low-spired (17 species, n = 264) or high-spired (15 species, n = 262) shell species, but there was when compared for each site (p < 0,01), indicating shell availability to be important. A good size correlation was found between 0. brevirostris and its associated shell suggesting that selection of available material and/or mutualism may occur

On the occurrence of Stenopus tenuirostris De Man, 1888 in Natal waters

The stenopodidean shrimp Stenopus tenuirostris De Man, 1888 is reported from Natal waters for the first time. This record represents a considerable southward extension of this uncommon tropical species for the African sub-region

Seasonal abundance, growth and production of Palaemon pacificus (Stimpson) in eastern Cape tidal pools

The shrimp Palaemon pacificus was sampled monthly from six tidal pools for two years. The shrimp populations varied seasonally with peak numbers and biomass found in summer (December to March) following a major influx of juveniles in November. These recruits were composed of discrete modal groups (bimodal) which spread and became less obvious by April (polymodal). Females had a greater mean size range than males. The sex ratio varied temporally and overall male: female ratios varied from 1:1,66 to 1:2,96. Average shrimp total length was significantly correlated to the average and maximum pool depth and to log pool volume while the average number of cohorts per year was found to be negatively correlated to these parameters. In smaller pools the shrimp disappeared above 20-30 mm but remained up to reproductive age in the deeper pools. Average residence time was positively correlated to pool depth, but not to log volume. Mean summer growth rates were almost double those in winter and were combined in the von Bertalanffy growth equation Lt = 64,51 (1-e-0,08902(t-0,1001)). P. pacificus   reaches first maturity from egg at six months for males and 12 months 2 for females. Mean annual production varied between pools from 0,22 to 2,0 g m -2 and biomass from 0,06 to 0,5 gm-2. The highest P/B ratio of 4,24 was obtained from the smallest pool which had a low biomass but high productivity and shrimp turnover

Tidal exchange of two decapod larvae Palaemon pacificus (Caridea) and Upogebia africana (Thalassinidae) between the Swartkops River estuary and adjacent coastal waters

Two 24-h plankton and seine-netting studies were carried out in the Swartkops estuary mouth, one during a spring-tide, the other on a neap-tide in order to determine larval movements of the prawns Palaemon pacificus and Upogebia africana in and out of the estuary. Two size classes of the former were caught, namely zoea 6 and adults. No zoeae 1, 2, 3, 4 or 5, post-larvae or juveniles were caught. Zoea 1 of U. africana are flushed out of the estuary into Algoa Bay on the nocturnal ebb-tide with very few returning on the subsequent flood-tide. Net fluxes were 97,1% and 99,9%. Larval life is then spent at sea after which the pre-postlarvae invade the estuary to reach the estuarine nursery areas. For P. pacificus this was during zoea 6, mainly on the nocturnal flood-tides. Net fluxes of P. pacificus zoea 6 were 95,8% and 100%. Most larval movement occurred at peak velocities as they utilize the tidal currents in dispersion and recruitment. Some P. pacificus zoea 6 are washed out again on ebb-tides, but lateral movement to the banks during slack-tide appears to be a mechanism to prevent this occurring. P. pacificus adult movement in the mouth region was better cor-related with nocturnal activity than with tidal movements