Orchestia

Abstract: Orchestia grillus efficiently feeds upon microorganisms attached to ingested Spartina alterniflora litter, but does not digest litter itself. Microorganisms respond to Orchestia grazing with increased metabolic activity, reflected in accelerated decomposition of the nitrogen fraction of litter and increased microbial biomass. Increased microbial activity may be partly a function of ammonia excretion and higher diffusion rate due to animal movement, but mainly it is a direct response to grazing. Microbial biomass increases with grazing because the pool of available nitrogen becomes larger. A model postulating interactions between Orchestria, Spartina litter and attached microorganisms is presented.

Abstract: The way leaf shredders handle and shred leaves under laboratory conditions and in the mangrove forest at Myora Springs, Queensland, Australia, was investigated during the period 1980 to 1984. Field observations on the behaviour of the crab Sesarma erythrodactyla during low tide revealed that this species spends most of its time foraging over mud. S. erythrodactyla eats mangrove leaves where they fall or drags them into burrows or hollow logs. Through their feeding activities, crabs (S. erythrodactyla, Metopograpsus frontalis, Helice leachii, Clistocoeloma merguiensis, Leptograpsus variegatus, Paragrapsus laevis, Ilyograpsus paludicula), isopods (Exospaeroma alata, Campaecopia sp.) amphipods (Orchestia sp., Melita sp.), and a capitellid polychaete (Capitellides sp.), break down whole mangrove leaves into small particles. The way in which a leaf is broken down by leaf-shredders influences the size and composition of the particulate organic matter (POM) in the environment. The POM egested by leaf-shredders varies from 32 to 1171 μm. POM in this size range occurs in the guts of 38 invertebrate species which feed on mud, suggesting that they depend on the plant detritus produced by leafshredders as a source of food. Leaf-shredders therefore constitute a primary link in the marine food web of mangrove forests.

Abstract: Based on new talitrid amphipod collections from South Africa one new genus, Capeorchestia gen. nov., and one new species, Africorchestia meridionalis sp. nov., are described and Africorchestia quadrispinosa (K.H. Barnard, 1916) is redescribed. Eorchestia Bousfield, 1984 is redescribed. Based on this redescription Orchestia dassenensis (K.H. Barnard, 1916) is moved to Eorchestia and the Tasmanian species Eorchestia palustris Richardson, 1993 and E. rupestris Richardson, 1993 are moved to Microrchestia Bousfield, 1984. The current knowledge about the ecology of Capeorchestia capensis (Dana, 1853), Africorchestia quadrispinosa and A. meridionalis is summarized.