Size at onset of maturity of spiny lobsters Panulirus homarus homarus at Mambrui, Kenya

The size at onset of sexual maturity of the scalloped spiny lobster Panulirus homarus homarus captured off Mambrui, Kenya, was estimated from ovary condition, presence of external eggs and/or spermatophore remains (berry method), and presence of well developed ovigerous setae (setal method). The predictive utility of morphometric relationships between carapace length (CL), pleopodal exopodite length (exopod length) and second leg length were assessed by fitting linear regressions to lobsters categorised as immature and mature, and comparing the intersect values with other estimates of functional maturity. The smallest female with mature ovaries was 52mm CL, compared with the smallest specimens of 50mm with ovigerous setae and 54mm with external eggs. Logistic curves indicated that 50% maturity was reached at 52.6mm (setal method) or 63.4mm (berry method) during the main breeding season. Female exopod lengths suggested that functional maturity was reached at 50.5mm and male leg lengths increased significantly upon reaching 57.5mm. The analyses indicate that measurements of female exopods and male second leg length can be used as cost-effective methods to estimate size at functional maturity of P. homarus homarus in Kenyan waters, and that these indices can augment estimates based on the presence of external eggs and ovigerous setae.Keywords: allometry, Kenya, Panulirus homarus, sexual maturityAfrican Journal of Marine Science 2006, 28(1): 51–5

What drives ontogenetic niche shifts of fishes in coral reef ecosystems?

Ontogenetic niche shifts are taxonomically and ecologically widespread across the globe. Consequently, identifying the ecological mechanics that promote these shifts at diverse scales is central to an improved understanding of ecosystems generally. We evaluated multiple potential drivers of ontogenetic niche shifts (predation, growth, maturation, diet shifts, and food availability) for three fish species between connected coral reef and nearshore habitats. In all cases, neither diet compositional change nor sexual maturity functioned as apparent triggers for emigration from juvenile to adult habitats. Rather, the fitness advantages conferred on reef inhabitants (that is, enhanced growth rates) were primarily related to high prey availability on reefs. However, there exists a clear trade-off to this benefit as survival rates for small fishes were significantly reduced on reefs, thereby revealing the potential value of (and rationale behind high juvenile abundances in) nearshore habitat as predation refugia. We ultimately conclude that predation risk functions as the primary early life stage inhibitor of ontogenetic niche shifts towards more profitable adult habitats in these systems. Furthermore, this study provides a case study for how complex, meta-dynamic populations and ecosystems might be better understood through the elucidation of simple ecological trade-offs.I. A. Kimirei, I. Nagelkerken, M. Trommelen, P. Blankers, N. van Hoytema, D. Hoeijmakers, C. M. Huijbers, Y. D. Mgaya, and A. L. Rype