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Biology of <i>Hymenosoma orbiculare </i>Desm in Lake Sibaya


B.J. Hill
A.T. Forbes

Abstract

Hymenosoma orbiculare, (Crustacea: Brachyura: Hymenosomatidae) a crab common in estuaries in south- and south east Africa occurs in freshwater in Lake Sibaya at depths down to 40 m. This is the only known freshwater record of this species. The population density in the lake is fairly uniform declining in shallow and very deep water with an a11erage- of 2,5 m-2 (134 Jm-2). The proportion of females in the population increases with depth. Females carry fewer eggs than estuarine populations and breeding appears to occur throughout the year. Both zoeae and adults differ morphologically from estuarine populations. These differences in breeding, biology and morphology raise the possibility that the Lake Sibaya population is specifically distinct from estuarine populations and this is discussed in relation to other instances of hymenosomatid invasion of freshwater habitats.

Hymenosoma orbiculare, (Crustacea: Brachyura: Hymenosomatidae) 'n krap wat algemeen voorkom in riviermondings in suid en suidoos Afrika, word ook aangetref in die varswater van die Sibayameer tot op 'n diepte van 40 m. Dit is die enigste bekende geval van hierdie spesie se voorkoms in varswater. Die populasiedigtheid is redelik eenvormig en neem af vanaf vlak tot baie diep water met 'n gemiddeld van 2,5 m-2 (134 Jm -2). Die verhouding van wyfies tot mannetjies neem toe met toename in diepte. Wyfies dra minder eiers as die in riviermondings en voortplanting vind blykbaar dwarsdeur die jaar plaas. Seide die zoeae en volwassenes verskil morfologies van die in riviermondings. Hierdie verskille in teling, biologie en morfologie dui op 'n moontlike spesieverskil tussen die bevolking in die Sibayameer en die in riviermondings. Hierdie aspekte word bespreek in die lig van ander gevalle van toetreding tot varswaterhabitatte onder die Hymenosomatidae.


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eISSN: 2224-073X
print ISSN: 1562-7020