Abstract

 
 
 The thick-shelled clam Meretrix morphina, previously referred to as Meretrix meretrix, now occurs in the west Indian Ocean region, along the eastern seaboard of Africa, from the Red Sea to the Mlalazi Estuary, close to the Tugela River. Its presence in South Africa is only of recent recording. Meretrix morphina was detected for the first time in Lake St Lucia in 2000. The population declined and was not detected from 2005 until 2011, most likely as a result of a severe drought that resulted in widespread desiccation and hypersalinity in the lake. The system then experienced increased freshwater input resulting in lower salinities from 2011 until 2014, during which time M. morphina reappeared and their population gradually increased. In 2015, M. morphina became abundant in St Lucia, attaining unprecedented densities of 447 ind./m2. Biomass, expressed as a fresh weight, varied in the different basins of St Lucia, ranging from 195 g/m2 at Lister’s Point to 1909.8 g/m2 at Catalina Bay. However, in 2016, when drought conditions returned, M. morphina disappeared. This species appears to thrive under brackish salinities and high temperatures. It is able to establish large populations with high biomass and can become dominant. However, M. morphina is sensitive to desiccation and hypersaline conditions. This clam has substantial commercial value and is exploited along the African east coast, particularly in Mozambique. In future, it may feature more prominently in South African estuaries. However, the ecology of M. morphina is still largely unknown.
 
 
 
 
 Significance: 
 
 
 
 First record of population irruption of M. morphina in South Africa.
 Report on the largely unknown ecology of a commercially valuable bivalve.
 Update on the taxonomy and poleward spread of M. morphina.
 
 
 
Highlights
The thick-shelled clam Meretrix morphina (Lamarck, 1818) has previously been reported erroneously as Meretrix meretrix (Linnaeus, 1758) in the western Indian Ocean[1,2] because of an invalid synonymy which has only recently been rectified[3]
This occurrence represents another case of poleward range expansion, probably in response to global warming, as many such cases have already been documented along the South African coastline.[5]
The presence of M. morphina in Lake St Lucia and Mlalazi Estuary adds to the growing record of species displaying a poleward migration in South Africa.[5,20]
Summary
The thick-shelled clam Meretrix morphina (Lamarck, 1818) has previously been reported erroneously as Meretrix meretrix (Linnaeus, 1758) in the western Indian Ocean[1,2] because of an invalid synonymy which has only recently been rectified[3]. The species is restricted to the shores of the west Indian Ocean, but has occasionally been reported from South African coastal waters, mainly as dead shells.[1] Branch et al.[2] and Kilburn and Rippey[4] have stated that M. morphina does not occur naturally south of Maputo Bay, Mozambique; this species has recently been recorded in the St Lucia and Mlalazi estuaries. For a detailed review of hydrodynamics in St Lucia see Stretch et al.[9] and Lawrie and Stretch[11]
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