Consistent spatial patterns of arrival of larvae of the honeycomb barnacle Chamaesipho tasmanica Foster and Anderson in New South Wales

Genetic connectivity in the broadcast-spawning coral Acropora digitifera analyzed by microsatellite markers on the Sekisei Reef, southwestern Japan

Worldwide decline has been observed in coral populations due to environmental changes at both regional and global levels. Since corals have a typical metapopulation structure, the recovery of severely disturbed local populations depends on larval recruitment from outside the disturbed region. Therefore, the connectivity among local populations may play a key role in the disturbance/ recovery dynamics of coral populations. On the basis of the simulation of surface currents, derived from ocean currents, we hypothesized that the genetic connectivity of coral populations is weak between the northern and southern regions of Sekisei Reef, Japan. To test this hypothesis, the connectivity of the population of the broadcast-spawning coral Acropora digitifera, one of the dominant corals in the Sekisei Reef, was analyzed at six sampling sites that were 5-25 km apart by using six microsatellite markers. The degree of genetic differentiation of A. digitifera was very low among all the sites (F(ST) from -0.006 to 0.015), and no evidence was found in support of our hypothesis. The relative lack of genetic differentiation of A. digitifera among the sites may be attributable to the large effect of wind-driven surface currents, which highly vary on an annual basis, on coral larvae or to the time gap between simulation based on ocean current patterns and accumulation of genetic differences. Considering the pattern of the genetic connectivity of A. digitifera in the Sekisei Reef, the sites that are located within less-disturbed areas may act as larval sources and should, therefore, be preserved.

Longevity determines sizes of an adult intertidal barnacle

The small honeycomb barnacle Chamaesipho tasmanica Foster and Anderson often forms continuous sheets covering the substratum at mid-shore levels of sheltered rocky shores but also occurs in sparse distributions on exposed shores, and on higher levels of sheltered shores, in south-eastern Australia. Larger barnacles are generally found in more exposed and higher areas. Effects of each site on ultimate sizes of barnacles were therefore examined by measuring growth of barnacles of the same cohort at sheltered and exposed shores from the end of 1989 to the end of 1992. Because Chamaesipho varied in size among sites, and because size was not necessarily representative of age, three growth models were proposed to explain this size differential. While rates of growth and periods of growth did not differ, differences in longevity were found to influence size so that larger barnacles survived longer and were more abundant on the more exposed shores. In fact, when barnacles were aged, it was found that a greater proportion of older barnacles (>3 years of age) occupied these exposed areas. It seems that site-specific characteristics influenced longevity in some places so that larger Chamaesipho continued to predominate in these areas.

Settlement in different-sized patches by the gregarious intertidal barnacle Chamaesipho tasmanica Foster and Anderson in New South Wales

Most models on settlement of open marine invertebrate populations are based on space-limitation. These models, however, do not recognise that free space may not drive the demography of populations when larval numbers are small or when larval supply varies along a gradient in the habitat. They also do not incorporate the effects of larval choice when settling. It has been hypothesised that, in gregarious barnacles, the effects of adult conspecifics, rather than available free space, may play a primary role in settlement. That is, cues from adults along perimeters of patches, rather than space available, may enhance colonisation. This study therefore aimed to distinguish between these separate influences on populations of Chamaesipho tasmanica, a gregarious barnacle characterised by relatively few larvae arriving to settle each year. Patches of 6, 3 and 1.5 cm diameter were cleared within aggregations of barnacles at three heights (Low, Mid, Upper) of Chamaesipho's distribution at two sites and during 2 years of settlement. Total numbers of settlers in each year were manipulated to determine the separate influences on settlement due to availability of substratum or the effects of conspecific adults. To test for the effects of available free space, numbers of settlers per unit area were analysed. To test for gregarious effects due to the presence of adults, numbers of settlers per unit perimeter were analysed. While available substratum was found not to affect settlement of this barnacle, gregarious settlement in response to adults at perimeters of patches was thought to be confounded by differential larval supply and differential conspecific cues among heights on the shore. Results from this study therefore have important implications for survival of gregarious populations following disturbances, especially in species where larval supply is poor.