A Not So “Boring” Interaction

Sometimes words can take on very different meanings depending on context – a term might mean something very different in everyday usage as compared to a scientific application. If someone heard a bivalve described as “boring” in passing, they might assume that the speaker is referring to a less-than-interesting clam or mussel. However, “boring” can also refer to bioerosion, the process of living organisms breaking down calcium carbonate. The authors of this new paper (Microalgae, a Boring Bivalve, and a Coral – A Newly Described Association Between Two Coral Reef Bioeroders Within Their Coral Host) describe a novel species interaction that is anything but boring!

Bioeroding organisms can range in size from large to small, occupy different habitats, and use varied types of mechanical and chemical methods of breaking down calcium carbonate structures. Depending on the ecological interactions, bioeroders may enhance or suppress erosion within a coral reef habitat. One charismatic bioeroder is the excavating parrotfish, which breaks down carbonate and helps produce sediment in reef habitats while also helping determine which organisms live in a particular reef habitat. Indeed, the relationships between a specific bioeroder and a coral can be complicated, often involving multiple participants and a suite of costs and benefits within each set of interacting species. This study focuses on the interactions between boring bivalves (mussels in the subfamily Lithophaginae), microalgae, and a coral (Isopora palifera) in the Heron Island reef lagoon off of the east coast of Australia.

Isopora palifera has variable morphology and often hosts boring mussels; the authors also observed dense green “halos” of microalgae around the bore holes created by the mussels. They conducted surveys of 45 I. palifera colonies at low tide, leaving 15 m between patches to help reduce the likelihood of sampling clonal colonies, and collected coral branches with and without bivalve inhabitants. Sample processing in the lab included removal of photosynthetic dinoflagellates and macroborers, isolation of endolithic microbial material, and the measurement of coral skeletal volume, chlorophyll concentration, and biomass.

Coral branches with bivalve inhabitants had a greater endolithic microbial biomass than those without a macroborer (mussel) and the mean chlorophyll a and b concentrations were greater as well. Chlorophyll b is the primary photosynthetic accessory pigment found in microendolithic green algae living in coral skeletons, which suggests that these microalgae might account for much of the microbial material isolated.

When analyzing a symbiotic relationship, it is always important to consider the costs and benefits to participants. In this system, the bivalve borer might add nitrogen into the coral tissue, which could suggest a mutualistic relationship between the coral and the bivalve. Carbon dioxide produced by the bivalve could diffuse into the skeleton and provide photosynthetic microbes with an additional source of this important input for photosynthesis. The microalgae, in turn, might supply the bivalve with oxygen, a byproduct of photosynthesis. But there are also costs embedded within these relationships – for example, the very act of boring could weaken the structure of coral and invite other microborers in by lowering the amount of energy required for successful colonization. 

Additional work on this and other similar coral reef systems will help clarify the roles of the different participants and elucidate the associated costs and benefits. With additional information about the roles of bioeroders on reefs, it may become increasingly possible to tease apart whether each particular interaction represents a mutualism (where the benefits outweigh the costs for both participants), commensalism (where one participant benefits and the other is unaffected), or parasitism (where one participant benefits and the other is harmed). 

Katie Dobkowski is a marine biologist and seaweed enthusiast. She is currently a visiting assistant professor of marine ecology at Bates College in Lewiston, ME. Her website is kdobkowski.com

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