(Georgia Institute of Technology, 2002-01-11)
Bullard, Stephan G.; Hay, Mark E.
Difficulties associated with manipulating plankton in situ have limited the ability of investigators to assess among-habitat variation in predation risk for plankton. We used plankton tethering units (PTUs) to tether zooplankton in a variety of reef and seagrass habitats, and used field and laboratory assays to test PTUs for tethering artifacts. Tethering did not affect the survivorship of 5 species of plankton (sizes <1 to 6 mm), indicating that the method works with a range of planktonic organisms. We then used the reef mysid Mysidium columbiae in additional assays and found that: (1) mysids remained on PTUs unless they were attacked by predators; (2) PTUs did not prevent planktivorous fishes from consuming tethered mysids; (3) untethered mysids commonly evaded predators, while mysids on PTUs did not; (4) the same types of predators consumed untethered and tethered mysids in the field; and (5) fishes were neither attracted to nor repelled from PTUs. We used PTUs and mysids to assess predation risk for plankton over various coral reef or seagrass habitats. Risk of attack varied among habitats and was correlated with abundance of planktivorous fishes. On the reef, attack rates were most intense over a topographically complex reef ledge, less intense over the less structurally complex center of the reef, and least intense over a structurally simple sand plain. Within the seagrass bed, attack rates were highest at the edge of the bed and less intense to the center of the seagrass bed and over an adjacent sand plain. Thus, attack rates at these sites varied tremendously over small spatial scales (meters).
(Georgia Institute of Technology, 1999-12-30)
Bullard, Stephan G.; Lindquist, Niels Lyle; Hay, Mark E.
Predation is believed to be a major source of mortality for larvae of benthic invertebrates, but the palatability of larvae commonly found in the water column has rarely been assessed. Larval palatability assays were conducted by collecting live invertebrate larvae from a temperate field site and offering them to a suite of common predators (the fishes Lagodon rhomboides, Leiostomus xanthurus, and Monacanthus hispidus and the hard coral Oculina arbuscula). By crushing larvae that were rejected intact and re-offering them to predators, it was possible to distinguish between defenses based on morphological and chemical characteristics of the larvae. Additionally, abundance data were collected for taxonomic groups of larvae at our sampling location. The majority of invertebrate larvae were palatable to consumers. Most predators readily consumed polychaete larvae, barnacle nauplii, bivalve veligers, shrimp zoeae, crab megalopae, phoronid actinotrochs, and hemichordate tornaria (which together accounted for 65% of meroplankton abundance), suggesting that these larvae lacked effective morphological or chemical defenses. Against at least 1 fish predator, a significant number of gastropod veligers, barnacle cyprids, crab zoeae, and stomatopod larvae (which accounted for 34% of meroplankton abundance) appeared to be morphologically defended. Larvae from these groups tended to be rejected whole, but were consumed by fishes once they were crushed. A significant number of nemertean pilidia, asteroid bipinnaria, and cnidarian planulae (which accounted for only 0.2% of meroplankton abundance) were rejected both whole and crushed, suggesting that some species or individuals within these taxa may be chemically defended. Thus, the majority of larvae from this assemblage of temperate meroplankton lacked physical or chemical defenses against potential predators (3 fishes and 1 cnidarian). Among the remaining larvae, physical resistance to predators was much more common than chemical resistance.