Variation in the growth and survival of the tropical vermetid gastropod Ceraesignum maximum is driven by size, habitat, and density

Abstract

The vermetid Ceraesignum maximum is a sessile tube-dwelling snail with a wide geographic distribution that can strongly reduce the growth and survival of corals, alter coral morphology, and shift coral species composition. Thus, this species has the potential to drive dramatic changes in coral reef ecosystems; yet, very little basic demographic information exists for C. maximum; nor do we know factors that potentially contribute to variation in vital rates. This study estimated individual growth (change in shell aperture diameter) and survival of focal (tagged) vermetids (n = 297) over 646 days, to test hypotheses that growth and/or survival rates were size-dependent, density-dependent, and/or habitat-dependent. This study was conducted on 80 haphazardly selected patch reefs, split evenly among eight discrete sites. The design also incorporated an experimental density reduction (e.g. on half the study reefs at each site divers reduced the densities of vermetids). Vermetid growth rates decreased with body size, and the slope of this relationship was steeper for reefs with experimentally reduced densities. Vermetid growth decreased weakly with local biomass of vermetids (i.e. estimated biomass within a 20 cm radius of focal individuals) but did not vary with local substrate composition. Vermetid survival increased with body size and local biomass; the relationship with biomass was steeper for reefs where vermetid densities had been experimentally reduced. Survival was also greater on living coral Porites lobata (where coral growth covered longitudinal extensions of vermetid shells) relative to dead coral substrate (where vermetid shells extensions remained exposed). These data suggest little scope for regulation of vermetid population growth via direct density dependence in demographic rates. However, given that vermetids can kill corals, and vermetid survival probabilities appear to depend upon coral cover, it is predicted that vermetids may experience cyclical dynamics that are coupled to the dynamics of living coral.