Density Of Small Individuals Research Articles

Influence of environmental patterns on gonadosomatic index and early life stages of Paracentrotus lividus in Corsica (Mediterranean Sea)

Spatio-temporal patterns of the gonadosomatic index (GSI) and life cycle stages of the sea urchin Paracentrotus lividus were studied at two Corsican near sites which differ in terms of protection status. The results highlighted an annual cycle of the GSI with spring spawning (between May and June) and presumed lesser autumn spawning (October or November). Spatial patterns revealed homogeneity in larval densities between the sites whereas differences were observed for settlers, recruits, sub-adults and adult stages. Temporal variability was detected for all stages with highly variable patterns for larval stage and settlement. This study suggests the presence of sporadic spawning and/or the arrival of allochthonous larvae to the sites. The recruitment intensity was highly variable between the two studied years; in 2016, the peak recorded was five-fold higher than in 2015. This study confirmed the importance of habitat diversity on the density of small individuals. The site’s protection status also influenced the structuring of P. lividus indirectly as the highest densities were recorded at the site where fishing was allowed. Finally, this work revealed the difficulty of observing the temporal evolution of the P. lividus life cycle in the natural environment.

Distribution and Biomass of an Estuarine Ctenophore Population, Mnemiopis leidyi (A. Agassiz)

For an estuarine population of the ctenophoreMnemiopsis leidyi (A. Agassiz), density of small individuals, indicative of recent spawning, and total biomass were measured on 32 dates over 20 months. These observations were related to the environmental parameters of temperature, salinity, daylight, and presence of a predator.