AIR-GAPING BY THE RIBBED MUSSEL, MODIOLUS DEMISSUS (DILLWYN): EFFECTS AND ADAPTIVE SIGNIFICANCE

¹ Department of Biological Sciences, University of Delaware, Newark, Delaware 19711

1. Normally, the intertidal mussel, Modiolus demissus, does not close its valves when exposed to air by low tides: it air-gapes. Desiccation resulting from water loss is a physical phenomenon and a surface-to-volume relationship causes small mussels to reach the median lethal weight loss of 36-38% more rapidly than large mussels.

2. Evaporative cooling is demonstrable, but no significant difference in high tempertaure survival was observed between gaped and non-gaped mussels. The LD₅₀ for a 10-hour heat stress lies between 36.4 and 37.8° C. and large mussels are more labile than small ones to thermal stress.

3. Median survival times of mussels in gaseous environments are proportional to the volume of oxygen present. Observations of lowered rates of respiration in air are explained by lowered oxygen tensions in mantle cavity fluid in air-gaping mussels.

4. Mussels living in moist marsh mud occupy a higher intertidal position than those living on bridge pilings which are more exposed to the aerial environment. When the intertidal position of bridge mussels is examined as a function of size, the population lies within an ellipse. The upper surface is generated by physical factors: desiccation and thermal stress. Predation and competition are proposed as biotic factors responsible for the lower surface.

5. Air-gaping is probably an adaptation for aerial respiration with desiccation as an unavoidable side effect.

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