Common names: Furry marsh crab, marsh crab (Eng.); moeraskrap (Afr.)
One of the most fascinating creatures in the mangrove ecosystem is the crab species, Parasesarma catenatum. This crab digs sturdy burrows into the mud substrate and is often spotted swarming around pneumatophores – the upward-growing lateral roots of mangroves.
They feed on detritus and collect mangrove leaves to store in their burrows. P. catenatum is an amphibious species, which can be found basking on the mudbanks on sunny days. They walk into the water and remove a considerable amount of mud to clean themselves.
Description/How to recognise a…
Furry marsh crabs have a square carapace (hard upper shell of a crustacean) with lateral grooves that is yellow to brown in colour. Their nippers are also yellow-brown, equal in size, and have a distinct furry lining around the hinge.
These crabs are relatively small and adult males can reach a maximum carapace width of 24.5 mm (Branch et al. 2017). This species exhibits sexual dimorphism, with the most noticeable difference between the sexes being the large chelipeds of males. Both male and female crabs can tolerate a range of temperatures and salinity levels.
Getting aroundParasesarma catenatum can be seen swimming or crawling around during the day. At night, they remain submerged and hide under rocks or other objects.
Furry marsh crabs exhibit acoustic behavior, although they don’t produce vocal sounds. The sounds are produced during mating or when they defend their burrows. The male crab makes a tapping sound by striking two of its legs together.
Furry marsh crabs are endemic to the parts of Africa’s southern and southeastern coasts, inhabiting estuaries from the Breede River, South Africa, to Inhaca Island, Mozambique (Day 1974). In southern Cape estuaries, it is found in high densities within Spartina saltmarshes, while in the Eastern Cape, KwaZulu-Natal and southern Mozambique, it is associated with mangrove swamps (Paula et al. 2003).
Parasesarma catenatum lives in salt marshes and mudflats near the shore (Branch et al. 2017). It digs vertical burrows 15–20 cm deep with side entrances 2–4 cm wide. Some burrows are on the surface, while others are below mudflats or cliff faces. The cliff face burrows are wider and have tunnels going as far back as 40–50 cm (Alexander & Ewer 1969).
Previous studies have shown that crabs of the Sesarmidae family (sesarmid crabs) consume plant material such as leaves, fruits and propagules, and prefer decomposed leaves over senescent or mature ones (Giddins et al. 1986; Chen & Ye 2008). Parasesarma catenatum, for instance, feeds on decomposed leaves of riparian trees found on land and decomposed and mature leaves of Spartina maritima. It picks material from the substrate using its chelipeds (Alexander & Ewer 1969).
Recent research using fatty acid and stable isotope analyses has confirmed that Parasesarma catenatum includes salt-marsh plants in its diet in various estuarine areas in South Africa (Paterson & Whitfield, 1997; Bergamino & Richoux 2015).
The feeding behaviour of Parasesarma catenatum, combined with its distribution in the field, exposes the crabs to potential food sources such as bacteria and benthic algae, which can help fulfil their nitrogen needs (Bouillon et al. 2002).
Sex and life cycles
Parasesarma catenatum breed at different times across South Africa, depending on the region, with their breeding season starting in August and slowing down between February and March (Emmerson 1994). The breeding process is delayed and distinctive as the intertidal zone increases in elevation. The male crab transfers sperm to the female using a modified set of antennae, and the female carries the fertilised eggs in her abdominal flap for up to two months. The number of eggs can be extensive, reaching into the thousands.
The larval development has four zoeal and a megalopa stage as described by Pereyra-Lago (1987):
(1) Zoea (free-swimming larvae)
The dorsal spine is curved backwards, with a slight frontal depression in the middle. The spines protrude downward, and the front contour is convex. There are no side spines. Pairs of small bristles are located laterally at the base of the dorsal spine and on the front above the eyes. The abdomen consists of five segments plus the telson (last abdominal segment).
(2) Zoea II
The carapace of the crab possesses two small bristles (setae) located on the midfrontal region, along with two larger feathery setae found on the posteroventral margin. Ventral buds of small pereiopods (legs) are also observed. On the abdomen, there is a solitary seta located dorsally at the posterior margin of the first segment. Additionally, the horn-shaped telson exhibits a slight outward bend towards the end.
(3) Zoea III
The carapace becomes more elongated towards the posterior compared to earlier stages. The dorsal spine is wider in proportion along most of its length. On each side of the posterior margin, there are four (sometimes five) feathery setae. The eyes are fully developed. The pereiopods are more advanced, with the first leg showing the initial signs of chelae (claw) segmentation.
(4) Zoea IV
The carapace is characterised by its subquadrate (squarish) shape. The distal portion of the dorsal spine is curved inward. The rostral (front end) spine has a widened base that extends the same length as the antennule (small antennae). On both sides of the posterolateral margin, there are eight plume-like setae. The pereiopods are well-developed and extend below the carapace.
At this stage the carapace of the crab is spherical, smooth and flattened, with a short rostrum that bends downward. The front part of the carapace is narrow, while the circular edge is wide and terminates in a rounded angle. The sides and back of the carapace are adorned with numerous setae. Additionally, the abdomen consists of six segments, concluding with the telson.
Females must leave the shelter of their burrows and move to the water’s edge to release zoeae and are vulnerable to predation at this time. The greater the distance from the water, the greater the predation risk (Emmerson 1994). The newly hatched larvae of this species are exported from the estuarine environment to the sea for development (Paula et al. 2003).
The big picture
Friends and foes
According to Alexander and Ewer (1969), Parasesarma catenatum and Cyclograpsus punctatus H.Milne Edwards, 1837 can be found living together in the Kowie River of the Eastern Cape.
The two species can be found in various locations and often share burrows in muddy riverbanks. Specifically, the burrows of Parasesarma catenatum can be found among the Spartina and Sarcocomia mangroves, as noted by Hodgson (1987).
Parasesarma catenatum is a resilient species that thrives in estuarine environments due to its unique respiratory and water-saving abilities. It can obtain water from the damp sediment and can thus stay out of free water for relatively long periods of time (Alexander & Ewer 1969).
Although it primarily feeds on mudflats, it seeks refuge in burrows for protection from various threats and extreme temperatures. Additionally, it provides high relative humidity for aerial respiration and reduces water loss across the gill surfaces to prevent desiccation.
When Parasesarma catenatum gets dirty, it cleans itself with chelipeds (claws). It walks into the water and rubs the chelipeds together on both sides, scraping any clumps of mud off onto the surface beneath (Alexander & Ewer, 1969). The chelipeds are also used as tweezers to remove mud from various body areas, including the mouthparts, respiratory surfaces, undersides and legs.
When threatened, Parasesarma catenatum quickly escapes by running and may reappear at another entrance, only to retreat again if pursued. In response to continued stimulation, the crab sidesteps, always keeping its chelae toward the perceived threat. It will eventually launch a forceful attack with its claws if unable to evade.
In male specimens, the exhibition of the chelae is occasionally accompanied by rubbing the heavily tuberculated outer surfaces of the dactylus and propus parts of the cheliped together.
Poorer world without me
True crabs, also known as brachyurans, play a crucial role in maintaining the mangrove ecosystem (Day 1981). Sesarmid crabs aid in processing leaf litter, retaining nutrients and impacting the aquatic environment. This process helps retain essential nutrients and impacts the particulate organic matter in the surrounding aquatic environment (Ashton 2002 ; Chen & Ye 2008).
Their feeding activities contribute to the ecosystem’s composition and serve as a habitat and food source for other inhabitants (Camilleri 1992). Furthermore, the faeces produced by these crabs serve as a habitat and food source for bacterial activity, eventually becoming food for small detritivores (Lee 1997).
Burrowing species also facilitate aeration and oxygenation of the dense mangrove mud, which is significant for the growth and development of mangrove seedlings (Steinke et al. 1993).
People and I
Currently, there is a lack of research exploring the dynamics between people and furry marsh crabs.
Conservation status and what the future holds
Parasesarma catenatum has not been assessed according to the IUCN Red List criteria.
The family Sesarmidae currently consists of around 324 species in 33 genera. There are currently five species of Parasesarma in the west Indian Ocean: P. guttatum (A. Milne-Edwards, 1869), P. leptosoma (Hilgendorf, 1869), P. catenatum (Ortmann, 1897), P. samawati (Gillikin & Schubart, 2004) and the newly described P. gazi Cannicci, Innocenti & Fratini, 2017.
Scientific name and classification
Species: P. catenatum (Ortmann, 1897)
Synonym: Sesarma catenata Ortmann, 1897
References and further reading
- Alexander, S.J. & Ewer, D.W. 1969. A comparative study of some aspects of the biology and ecology of Sesarma catenata and Cyclograpsus punctatus M.Edw., with additional observation on Sesarma meinerti De Man. Zoologica Africana 4: 1–35.
- Ashton, E. C. 2002. Mangrove sesarmid crab feeding experiments in Peninsular Malaysia. Journal of Experimental Marine Biology and Ecology 273: 97–119.
- Bergamino, L. & Richoux, N. 2015. Spatial and temporal changes in estuarine food web structure: differential contributions of marsh grass detritus. Estuaries and Coasts 38: 367–382.
- Bouillon, S., Koedam, N., Raman, A. V. & Dehairs, F. 2002. Primary producers sustaining macro-invertebrate communities in intertidal mangrove forests. Oecologia 130: 441–448.
- Branch, G.M. & Grindley, J.R. 1979. Ecology of southern African estuaries. Part XI. Mngazana: a mangrove estuary in Transkei. Suid-Afrikaanse Geografiese Tydskrif 14(3): 149–170.
- Branch, G.M., Griffiths, C., Branch, M. & Beckley, L. 2017. Two Oceans: A guide to the marine life of southern Africa. 3rd edition. pp. 464. Penguin Random House, South Africa..
- Camilleri, J.C. 1992. Leaf-litter processing by invertebrates in a mangrove forest in Queensland. Marine Biology, 114: 139–145.
- Chen, G.C. & Ye, Y. 2008. Leaf consumption by Sesarma plicata in a mangrove forest at Jiulongjiang Estuary, China. Marine Biology 154: 997–1007.
- Day, J.H. 1974. A guide to marine life on South African shores. 2nd pp. 1–229. A.A. Balkema, Publishers, Rotterdam, The Netherlands.
- Day, J.H. 1981. The estuarine fauna. In: Day, J.H., ed., Estuarine ecology: with particular, reference to southern Africa. Cape Town: Balkema.
- Emmerson, W.D. 1994. Seasonal breeding cycles and sex ratios of eight species of crabs from Mgazana, a mangrove estuary in Transkei, southern Africa. Journal of Crustacean Biology 14: 568–578.
- Giddins, R.L., Lucas, J.S., Neilson, M.J. & Richards, G.N. 1986. Feeding ecology of the mangrove crab Neosarmatium smithi (Crustacea: Decapoda: Sesarmidae). Marine Ecology Progress, Series 33: 147–155.
- Hodgson, A.N. 1987. Distribution and abundance of the macrobenthic fauna of the Kariega estuary. South African Journal of Zoology 22 (2): 153–162.
- Lee, S.Y. 1997. Potential trophic importance of the faecal material of the mangrove sesarmine crab Sesarma messa. Marine Ecology Progress Series 159: 275–284.
- Paterson, A.W. & Whitfield, A.K. 1997. A stable carbon isotope study of the food-web in a freshwater-deprived South African estuary, with particular emphasis on the ichthyofauna. Estuarine, Coastal and Shelf Science 45: 705–715.
- Paula, J., Dornelas, M. & Flores, A. 2003. Stratified settlement and moulting competency of brachyuran megalopae in Ponta Rasa mangrove swamp, Inhaca Island (Mozambique). Estuarine, Coastal and Shelf Science 56: 325–337.
- Pereyra-Lago, R. 1987. Larval development of Sesarma catenata (Brachyura, Grapsidae, Sesarminae) reared in the laboratory. South African Journal of Zoology 22: 200–212.
- Steinke, T.D., Rajh, A. & Holland, A.J. 1993. The feeding behaviour of the red mangrove crab Sesarma meinerti De Man, 1887 (Crustacea: Decapoda: Grapsidae) and its effect on the degradation of mangrove leaf litter. South African Journal of Marine Science 13: 151–160.
Author: Relebohile Lesuthu-Ntoyi