| Zusammenfassung: | Larvae of several marine invertebrates are known to alter their morphology adaptively as a response to changes in conditions, such as food and predator density, within plankton communities. In contrast, nothing is known about plastic re sponses to the density of competitors, which could signal the potential for food limitation or predation risk. We studied effects on trophic morphology of culturing sand dollar pluteus larvae with limiting or saturating food and at low or high density using 1 of 3 potential competitors: feeding conspecifics, feeding heterospecifics, and non-feeding hetero specifics. We hypothesized that feeding competitors would induce a morphological response similar to limiting food, while the effect of non-feeders would depend on whether larvae respond directly to higher density or indirectly to food reduction. Plutei cultured with limited food grew longer feeding arms and shorter stomachs, though only in higher density treatments. Whereas the response to food involved a tradeoff between skeleton and stomach growth, larvae responded to high density by in creasing in vestment in both, reflecting a more complex set of energetic tradeoffs. Similar responses to the presence of feeding and non-feeding larvae implicate a sensory mechanism involving signals from potential competitors rather than a food de cline. Patterns in 2 literature surveys of laboratory experiments and field studies suggest that culture density is a critical but neglected aspect of experimental design in the study of early life-history stages. Larvae are most likely exposed to densities sufficient to induce plasticity following synchronous spawning, highlighting the importance of natural history in understanding density-dependent effects on developmental plasticity.
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