Functional Ecology

the organism’s point of view

Tube-building organisms are important ecosystem engineers, but tube-building itself is also an intriguing adapation that diverse taxa have converged on. I am interested in how tubes function from the worm’s point of view, especially in feeding and avoiding predation. Worms in the Diopatra genus are particularly fascinating tube-builders because they industriously decorate their tubes with fragments of algae, shell, and other debris--they use their palps and jaws to pick up and position material while gluing it into the tube wall with secreted mucous. Diopatra species eat the attached algae and smaller organisms living on it, making tube-decorating akin to gardening. (Tube decoration has also been hypothesized to provide camouflage, but my experimental manipulations of tube decration suggest this may not be the case.) This behavior facilitates algal communities, increasing the diversity and abundance of algae and associated fauna in soft-sediment habitats.

I have recently shown that Diopatra cuprea exhibits strong geographic patterns in decorating behavior across a latitudinal gradient in the Northwestern Atlantic. This behavioral pattern has enormous implications for the composition and structure of habitats across the gradient, but also suggests that the selective benefits of tube decorating varies among habitats. Clearly, there are aspects of the tube-worm-environment relationship that we have yet to understand. I plan manipulative experiments to further explore the mechanisms underlying geographic variability in tube decorating behavior.

Decorating behaviors are surprisingly widespread in the animal kingdom: insects, arachnids, decapods, gastropods, polychaetes, echinoderms, and vertebrates all include taxa that festoon their bodies (or their biogenic structures) with foreign matter. For invertebrates, decoration usually provides camouflage and/or a food cache. Overall, decorating has arisen repeatedly in diverse ecological, morphological, and taxonomic backgrounds, generally seems to enhance fitness, and only sometimes requires specialized morphological pre-adaptations. If decorating is so ‘evolve-able’ and useful, then why isn’t it more common? I hypothesized that the energetic costs of decorating offset the benefits, making it selectively advantageous in only a narrow range of circumstances.

I adopted decorator crabs (Oregonia gracilis) as a manipulable system for examining the ramifications of decorating. These spider crabs attach sponges, algae, and bryozoans to specialized velcro-like hooked setae. By experimentally comparing starvation-induced weight loss in decorated versus undecorated crabs, I demonstrated that decorating does incur measurable energetic costs that appear to drive sexually dimorphic ontogenetic shifts in O. gracilis decorating behavior. Female O. gracilis decorate life-long, while males stop decorating at maturity--at the same time that their claws grow disproportionately large. Females prefer large-clawed mates, and large claws are costly to grow and maintain. Thus, adult male O. gracilis forego decorating in lieu of claw growth. Female O. gracilis, in contrast, have small claws and also brood their eggs for 11 months, thus experiencing strong pressure (and lower costs) to retain their camouflage.

Ontogenetic shifts in spider crab decoration appear to depend on the species’ body size at maturity. Both sexes in small-bodied species decorate life-long, and both sexes in large-bodied species stop decorating as adults. I found sexually dimorphic decorating only in medium-sized species like O. gracilis.  Thus, spider crab decorating phenotypes reflect a complex interaction between body size, metabolism, predation risk, and sexual selection in shaping behavioral and morphological patterns both within and among species. 

What do organisms do, how do they do it, and why? Understanding the ecological and evolutionary drivers of behavioral and morphological phenotypes unites much of my work.

© S. Woodin