Glynn, P.W. 1983. Increased survivorship in corals harboring crustacean symbionts. Marine Biology Letters 4: 105-111.
Crabs and shrimp normally live among the branches of the coral Pocillopora damicornis. Coral colonies possessing crustaceans have a higher survival rate, previously attributed to their defense of the colony against predators. Are there other benefits for the host coral in the absence of predators?
To test this, samples of coral were collected and observed for three months in tanks. Survival, growth, crustacean damage and mucus production were measured. The control group possessed symbiotic Trapezia crabs and Alpheus shrimps; these crustaceans were removed from the experimental group
Of the colonies without crustaceans, 31% experienced massive tissue loss. Polyps retracted, a non-viscous film formed, the tissue disintegrated, then was shed. Significant tissue loss was not observed in the corals possessing crustaceans. The healthier state may be due to increased mucus production and secretion (19% higher). The mucus acts to flush microorganisms, sediment and invading larvae from the tissue, avoiding anoxic conditions and infestations. In laboratory testing, tissues of corals without crustaceans were colonized by ciliate protozoans, and 10X more bacteria were present than in the control corals. The higher rate of mucus secretion may be caused by the healthier state of the coral and/or stimulation of production by crustacean activity. The animals' activity also resulted in some damage to the coral, but this soon healed.
Overall, the Pocillopora colonies harbouring crabs and shrimp demonstrated higher survival and growth rates than those deprived of crustaceans. The relationship is evidently beneficial for the host coral even in the absence of predators.
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