MARINE ECOLOGY PROGRESS SERIES Vol. 224:115-131, 2001.
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,
* Present address: Center
for Population Biology,
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 'desperate' 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