Surveys conducted in the south-western Dominican Republic showed that the penshellPinna carneais a consistent component of seagrass beds and is absent in adjacent sandflats. Population densities were low (0.012–0.076·m−2) and the size–structure skewed towards large individuals. Trials with different types of substrata in caged spat collectors, involving a combination of sand, seagrass blades and roots, and an artificial treatment by itself, indicated no settlement preference for any of the substrata tested. Comparison of additional spat collectors (caged and uncaged) indicated high predation losses (84%) for recently settled penshells. Experiments with penshells measuring 10–30, 50–70 and 90–110 mm (anterior to posterior dorsal tip) transplanted to plots in a seagrass bed and sandflat showed that predation losses decreased with increasing size and were much less in seagrass than in the sandflat. In 10-day trials, survival in the three size-groups was 27-fold greater in the seagrass bed than on the sandflat. During 3-day trials in the sandflat, survival increased from 6% for 50–100 mm penshells to 93% for 150–170 mm penshells. At the end of 100-day trials, during which the study area was subjected to Hurricane Dennis, the only surviving penshells were large individuals (90–110 mm) that had been transplanted to the seagrass bed. All individuals transplanted to the sandflat went missing. Growth measurements showed that small penshells grow rapidly (up to 2.2 mm·d−1), but the growth rate drops markedly at ~150 mm. Rapid juvenile growth may be a strategy for reducing the period of high vulnerability to predators. The high proportion of large individuals in the population likely represents the accumulation of successive recruitments as growth slows in older penshells.Pinna carneais mainly restricted to seagrass beds because they provide more protection from predators than adjacent habitats. Moreover, the consolidation of sediments in seagrass beds by roots and algal rhizoids provides a degree of protection from physical disturbances such as hurricanes. The advantages provided by the seagrass habitat come at a cost because we detected a mortality factor in the seagrass bed (possibly related to the ~3-fold higher silt load) that was absent on the sandflat.
Examination of the mantle cavity of 310 penshells Pinna carnea, collected from seagrass beds in southwestern Dominican Republic, revealed the presence of four species from three phyla as potential symbionts. The shrimp Pontonia sp. was found in 178 penshells, the cardinalfish Astrapogon stellatus in 18, an anemone (order Actiniaria) in two, and a pea crab (family Pinnotheridae) in one. The penshell likely provides these organisms with a refuge from predators. It also provides nutrition for shrimp as they consumed food trapped in the mucus secreted by the penshell host. All of the cardinalfish, and 91% of the shrimp, were found in penshells measuring >150 mm in hinge length. At approximately 150 mm, the shells of P. carnea thicken and so flatten less when the valves are closed. As a result, larger penshells provide more mantlecavity space for accommodating organisms. Shrimp and cardinalfish size increased with the size of the host penshell. The positive relationship between shrimp size and penshell size, a prevalence of male-female pairs of shrimp and other field observations, suggest that a longterm association exists between shrimp and penshells, and that the mating system of the shrimp involves social monogamy.
Abstract-We developed an alternative capture-and-release method for sharks using a simple poker-andhook tool for divers to quickly capture nurse sharks (Ginglymostoma cirratum) entering the intake canal of a nuclear power plant in Florida. The capture technique consists of using a short metal rod (poker) with a barbless J-hook (size 10/0) to snag the base of a shark's tail (caudal peduncle), then safely hauling the individual into a boat by the buoy line that has been hooked to its tail. We captured 20 nurse sharks ranging from 11.8 to 80.3 kg, and 9 individuals were monitored over time. Six sharks were released into the canal and 3 sharks were put in an open tank for 23-24 days for daily observations. All hook wounds resulting from this technique were assessed between 9 and 42 days, and no ill effects were observed throughout the study period. The capture method caused a small superficial wound to the caudal peduncle that averaged 11.3 mm 2 (standard deviation 8.7; n=23). Wound closure was observed after 9 days and re-epithelialization was almost complete (or the wound had completely healed) between 22 and 42 days. Landing nurse sharks this way is less traumatic than traditional methods (e.g., angling, netting). This study provides preliminary information on, and validates, the use of this tool as an efficient and less invasive capture method than traditional methods and as a method that could be applied to broader areas of shark research.
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