Space as a limiting resource among sessile benthic invertebrates: Zebra mussels, freshwater sponges and bryozoans

Thomas Edward Lauer, Purdue University

Abstract

Four species of freshwater sponges (Porifera) and four species of bryozoans (Bryozoa) were identified from six southern Lake Michigan harbors growing in association with the recently introduced zebra mussel (Dreissena polymorpha). Most hard substrates found within the harbors, including wooden pier posts, steel revetment walls, and stone rip-rap, were completely covered by these sessile benthic organisms, suggesting attachment space was a limiting resource. Sponges prefer vertical or overhanging surfaces for attachment sites, and were not found on the soft silt harbor bottoms. Competition experiments using removal techniques in Michigan City and East Chicago harbors indicated sponges were uninhibited by zebra mussel presence and overgrew zebra mussel shells inhabiting hard vertical substrates. In contrast zebra mussels did not overgrow sponge colonies, but did show abilities to re-capture hard substrata if relinquished by the sponge. Sponges as well as the bryozoan, Lophopodella carteri, also monopolized attachment space, rendering it unusable for settling zebra mussel larvae. Conversely, zebra mussels enhanced the successful recruitment of sponge larvae by providing preferential settlement sites. Epibiont growth of freshwater sponges on adult zebra mussel shells negatively impacted the growth and survivability of zebra mussels. Sponge-covered zebra mussels showed a significant reduction in glycogen content $(p=0.003),$ a loss of soft tissue $(p=0.007),$ and an increase in mortality $(p=0.001)$ when compared to non sponge-covered mussels. Predation of zebra mussels by the round goby (Neogobius melanostomus) and the rusty-sided crayfish (Orconectes rusticus) occurred, but did not have an demonstrable effect on the mussel population. In general, sponges exhibit competitive dominance over zebra mussels from the initiation of sponge somatic growth in the spring to sponge growth cessation in autumn. Zebra mussels are able to co-exist in these same habitats due to abiotic factors regulating sponge populations, providing zebra mussels a reprieve during selected portions of the year. The trophic interactions with sponges and bryozoans suggest mechanisms for zebra mussel population regulation, despite the apparent dominance of the benthic community by the zebra mussel.

Degree

Ph.D.

Advisors

Spacie, Purdue University.

Subject Area

Ecology|Freshwater ecology

COinS