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Marine Ecology Progress Series

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MEPS 224:115-131 (2001)  -  doi:10.3354/meps224115

Settlement of the gregarious tube worm Hydroides dianthus (Polychaeta: Serpulidae). II. Testing the desperate larva hypothesis

Robert J. Toonen*, Joseph R. Pawlik

Department of Biological Sciences, Center for Marine Science Research, University of North Carolina at Wilmington, Wilmington, North Carolina 28403-3297, USA
*Present address: Center for Population Biology, Section of Evolution and Ecology, University of California, Davis, California 95616, USA. E-mail:

ABSTRACT: We have documented patterns of gregarious and nongregarious settlement among the larvae of the gregarious tube worm Hydroides dianthus (Verrill, 1873) and shown that larvae do not exhibit decreased substratum-specificity throughout a prolonged planktonic period regardless of prior exposure to experimental substrata. Previous investigations with barnacles and polychaetes have suggested that the colonization of new substrata occurs because larvae become less discriminating as they age, i.e. that they become Œdesperate¹ to settle after searching unsuccessfully for conspecifics for some period of time. This hypothesis, first proposed by Knight-Jones & Wilson, is based on an energetic model in which non-feeding (lecithotrophic) larvae continue to search for specific substrata as long as their energetic reserves allow, but begin to accept sub-optimal habitat rather than exhaust their reserves and die without metamorphosing. Here, we examined whether it is possible to induce decreased substratum-specificity among competent larvae of the gregarious tube worm H. dianthus, which has feeding (planktotrophic) larvae. We show that neither altered feeding regimes nor larval starvation lead to decreased substratum-specificity among competent larvae; although larvae maintained at lower food concentrations take longer to reach competency, the qualitative patterns of settlement on biofilm and conspecifics is unaltered by feeding regime. Furthermore, starving competent larvae results in a loss of competency rather than larval desperation. Larvae belonging to different size classes (<79, 80 to 99, 100 to 126, and >127 µm) showed similar patterns of settlement, and differences among sibling cultures with different mean larval sizes resulted from a decrease in the proportion of larvae settling in response to conspecifics rather than an increase in the proportion settling in response to biofilm. We examined a number of obvious life-history characters for correlations with the tendency for larvae to settle nongregariously, and found that although a variety of life-history traits showed significant correlations, only the total number of eggs spawned by a dam was significantly correlated with the proportion of larvae settling in response to biofilm (r2 = 0.19), and the slope of this relationship was negative. These results are again diametric to predictions of the desperate larva hypothesis, and indicate that larval desperation is unlikely to be a general explanation for the initiation of monospecific aggregations of fouling marine invertebrates.

KEY WORDS: Colonization · Gregarious settlement · Habitat choice · Hydroides dianthus · Polychaete

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