Life history patterns, social group structure, and mating system of Pederson cleaner shrimps Ancylomenes pedersoni
Type of DegreePhD Dissertation
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Cleaner organisms perform key functional roles in reducing rates of parasitism in marine communities. Pederson cleaner shrimps Ancylomenes pedersoni are major cleaners of reef fishes in the tropical western Atlantic, and form obligate symbioses with host sea anemones. They usually occur in social groups of several different-sized individuals per host; a size-dependent social hierarchy has been proposed to structure these groups, but no quantitative data exist to support this idea. Information about their life history traits and mating system also would contribute to understanding how symbiosis impacts life history evolution in crustaceans, but little is known about patterns of growth and reproduction in this anemoneshrimp. I quantified growth, sexual reproduction, senescence, mortality and social interactions in individuals of A. pedersoni under laboratory conditions, and their abundance and population size structure, including group dynamics, on coral reefs at St. Thomas, U.S. Virgin Islands. Individuals grew rapidly when young, then slowed their growth after reaching sexual maturity at ~ 6 months. Individuals were gonochoric with sexual dimorphism, as shown through long-term growth measurements and experimentally-manipulated groups of shrimps observed for extended periods to document sexual habits. Females were larger than males, and exhibited continuous reproductive cycles in the laboratory. Prior to death at < 2 years, members of both sexes exhibited senescence during which they ceased reproducing, shrank (females only), changed body coloration, and decreased their activity levels over ~ 1-4 weeks. Field populations were abundant and composed mostly of juveniles during both years examined. Populations appeared to be stable, with individuals reaching maximum yield at ~ 4 months of age. On some host sea anemones in the field, shrimp social groups contained individuals that were similar in body size. As such, Pederson shrimps formed loosely-structured social groups, which did not exhibit rigid structure in terms of their relative body sizes on each host sea anemone. However, these shrimp social groups were spatially structured; the distances of individuals from the host anemone tentacles decreased significantly with shrimp body size. Large individuals (usually gravid females) occupied habitat on the anemone tentacle crown, while smaller group members perched on the surrounding substrate, with the smallest individuals located the farthest distance from the host. Under laboratory conditions, these shrimps engaged in size-structured behavioral dominance hierarchies, in which large individuals excluded smaller ones from access to resources (food and habitat). Large females both approached and cleaned client fish models (potential sources of food in the form of ectoparasites), and also attacked and chased smaller shrimps (thus excluding them from prime positions in the culture tank), significantly more frequently than did smaller females, males, and juveniles. Behavioral interactions explained observed microhabitat use patterns of shrimps on hosts in the field. I conclude that a size-based behavioral dominance hierarchy in social groups of Pederson shrimps allows large individuals to monopolize food resources in the form of client fish ectoparasites, and also to occupy habitat space near the centers of host anemones which provides more shelter than do peripheral microhabitats. I conclude that obligate symbiosis with large sea anemones, and cleaner mutualism with reef fishes, both contribute to explaining aspects of the life history of Pederson shrimps, especially their apparent mating system of pure-search polygynandry. This life history information also provides a scientific basis for sustainable fishery management and aquaculture of this key coral reef organism.