MARINE ECOLOGY PROGRESS SERIES Vol. 224:103-114, 2001
Settlement
of the gregarious tube worm Hydroides
dianthus (Polychaeta: Serpulidae).
I. Gregarious and nongregarious settlement
Robert J. Toonen*,
Joseph R. Pawlik
Department of Biological
Sciences, Center for Marine Science Research,
* Present address: Center
for Population Biology,
ABSTRACT: We conducted still-water time-course
experiments on cultured larvae of the serpulid polychaete Hydroides
dianthus (Verrill, 1873) to examine timing and
patterns of gregarious and nongregarious settlement
to better understand the conditions under which larvae of a gregarious species
colonize new habitats. We first confirmed that these worms are aggregated in
the field, and quantified patterns of association with a number of other
common intertidal species. Patterns
of negative association with spatial competitors is unlikely to have
resulted from larval avoidance during settlement, because we found no
difference in larval settlement in response to conspecifics
with or without the solitary tunicate Ascidia interrupta
present. We then examined the settlement of competent larvae in response
to biofilm and conspecifics
to determine the conditions under which the larvae of this gregarious species
settle nongregariously. Larvae settled concurrently
in response to both biofilm and conspecifics,
starting approximately 4 d after fertilization, and response was of similar
magnitude to both biofilm and to conspecifics
for the first couple of days post-competency. While settlement in response to conspecifics continued in subsequent assays carried out
over 70 d (at which time larvae began to die in culture), settlement in
response to biofilm decreased abruptly after the
peak, and ceased by Day 14. Settlement patterns were qualitatively similar
regardless of previous exposure to substrata; similar results were obtained
whether the whole population of larvae in culture was denied access to or daily
provided access to 1 or both experimental substrata. Furthermore, these
patterns of settlement did not change as larval planktonic
period was prolonged. Of larvae settling during 24 h sample assays, the
majority of larvae metamorphosed in response to biofilm
during the first 6 h, whereas metamorphosis in the final 6 h (between 18 and 24
h) of the assays was primarily in response to conspecifics.
These results are opposite to the pattern predicted by the desperate larva
hypothesis, because some larvae settled in response to biofilm
even after rejecting conspecifics the previous day.
In fact, cultures of competent larvae appear to become more specific in terms
of larval settlement in response to live adult conspecifics
as the planktonic period is artificially prolonged.
KEY WORDS: Colonization, Gregarious settlement, Habitat
choice, Hydroides dianthus, Polychaete