Date of Award
© 2011 Charlotte Regula-Whitefield
Master of Science in Marine Sciences
Evaluating the net interaction between symbionts can be challenging when one participant is a colonial animal with polymorphic zooids, because each zooid type has unique functions. The colonial hydroid Hydractinia polyclina has three distinct zooid types, each of which contributes particular components to the interaction with host hermit crabs. Of these three zooid types, the function of spiralzooids is not well understood. Previously, spiralzooids have been proposed to contribute a strong negative interaction component by directly reducing their host hermit crab’s reproductive output. However, this hypothesis is not supported by past or current data. I propose that spiralzooids instead function to prevent hosts from foraging on the colonies in their own shells.
I conducted a series of experiments and surveys that explored spiralzooid distribution, structure, and function. Spiralzooid distribution at the species level was quantified through an examination of the scientific literature, which documented that spiralzooids only form in colonies living on hermit crab occupied gastropod shells, regardless of crab species or geographic region. Within the local species H. polyclina, only colonies that were living on gastropod shells occupied by hermit crabs contained spiralzooids, regardless of collection site or colony gender.
Next, I assessed spiralzooid structure. Spiralzooids had a mean length of 1.17 ± 0.62mm and occurred at a mean density of 1.91 ± 1.03 per mm2 along the aperture edge. Spiralzooids contain microbasic eurytele nematocysts, organized into nematocyst batteries, and battery abundance at zooid tips can be categorized into four levels. These nematocyst batteries adhered to hermit crab bodies and appendages, and adherence did not vary significantly among body parts.
Spiralzooid function was studied through several experiments. Increased hermit crab contact and the presence of a host stimulated spiralzooid formation throughout a shell, in parts of the colony that are normally devoid of spiralzooids. Although hermit crabs are active scavengers, those that are symbiotic with H. polyclina have never been documented consuming their own epibiont colonies. Yet, hermit crabs are commonly observed feeding on polyps in colonies on other shells. If spiralzooids prevent crabs from foraging on the colonies on their own shells, then crabs should respond to contact with spiralzooids. The act of spiralzooid lashing, as tested by probing hydroid covered shells (with a bare shell control), significantly altered hermit crab behavior. Six crab behaviors were in turn analyzed to determine their effects on initiating spiralzooid lashing. Foraging on H. polyclina stimulating spiralzooid lashing significantly more frequently than other hermit crab behaviors.
Lastly, hermit crab prey caloric values compiled from the literature were compared to the empirically determined caloric value for H. polyclina (4,011.55 ± 65.47 cal/g dry wt); hydroids ranked in the top 10% of potential hermit crab prey. In light of these findings I suggest the data support my guiding hypothesis that spiralzooids prevent hosts from foraging on their own colonies. Under this proposed function, I suggest that gonozooids actually contribute a weak positive interaction with the host crab by potentially providing caloric value, and spiralzooids in turn contribute a weak negative interaction. Therefore, the resulting net interaction between H. polyclina and Pagurus longicarpus should be considered commensal, or weakly mutualistic.
Regula-Whitefield, Charlotte M., "Invertebrate Zooid Polymorphism: Hydractinia Polyclina And Pagurus Longicarpus Interactions Mediated Through Spiralzooids" (2011). All Theses And Dissertations. 37.