Charles L. Griffiths

Marine alien species of South Africa — status and impacts

The current status of marine alien species along the South African coast is reviewed and the ecological and economic impacts of these invasions are discussed. In all, 10 confirmed extant alien and 22 cryptogenic species are recorded from the region. All 10 alien species support well-established populations and the majority of these remain restricted in distribution to sheltered bays, estuaries and harbours. Only one species, the Mediterranean mussel Mytilus galloprovincialis, has spread extensively along the coast and caused significant ecological impacts. These include the competitive displacement of indigenous species and a dramatic increase in intertidal mussel biomass. These changes have also increased available habitat for many infaunal species and resulted in enhanced food supply for intertidal predators. Considerable economic benefits have also resulted from this invasion because M. galloprovincialis forms the basis of the South African mussel culture industry.

Kelp Wrack and the Flow of Energy through a Sandy Beach Ecosystem

One of the most distinctive and frequently cited features of exposed sandy beaches as ecosystems is their almost complete lack of in situ primary production (e.g. Brown, 1964; Munro et al., 1978; McLachlan et al., 1981a). This results directly from the mobility of the sediments, which renders them unsuitable for the attachment of macrophytes or the development of dense benthic diatom communities. In the absence of these potential food resources macrofaunal organisms on exposed beaches must obtain their nutrition from imported materials. These may take the form of finely divided detritus or phytoplankton particles, many of which may originate from rich blooms of phytoplankton in the surf zone (McLachlan et al., 1981b), or large items of carrion. The macro-fauna associated with these food resources normally comprises two major trophic elements, namely filter feeders, including bivalves (such as Donax) and crustacea (such as Emenita and Gastnosaccub); and predator/scavengers (e.g. Bullia, Ocypode,various isopods and polychaetes). The proportion of these two trophic elements differs widely, depending on the relative availability of plankton and detritus versus carrion (McLachlan et al., 1981a), but filter feeding bivalves are frequently the overwhelming dominants in terms of biomass (e.g. McLachlan et al., 1981b).

The relative significance of bacteria, meio- and macrofauna on an exposed sandy beach

A survey of the intertidal biota of a sandy beach on the west coast of South Africa has shown standing stocks of macrofauna, meiofauna and bacteria of 241.23, 200.17 and 663.07 g dry wt m-1 of shoreline respectively, an approximate biomass ratio of 1:1:3. The distribution of the macrofauna was the reverse of the usual pattern, with highest biomass occurring at the level of the current driftline. This appears to be related directly to the ready availability of food in the form of drift algae. Peak meiofaunal numbers were also found below the driftline and it is proposed that meiofaunal distribution is governed by dissolved organic matter (DOM) levels in the interstitial environment. Bacteria were abundant up to 1.2 m below the sediment surface, with the highest concentrations occurring at low tidal levels. The significance of the various biotic components in the energetics of the sandy intertidal is discussed. Turnover estimates suggest that bacteria may account for about 87% of annual production, with meiofauna and macrofauna making up 10 and 3% respectively. Despite this overwhelming importance of bacteria, the macro-and meiofauna probably play a vital role in making small organic particles available to bacteria for mineralization and in optimising conditions for microbial growth.

Some relationships between size, food availability and energy balance in the ribbed musselAulacomya ater

The effects of body size and suspension density on filtration rates, assimilation efficiencies and respiration rates in the ribbed musselAulacomya ater (Molina) have been determined by means of short-term laboratory experiments. Filtration rates accelerate rapidly in response to increasing algal concentration up to approximately 10×106 cellsDunaliella primolecta l-1, beyond which a plateau is approached. Percentage increments are greatest in small individuals. Assimilation efficiencies are independent of body size, but decline rapidly with increasing ration to approach zero above 32×106 cells l-1. Increases in respiration rate accompany increments in filtration rate in all but the smallest size class tested. Filtration, assimilation efficiency and respiration measurements are used to calculate ingestion rations, assimilation rations and scope for growth for mussels of different sizes over a range of algal concentrations. Scope for growth, expressed as percentage change in body energy per day, is a declining function of body size, but larger individuals achieve their maximum growth rates at lower ration levels than smaller ones. Growth efficiency is independent of body size, and is maximal at 5×106 cells l-1, where 29 to 43% of ingested ration is converted into body energy. The applicability of these experimental results to natural ecosystems is discussed.

The effect of size and temperature on the predation of cockles Cerastoderma edule (L.) by the shore crab Carcinus maenas (L.)

Laboratory studies demonstrated that shore crabs Carcinus maenas (L.) can consume <40 cockles Cerastoderma (= Cardium) edule (L.) per individual · day−1. Various predation techniques used by the crabs are reported. The time required to open and consume individual cockles increased exponentially with prey size. Small (<l5-mm shell height), easily broken cockles appeared to be the most profitable in terms of energy acquisition per unit of handling time, the optimal size of prey increasing with predator size. With unlimited prey available, however, crabs selected prey of mean size smaller than these predicted optima, and much below the maximum size they were capable of opening. Feeding rates, both in terms of cockles ingested or energy intake per day, rose steeply with increasing temperature, but the size range of prey consumed remained unchanged. These data strongly suggest that Carcinus maenas is a potentially important predator of small cockles, particularly during the wanner summer months.

Energy expended on growth and gonad output in the ribbed musselAulacomya ater

AbstractLength/weight relationships have been computed for shell, flesh and byssus ofAulacomya ater (Molina) and energy values used to convert the weights to energy equivalents. Shell accounts for some 26% of total body energy, while the contribution of the byssus declines from 15 to 8% during growth. Observations of juvenile growth rates have been used to generate a Gompertz growth equation which predicts attainment of maximum length (90 mm) after 11 years. Reproductive condition has been assessed by monitoring seasonal fluctuations in the flesh weight of standard-sized individuals, calculated from monthly length/weight regressions. There appear to be three spawnings, of variable date and intensity, each year. From the above data, annual energy expenditure on growth and gonad output has been calculated for individuals of various sizes. The ratio of total production to biomass

The interactive roles of predation and tidal elevation in structuring populations of the edible cockle, Cerastoderma edule

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Factors affecting the distribution, abundance and diversity of rock-pool fishes on the Cape Peninsula, South Africa

is a declining clining function of shell length, dropping from 29.5 at 5 mm to 0.8 at 85 mm. The proportion of total production expended on gamete output increases steadily from 25% at attainment of maturity (15 mm) to 81% at 85 mm length. The considerable effects of changing size composition on the amount and type of production in natural populations are discussed.