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ItemSusceptibility of invertebrate larvae to predators: how common are post-capture larval defenses?(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.
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ItemReduced mobility is associated with compensatory feeding and increased diet breadth of marine crabs(Georgia Institute of Technology, 1999-11-03) Stachowicz, John J. ; Hay, Mark E.Direct effects of predation have been widely recognized as important in affecting prey population dynamics and evolution. However, less attention has been devoted to the consequences of indirect effects of predators on prey behavior. For example, to avoid predation many animals restrict their activities to physical refugia and adopt low-mobility lifestyles, yet the consequences of these anti-predator behaviors for foraging and diet selection are relatively unknown. In this study we examine the relationships between mobility, feeding preferences, and compensatory feeding for 3 species of marine decapod crabs feeding on seaweeds in North Carolina, USA. Low mobility and high site fidelity of crabs were associated with a broad, non-selective diet and compensatory feeding. The majid Mithrax forceps exhibited the lowest mobility, highest site fidelity, and least selective diet of the 3 species, whereas another majid Libinia dubia was intermediate in both mobility and selectivity, and the xanthid Panopeus herbstii had the greatest mobility and narrowest diet. Of these 3 crabs, only M. forceps compensated for low food quality by increasing consumption rates in single food-species feeding assays. This may be because M. forceps is resistant to (or tolerant of) seaweed chemical defenses, while other crab species are not. The ability to consume, and presumably subsist on, a wide variety of potential foods including those defended from more mobile consumers may facilitate a low-mobility lifestyle, allowing the crab to minimize movement and reduce exposure to predators. Low mobility and high site-fidelity may thus facilitate the formation and use of associational refuges with sessile benthic organisms that are resistant to predators; these associations can have important community and ecosystem-wide consequences.
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ItemMutualism and coral persistence: the role of herbivore resistance to algal chemical defense(Georgia Institute of Technology, 1999-09) Stachowicz, John J. ; Hay, Mark E.Because seaweeds uncontrolled by herbivores can overgrow and kill corals, competition can exclude corals from temperate latitudes where herbivores generally fail to control seaweed biomass. In this study, we show that the coral Oculina arbuscula persists on reefs in temperate North Carolina where seaweeds are common by harboring the omnivorous crab Mithrax forceps, which removes seaweeds and invertebrates growing on or near the coral. In the field, corals from which crabs are experimentally removed develop a dense cover of epibionts, exhibiting reduced growth and increased mortality relative to corals with crabs, which remain epibiont-free. This crab is unusual in that it readily consumes all local seaweeds in laboratory choice assays and is not deterred by chemical defenses that suppress feeding by local fishes, sea urchins, and other crabs. This allows Mithrax to defend corals from overgrowth by chemically noxious seaweeds like Dictyota and Sargassum that are avoided by most local herbivores. However, further field manipulations under reduced light conditions demonstrate that the outcome of the crab–coral interaction is context-dependent: crabs only benefit corals in well-lit areas where seaweeds are abundant. Field observations and tethering experiments show that, by living in association with Oculina, Mithrax gains a refuge from predation. Additionally, crabs grow faster when associated with live corals than with structurally equivalent dead corals, apparently because they are able to consume lipid-rich coral mucus. This mucus provides a dietary supplement that may help corals attract and retain low-mobility symbionts such as Mithrax, securing for the coral long-term and predictable protection against competitors. Structurally complex but competitively inferior organisms, like some corals and coralline algae, provide the biogenic habitat complexity upon which many other species depend. Because these sessile organisms may be dependent on symbionts to remove superior competitors, mutualisms can play an important, but currently underappreciated, role in structuring marine communities where biotic interactions are intense. In this study, the mutualism between Oculina and Mithrax promotes the persistence of both species in habitats from which they might otherwise be excluded by competition and predation.
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ItemReducing predation through chemically mediated camouflage: indirect effects of plant defenses on herbivores(Georgia Institute of Technology, 1999-03) Stachowicz, John J. ; Hay, Mark E.Herbivores often specialize on particular hosts that provide both food and shelter from natural enemies. It is thus often unclear whether a plant’s value as a food or its value as a safe shelter has played a larger role in selecting for specialization. Decorator crabs offer a novel opportunity to investigate the relative effects of diet vs. natural enemies in selecting for specialization because these crabs place plant “shelter” on their backs as camouflage but need not use these plants as food, thus decoupling the plant’s value as a food from its value as a shelter. In this study, we show that juveniles of the decorator crab Libinia dubia selectively decorate with the chemically defended brown alga Dictyota menstrualis but treat this plant as a low-preference food. Common omnivorous fishes that are potential predators of Libinia avoid consuming Dictyota due to the alga’s potent chemical defenses. In the field, juvenile crabs decorated with Dictyota experience significantly less predation than crabs decorated with an alga that is not chemically noxious to local fishes, and the Dictyota metabolite that most strongly deters feeding by fishes is the specific metabolite that determines decoration choice by this crab. Thus, by behaviorally sequestering defenses from this chemically noxious plant, these small crabs reduce their susceptibility to predation. In the presence of predators, juvenile crabs reduce their rate of feeding, but not of decorating, suggesting that antipredator behavior such as decorating takes precedence over feeding. In addition, only juvenile crabs that are of a size that can be consumed by local fishes decorate at all. Adult crabs that have carapace widths exceeding the gape size of co-occurring fishes do not decorate in either the field or the laboratory. Apparently predation, rather than diet selection, drives decoration specialization in Libinia, highlighting how indirect effects of plant secondary chemistry can impact herbivore behavior, ecology, and evolution.