De Villiers, Casper J.

Nine stages and substages of post-, inter- and premoult were distinguished in the West Coast rock lobster Jasus lalandii by microscopic examination of the cuticle, epidermal retraction and setal development in the pleopods. The postmoult condition is characterized by progressive thickening of the setal walls and cuticle through to inter- moult. Premoult commences with apolysis (Stage Do), followed by setal development (Stage 01', 0(, 0,"') and culminates with cuticle deposition (Stage O2), The diagnostic features of the stages are generally similar to those of other decapod crustaceans.

A study was initiated in 1997 to assess the diversity, distribution and density of the benthic macrofauna of the soft intertidal sediments of the Knysna Estuary using the same collecting sites and methods reported by Day, Millard & Harrison in 1952. A two-way ANOVA has shown that there is no difference between species richness of belt transects reported in 1952 and 1997. A significant increase in species diversity as measured by the Shannon-Weiner Index (H') in quantitatively sampled sediments in the Zostera zone between 1952 and 1997 is due, it is argued, to intermittent increases in suspensoids in the water column, as a result of poor land-management practices in the catchments of the streams which have run into the estuary during this 50 year period.

Demonstrates that sand moisture is a factor determining the depth of burrowing in the isopod Tylos granulatus Krauss. Role of grain size and stratification; Inability of the animals to burrow in totally dry sand. Tylos granulatus, a sandy-beach isopod, prefers an environmental moisture range exceeding 3,4% but less than 13%. The depths to which the animals burrow are, at least partly, determined by the moisture gradient in the sand. They are, however, incapable of burrowing into totally dry sand. Animals alter their position in the sand in response to changes in moisture content so as to ensure exposure to suitable conditions.

The effects of different local food resources on the diets of some filipalpian stonefly nymphs (Aphanicerca spp.) were investigated. Animals from two headwater reaches, one primarily allochthonously driven and the other an open-canopied autochthonous-based system were compared. However, gut and stable carbon isotope analyses revealed no real differences in plecopteran diets between the systems, with leaf detritus being a major source of carbon, and algae contributing virtually nothing to their diets. δ15N analysis suggested that the animals obtained their nitrogen from sources other than leaf material itself, and it was hypothesized that this source was the microbial slime layer formed on decomposing leaves. It was concluded that the feeding behaviour of Plecoptera is consistent with that of shredders of leaf detritus, and as such they uphold River Continuum Concept predictions of large shredder populations being indicative of a predominance of coarse particles in headwater reaches. This study also highlighted the danger of making broad generalisations as to an organism's diet, based on the overall nature of the system, rather than on the micro- level of what may actually be available to small populations within a species.

The relationships between the standing stocks of deposit-feeding benthic invertebrates and benthic chlorophyll-a, phaeopigment and total combustible organic matter were investigated at a series of coastal lagoons and in the type of intertidal soft-sediment sites from which the lagoons originated. Across all the sites, in Norfolk, UK, an inverse relationship occurred between (a) the amounts of chlorophyll-a and of other potential food materials and (b) the degree of coverage by water. The biomass of consumers also decreased with increased water coverage, so that the lagoons supported less biomass than the adjacent high-level intertidal sites. Further, the deposit-feeder biomass supported by unit food decreased with extent of water coverage.There was no evidence of any relationship between deposit-feeder and food biomass within any single site, in spite of the study period being selected to be that in which there was maximum likelihood of competition for microphytobenthic food. Whilst chlorophyll concentrations may set the maximum achievable level of consumer biomass at these sites, including in the deeper lagoons setting very low potential maximum population densities, the seasonal abundance patterns of the deposit feeders appear to be determined by other factor(s). [ABSTRACT FROM PUBLISHER]

Feeding (filtration) activity and filtration rate were determined in Solen cylindraceus subjected to a variety of natural seston concentrations (5–500 mg 1−1). At concentrations 5–100 mg l−1 animals spent approximately 90% of the submerged periods filtering (filtration rate ca. 20 ml min−1). Above these seston concentrations filtration activity declined, until at 500 mg 1−1 animals were active for only 30% of the time (filtration rate ca. 10 ml min−1). At seston concentrations below 50 mg 1−1 pseudofaecal production was minimal (<10 mg h−1). Between 100 and 500 mg 1−1 the amount of pseudofaeces produced increased significantly to about 45 mg h−1. The formation and dissolution of the crystalline style and activity of the digestive gland was investigated in animals under natural conditions. The volume of the crystalline style was constant at all phases of the tidal cycle, and it was estimated that the style was completely replaced every 48 h. However, the rate of style production was greater during flood tides. Histological sections of the digestive glands did not reveal any change in the appearance of the digestive cells.

Male and female genital morphology of the spirostreptid millipede Orthoporus pyrhocephalus was examined by conventional and scanning electron microscopy techniques. The relative syncopulatory positions of the genitalia were established through dissections of pairs in copula. Male gonopods bear scoops and regions of pitted spines. both of which are similar in form to morphological structures implicated in insect sperm displacement. Consideration of the position of the oviduct‐spermathecal junction in females predicts that displacement is restricted to a sperm removal mechanism. The importance of spermathecal shape in determining sperm precedence patterns is discussed. The orientation of the genitalia during copulation indicates that sperm displacement is plausible and a mechanism of gonopod action is suggested.

1. Major reductions in catchment run-off, a result of frequent and prolonged drought conditions, together with extensive impoundment of rivers and streams, has led to concern about the possible negative effects on downstream estuaries. Preliminary studies have shown that changes in river flow and associated nutrient inputs have had a predominantly negative impact on the aquatic biota of Eastern Cape estuaries. Natural successions now have human imposed trajectories which are reducing variability and forcing both freshwater &#145;deprived&#146; and freshwater &#145;enriched&#146; systems into artificial cycles. 2. The Kariega and Great Fish estuaries in the Eastern Cape Province are used in a comparative manner to illustrate how differences in riverine inflow can influence the structure and functioning of selected biotic components in permanently open systems. Maximum chlorophyll a values in the freshwater deprived Kariega Estuary were 1 μg L-1, whereas maximum values in the freshwater enriched Great Fish Estuary were 22 μg L-1. Mean zooplankton biomass in the lower, middle and upper reaches of the Kariega Estuary was always below 50 mg m-3, whereas in the same reaches of the Great Fish Estuary, these values ranged from 256 to 4253 mg m-3. Similarly, mean ichthyonekton densities in the mouth region of the Kariega Estuary were 49 individuals per 100 m2 compared with 279 per 100 m2 in the Great Fish Estuary. 3. Temporal changes of riverine flow reinforce the importance of allochthonous inputs to the functioning of Eastern Cape estuaries. A comparison between a dry and wet period in the Kariega Estuary revealed mean nitrate and phosphate concentrations increasing from 5 to 101 μmol L-1 and from 1 to 5 μmol L-1, respectively. Phytoplankton stocks responded positively to freshettes in both the Kariega and Great Fish estuaries. Similarly, peaks in zooplankton biomass in the Great Fish Estuary increased rapidly in response to high food resource availability resulting from elevated river discharge. The higher ichthyoplankton and ichthyonekton densities in the Great Fish Estuary, when compared with the Kariega Estuary, were attributed to a combination of stronger olfactory cues for larval immigrants from the sea and elevated food stocks in the former system. Copyright © 2000 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The chapter, "Studies on estuarine macroinvertebrates" was written by the listed authors including Casper de Villiers (Douglas College Faculty). Estuaries of South Africa presents an authoritative and comprehensive review of the current status of the country's estuarine research and management. Information is provided on a wide range of topics, including geological, physical and chemical processes, diversity and productivity of plant and animal communities, interactions between estuarine organisms, and system properties, ecological modelling and current management issues. This broad scope is complemented by a comparative perspective, resulting in a volume which provides a unique contribution to the subject of estuarine ecology, relevant to all those working in this field throughout the world. --From publisher description.