Monday, April 14, 2014

Saint-Girons' Sea Krait uses nurseries

Sea kraits (Laticauda) are amphibious snakes widely distributed in the coral reefs of the East Indian and West Pacific Ocean. They forage in the ocean but return to land, usually coralline islets, for resting, shedding skin, an
d digesting prey. Their dependence on coastal terrestrial habitats puts them at risk because of  increasing human populations and rapid industrial and mining developments that threaten coastlines. Human activities also contaminate the prey consumed by sea snakes. Many populations of sea kraits collapsed during the last three decades, due to habitat loss and massive harvesting for the leather industry.

Sea kraits are oviparous, and they lay their eggs on land. Communal nesting in tidal caves has been observed in two species (L. semifasciata and L. schistorhyncha) in Philippines, Taiwan and Niue islands. This information is anecdotal and the oviposition sites remain unknown for the six other species of  Laticauda. Additionally there are no data concerning the ecology of neonates or juveniles.

In a new study Bonnet and colleagues provide the first ecological data available on juveniles and coastal nurseries and propose simple practical conservation actions.

The study revealed that in New Caledonia, the maintenance of most yellow sea krait populations depends on few coastal nurseries. The populations contains a very small number of neonates or juveniles, thus local recruitment is extremely low. Careful long-term surveys of tens of islets in the southwestern-lagoon enabled us to find only two important and highly localized nurseries, respectively situated on two islands separated by 161 km: Verte Islet in the North and Ouen Island in the South. Ouen Island was remarkable, however, for two reasons: first the very high number of neonates and juveniles associated with very small number of adults was very unusual; second, this island is surrounded by many very different colonies. Ouen Island is indeed situated in a very large part of the lagoon; the barrier reef is more than 60 km offshore. This region of the lagoon contains many islets, essentially or exclusively populated by adults, spread from the line-coast to the barrier reef. This pattern suggests that likely hundreds of females converge every year to Ouen Island to lay their eggs.

Sea kraits are characterized by a low fecundity. In L. saintgironsi, the mean clutch size is 3.3 and females breed every two years on average. Considering the crude neonate number we estimated (~1,700), more than 500 females laid their eggs in the nursery during the winter 2010. There is no colony that shelters such a corresponding high number of reproductive females. Adult sea kraits display a very high fidelity to their colony, but very few adult females have been observed in Ouen Island. Thus, most of the females traveled from distant sites (sometimes more than 50 km) toward Ouen Island, and later returned home after having laid their three eggs. Ouen Island and possibly other similar nursery locations (yet undiscovered) likely supply a large proportion of juveniles for most populations of yellow sea kraits.

The authors did not find the nests (probably deeply hidden into crevices) and hence could not measure the specific environmental conditions in the laying sites, peculiar environmental conditions that prevail in coastal versus offshore sites might explain why many gravid females converged to the nurseries. In oviparous reptiles, incubation requires well-buffered thermal and hydric conditions. Ouen Island is a large basaltic island, physically extremely different from the flat sandy coralline islets that shelter most colonies. Coastal sites catch greater amount of rain and ambient temperature is more stable compared to remote islets. Thus, rocky coastal shores may provide thermally buffered and relatively humid microhabitats that are suitable for incubation. Interestingly, in the Ouen Island nursery, during the egg-laying period in late November, the authors found seven adult sea kraits sheltered in crevices under large rocks situated 50 m inshore, six were females and five were gravid. In strong contrast, in offshore sandy islets, large protective rocky structures and neonates are absent.

The Ouen Island nursery plays another essential role. The clearly multimodal distribution of body sizes shifted over seasons and  recapture data suggests the existence of successive cohorts of young snakes. Many juveniles belonging to each cohort remained in the nursery over prolonged periods (several months). During this time they do feed and grow. This suggests that many juveniles utilize the nursery before dispersal across the lagoon. Although mortality accounted for an unknown proportion of the progressive decrease in the size of each cohort, dispersal was important in this process as revealed by the very low local recruitment rate and the very small number of adults. Ouen Island nursery functions as dispersal springboard for juveniles. In 2012, we recaptured an adult yellow sea krait marked as a neonate in Ouen Island in 2010 on an islet situated 18 km away.  This information is anecdotal, but this nonetheless demonstrates the great dispersal ability of very small sea kraits, and the fast growth rate of juveniles.

Protecting nurseries should be a conservation priority to protect laying females, nesting sites, neonates, and  juveniles during a prolonged pre-dispersal phase. In addition, building artificial laying sites in coralline islets may represent an option to promote recruitment in threatened populations; such constructions also offer appropriate shelters to adult snakes.

Citation
Bonnet X, Brischoux F, Bonnet C, Plichon P, Fauvel T (2014) Coastal Nurseries and Their Importance for Conservation of Sea Kraits. PLoS ONE 9(3): e90246. doi:10.1371/journal.pone.0090246

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