Abstract
The death of over 50% of corals during the 1982-83 El Nino and subsequent increases in populations of the bioeroding echinoid Diadema mexicanum have destroyed large areas of reef frameworks. At Uva Island, Panama, calcium carbonate erosion reached 14 kg/m$\sp2$/y, causing 22 mm/y vertical erosion and 44 mm/y recession of framework walls. Infauna, Diadema and fishes were the most important eroders, destroying 74%, 15% and 11% of frameworks. Pocilloporid corals and crustose coralline algae were the primary carbonate producers, contributing over 3 kg/m$\sp2$/y. A budget of carbonate production, erosion and sediment retention revealed net erosion of the 2.5 ha reef at 2.9 tonnes/y. The balance varied from 1.85 kg/m$\sp2$/y in the back reef to $-$9.68 kg/m$\sp2$/y in the seaward reef base.The association between damselfish and algal lawns, herein proposed as a multi-species (plural) mutualism, can protect reef frameworks from erosion. At the Uva reef, the damselfish Stegastes acapulcoensis defends territories against invading Diadema mexicanum diurnally, with a positive correlation between damselfish responsiveness and lawn quality, fish size and Diadema density. Diadema moved out of crevices and into damselfish territories at night, but avoided lawn algae, even in the absence of damselfish aggression. Laboratory experiments demonstrated that D. mexicanum avoid algal textures rather than chemical feeding deterrents.Most aggressive encounters between damselfish and Diadema occurred in the fore reef slope and seaward reef base--where erosion by D. mexicanum is greatest. Protection by the damselfish/algal lawn symbiosis reduced vertical erosion from 22 mm/y in unprotected regions to 6 mm/y in damselfish territories. In the absence of the damselfish-algal lawn symbiosis, total erosion at the Uva reef should increase by almost 2.1 tonnes/y. Under current conditions of low carbonate production and high echinoid erosion, the damselfish-algal mutualism is important in preserving reef frameworks.Approximately 1% of Diadema mexicanum at the Uva reef were parasitized by the prosobranch gastropod Echineulima thanuumi. The parasites castrate their hosts by devitalization and ingestion of gonadal tissues. Infested individuals were significantly smaller than the population mean. The parasite probably attaches to young echinoids and reduces host growth and/or longevity.