Sesarma Catenata Research Articles

Food preferences of the estuarine crab Sesarma catenata estimated through laboratory experiments

Feeding by sesarmid crabs on plants represents an important energy pathway within some estuarine ecosystems. We examined the trophic ecology of estuarine sesarmid crabs Sesarma catenata through a series of laboratory feeding-preference experiments. Our experiments considered decomposed and mature leaves of terrestrial riparian trees, marsh plants Chenolea diffusa and Sarcocornia perennis and the marshgrass Spartina maritima as potential food items. S. catenata preferred decomposed leaves of terrestrial riparian trees, followed by decomposed and mature leaves of S. maritima. We suggest that the low carbon:nitrogen (C:N) ratios of S. maritima and high bacterial production associated with decomposed terrestrial leaves may explain the trophic behaviour of S. catenata. The faecal production by S. catenata during these experiments confirmed the preferential assimilation of decomposed material by the crabs. By combining the consumption rates with an estimated density of S. catenata within the local estuary that it inhabits, we suggest that moderate proportions of the leaf material can potentially be consumed by this species (34% of total leaf litter), leaving substantial amounts of unconsumed leaf litter that may represent an important subsidy for adjacent environments. Our results validate previously published trophic data, showing the value of linking feeding experiments with biological tracers to improve food-web models.

SEASONAL BREEDING CYCLES AND SEX RATIOS OF EIGHT SPECIES OF CRABS FROM MGAZANA, A MANGROVE ESTUARY IN TRANSKEI, SOUTHERN AFRICA

Three breeding trends were observed in mangrove grapsids and ocypodids from Mgazana. First, significant changes were found from continuous breeding in the low shore (Macrophthalmus grandidieri) to seasonal breeding on the middle to high shore (Uca and Sesarma spp.). The higher on the intertidal a species occurred, the larger the crab and the more defined the breeding season (Sesarma meinerti). Small species (M. grandidieri) have adopted a strategy of continuous breeding to maximize reproductive output. The second pattern was a staggering of spawning peaks among seasonal breeders with increasingly later peaks the higher the crabs occurred on the shore. Sesarma catenata commenced breeding in August, while the high intertidal species Uca lactea annulipes was ovigerous in March. The third trend was a sharp decline in breeding activity in autumn (March) when temperatures dropped. For the grapsids and Uca vocans hesperiae, sex ratio peaks declined during ovigerous periods. With the exception of M. grandidieri and S. meinerti, there were significantly more males than females. Breeding periodicity may be controlled by a combination of factors, including latitude, temperature, food availability (both adult and larval), and intertidal zonation. Larval interspecific competition in coexisting guilds (Sesarma and Uca) could be minimized by staggering egg-bearing peaks, with breeding delayed and more defined higher on the intertidal.

Tidal exchange of larvae ofSesarma catenata(Decapoda, Brachyura) in the Swartkops estuary, South Africa

The tidal exchange of larvae of the salt-marsh grapsid crab Sesarma catenata was studied in the Swartkops estuary, a tidally driven, shallow estuary in Algoa Bay, South Africa. Plankton samples were collected bimonlhly during spring and neap tides from October to March at the tidal inlet. Samples were collected hourly for 25 h in February, and for 13 h in all other months. Hourly estimates of water flux through the tidal inlet of lhe esluary were calculaled using a generalized one-dimensional hydrodynamic model calibrated for lhe Swartkops estuary. Water flux estimates were used to calculate larval llux through the tidal inlet, Zoea I larvae of Sesarma catenata were found in all sampling sessions, with highest abundance during nocturnal ebb tides. Two seasonal peaks of abundance were detected in November and February-March. These coincided with recorded peaks of percentage abundance of gravid females in the adult population. Megalopa larvae were recorded mainly during flood tides from January to March. More than 70 % of all Sesarma catenata larvae moving through the tidal inlet of the Swartkops estuary in either direction were exported in any given sampling session. There was net export of zoea I larvae in all cases but one, and net import of megalopa larvae in all cases. The possibility that Sesarma catenata larvae may become entrapped in lhe surf zone circulation system throughout development, as a mechanism to avoid offshore dispersal, is discussed.

Predicted effects of a hypothetical oil spill on the saltmarsh crab Sesarma catenata

The effect of Qatar Light crude oil on the saltmarsh crab Sesarma catenata was investigated. These crabs are well adapted to the estuarine environment and mainly occur in the zone dominated by the saltmarsh grass Spartina maritima. They have an excellent aerial respiration and water-saving system and their burrows do not penetrate to water level. Static bioassays conducted showed that adult crabs are fairly resistant to the water-soluble fraction (WSF) of the crude oil, but that larval survival is affected at a much lower WSF concentration. Many of the crabs were, however, affected during the bioassays and may be considered as ‘ecologically’ dead. In the event of spilled oil entering the estuary it is predicted that oil would form a layer over the water contained in the burrows. Although oil initially evaporates quickly, the layer of oil would drastically retard the diffusion of oxygen into the water, while some of the oil would dissolve into the water. It is estimated that the concentration reached at the end of a tidal cycle would not produce mortality, but would affect the crabs' respiration. Because of the small burrow volume, the crabs would be expected to utilize all the oxygen in the burrows before the next high tide. However, before that occurred, and at a critical oxygen saturation, the crabs would be expected to leave the water and climb through the layer of oil. Most crabs that return to clean water immediately, would probably not be further affected, except that the oil in their branchial chambers would reduce the area available for oxygen exchange. In some crabs, carapace contamination induces moulting, which in turn would make them more sensitive to oil pollution. The crabs would avoid polluted sediment, but might be attracted to very low and very high concentrations. Crabs remaining in the oil would clearly soon die of the combined effect of toxicity and oxygen depletion. Oil occurring on the saltmarsh surface would be degraded relatively fast, but once it seeps into the anaerobic mud, degradation virtually ceases. Experimentally, the hatching success ofgravid females, normal development and size of larvae spawned is not affected by Qatar Light crude oil, but their photopositive response is affected. The dangers of transferring oil from damaged tankers in Algoa Bay is discussed and recommendations made.

Phototactic behaviour and the nature of the shadow response in larvae of the estuarine crabSesarma catenata

The phototactic behaviour of the first zoea of the estuarine crab Sesarma catenata (Brachyura, Grapsidae) was studied in the laboratory. Light and dark-adapted larvae showed onjy positive phototaxis when stimulated horizontally with light intensities ranging from 10-7 W m-2 to 1 W m. Peak percentage response was at 10-1 W m-2 and 10-4 to 10-2 W m-2 in light and dark-adapted larvae respectively. This pattern differs from that common to other brachyuran larvae in the absence of negative phototaxis at low light intensities in light- adapted larvae. The shadow response in S. catenata first zoea was studied in a natural light field. The nature of the shadow response is discussed according to larval responses in the horizontal plane. An interpretation of the mechanics of the shadow response is proposed which does not involve negative phototaxis. The consequences of the phototactic behaviour of larvae for dispersal in the Swartkops estuary are briefly discussed. According to their phototactic pattern, larvae would migrate to the top section of the water column and aggregate there during the day. This would increase their chances of being exported out of the estuary with the ebb tides.

Aspects of Sesarma catenata (Grapsidae, Crustacea) burrows and its implications in the event of an oil spill

The shape and dimensions of Sesarma catenata crab burrows were determined by means of polyester resin casts. Relationships between burrow entrance size and volume as well as burrow entrance size and size of the occupant crab were established. The ‘average’ crab burrow penetrated into the mud by a distance of 11·34±3·02 cm from the surface at an angle of 55·5° resulting in a depth of 8·94±3·54 cm with a volume of 15·68±9·50 ml. The drainage rates of artificial crab burrows were determined and it was established that, depending on distance from the salting cliff, these burrows were between 48·5% and 22·9% full of water at the end of the tidal cycle. The natural crab burrows contained between 0% and 5% water at this stage. The crab burrows penetrated an average of 14·9% of the distance between the surface and the sea water level. In the event of an oil spill the crabs will come into direct contact with the oil when the water drains from the burrows during the ebbing tide or when they climb out of the burrow under conditions of increasing hypoxia. This carapace contamination by oil will result in adverse effects on the physiology and behaviour of these crabs.

Distribution and abundance of the macrobenthic fauna of the kariega estuary

The Kariega estuary is an open estuary which is about 18 km in length. The freshwater inflow into the estuary is very sporadic and consequently the water is often hypersaline at the head. Quantitative and qualitative sampling has been undertaken to describe the distribution and abundance of the macrobenthos along the estuary. Of the 107 species that have been recorded, 80% can be classified as euryhaline. One of the possible reasons for the high species diversity, when compared to other eastern Cape estuaries, is the growth of Zostera capensis along the entire length of the estuary. The major contributors to the macrobenthic biomass are the crustaceans, Cleistostoma edwardsii, C. algoense, Upogebia africana, Sesarma catenata and Uca urvillei and the bivalve Solen cylindraceus. The latter is particularly abundant in the middle reaches of the estuary where it can reach densities of 400 m-2. Its successful colonization is attributed to food availability and stable physical conditions within the estuary which...

Larval development ofSesarma catenateOrtmann (Brachyura, Grapsidae, Sesarminae) reared in the laboratory

Sesarma catenata larvae were raised in the laboratory and the development stages described in detail. Larval development comprises four zoea and one megalopa stage. Morphological features of the first zoea and megalopa stages of Sesarma catenata are compared with those of other Sesarminae species described in the literature. The genera Sesarma, Metasesarma, and Bresedium appear to be closely related taxa within the subfamily. The diagnostic value of some features of larval morphology is briefly discussed.

Effects of Qatar light crude oil on adult saltmarsh crabSesarma catenataand implications in the field: respiration, oxygen diffusion and hypoxia

The effects of Qatar Light crude oil on the respiration, hypoxic death and behaviour under conditions of increasing hypoxia of the saltmarsh crab Sesarma catenata were determined. The rate of oxygen diffusion through layers of crude oil into the underlying seawater was also determined. Even a thin layer of the crude oil drastically reduced oxygen diffusion into the underlying seawater. Respiratory rate of the crabs was depressed at low concentrations of water soluble fraction (WSF) but stimulated at higher concentrations, these effects being more pronounced in males than in females. A synergistic relationship was found between the effects of low oxygen tension and WSF concentration that raised the threshold of dissolved oxygen at which death occurred. In the event of an oil spill, most S. catenata would deplete the available oxygen in their burrows before the next high tide. Death occurred at a concentration of dissolved oxygen of some 8,3 percent, but most crabs were found to leave their burrows and clim...

Respiratory responses of winter acclimated grapsoid crabs to a number of environmental parameters

1.1. The effect of light, temperature and salinity on the respiration of two winter acclimated, estuarine grapsoid crabs (Sesarma catenata Ortm. and Cleistosoma edwardsii McLeay) were investigated.2.2. Both species showed a clear cycle of respiratory activity with peaks during low tides when these occurred at night, and decreases during high tides irrespective of photoperiod.3.3. Oxygen consumption was directly related to temperature although Q10 was low (1,4) over the range 15 to 25°C,4.4. S. catenata showed small but significant increases in oxygen consumption in both dilute and concentrated seawater while C. edwardsii exhibited decreases in oxygen consumption under these conditions.

The Osmoregulatory Ability of Three Grapsoid Crab Species in Relation to Their Penetration of an Estuarine System

AbstractThe osmoregulatory abilities of the grapsoid crabs Cyclograpsus punctata*, Sesarma catenata andSesarma eulimene were studied comparatively in an attempt to explain, at least in part, their distribution in estuaries. Both survival and haemorymph osmotic pressure were used as indices of hyperosmotic regulatory ability.All three species were shown to be capable of regulating their haemorymph osmotic pressure against lowered external salinities. S. eulimene was found to be better able to control its haemorymph concentration in very low salinities than S. catenata, which would account for the former's occurrence in the upper reaches of estuaries. C.punctatus was shown to be capable of roaintaining its haemorymph osmotic pressure above that of the environmental medium in lowered salinities, but without the degree of constancy characteristic of the two Sesarma species. It does, however, show a marked ability to tolerate lowered internal osmotic pressure, an ability which appears to develop in response to...

The effects of Qatar Light crude oil on the saltmarsh crabSesarma caienataand its implications in the field: toxicity to adults and larvae

The toxicity of Qatar Light crude oil to adult and larval Sesarma catenata was determined under different conditions of test-solution preparation. Evaporation of the oil and its water-soluble fraction (WSF) was studied and an estimate made of oil content in artificial burrows approximating field volumes. In the experiments with adults, where crude oil was added to the surface of the water and WSF was not renewed, 50-per-cent mortality was not obtained, but the crabs were sub-lethally affected (righting response and escape mechanism). These sub-lethally affected crabs usually recovered after a few hours as the WSF of the crude oil escaped rapidly from the seawater, only 32 per cent remaining after 24 h. In the experiment where the experimental solution was renewed every 8 hours, the 24-h and 96-h LC50 values were 12,0 and 9,2 mg·ℓ−1. Ecological toxicity (animals affected, not killed) was in all cases lower than acute toxicity, larvae being much more sensitive than adults. In the event of an oil spill, it is unlikely that the oil in the seawater contained in the crab burrows will reach the high concentrations required to produce death in these crabs.

A Comparative Study of Some Aspects of the Biology and Ecology ofSesarma CatenataOrt. andCyclograpsus PunctatusM. Edw., with Additional Observations onSesarma MeinertiDe Man

In the estuary of the Kowie River in the Eastern Cape Province there occur two amphibious grapsoid crabs, Sesarma catenata Ort. and Cyc/ograpsus punctatus M. Edw. They live together in many places, commonly inhabiting the same burrows in the muddy banks of the river. The present exploratory study of these two species has been undertaken to enhance our knowledge of their biology as well as our understanding of the relations between these two very similar animals which live in such close proximity one to another. THE HABITAT The Kowie River empties into the sea at Port Alfred, and is broad, winding and shallow. The tidal limit lies 13 miles from the mouth and over most of this length extensive, gently sloping mud banks are left exposed at low tide. The level of mean high tide is marked by a low salting cliff which is surmounted with vegetation. At Port Alfred itself the river runs between steep, stony, artificial dykes and on either side lie lagoons (Fig. 1). The lagoons on the East Bank (1, 2 and 3) are almost completely cut off from the river although an exchange of water does take place via seepage points in the dyke. The two western lagoons (4 and 5) are fully tidal. Large areas of the intertidal zone in the estuary are colonised by the tall, coarse marsh grass Spartina capensis Nees (S. stricta Roth.). This usually disappears about the level of High Water Neaps (HWN), to be replaced by a zone of typical salt marsh plants, dominated by Arthrocnemum, especially A. pillansii Moss, which has long stems, prostrate and woody at the base and terminating in erect green succulent tips. With the Arthrocnemum there may be min­ gled Spartina and Triglochin bulbosum L., particularly in low wet situations, and towards High Water Springs (HWS) Chenolea diffusa Thunb. A short distance below HWS Limonium scabrum (Thunb.) O.K. appears and spreads upwards, presently to give way to grasses, shrubs and trees. The zonation of plants on the banks of four of the five lagoons is very much the same as that along the river banks. The banks of Lagoon 4 (known locally as Little Beach) are sandy and devoid of vegetation. The two lagoons which lie furthest from the sea (3 and 5) contain masses of drifted algae, in particular the slender, branched brown alga Graci/aria verrucosa (Huds.) Papenf. The distribution of the two crabs in the area round Port Alfred is shown in Fig. 1. Beyond the town, where the river is not restrained by artificial dykes, the two species occur together