Selective Herbivore Increases Biomass of Its Prey: A Chiton‐Coralline Reef‐Building Association

The intimate association between a selective herbivore (the chiton Choneplax lata) and its primary prey (the crustose coralline alga Porolithon pachydermum) results in increased biomass and accretion of the alga. This process, over ecological and geological time scales, comprises a major component of Caribbean reef—building systems. Manipulative experiments showed that as the chiton grazes the alga it stimulates new meristematic activity and removes sporlings of the competitively superior frondose and filamentous algae, thereby increasing the survival rate of P. pachydermum on the intertidal reef crest. Furthermore, in the absence of C. lata, overgrowths of frondose and filamentous epiphytes provide an attractive food source for parrotfishes (Scaridae), which accelerates bioerosion of the coralline reef—crest structure due to the deep rasping action of feeding activity. Algal removal experiments suggest that the role of P. pachydermum is to provide a predictable food source and refuge substratum, which increases survivorship of the burrowing chiton by minimizing expenditure of energy during foraging and risk of predation. The chiton/coralline alga association is abundant throughout tropical western Atlantic islands and augments reef—building processes on the shallow algal crest portion of Caribbean reefs. Cover of the Choneplax/Porolithon association in the Belize Barrier Reef crest averages 13% (maximum to 70%) with a mean chiton density of 664 individuals/m2 within the association. On average, the extensive networks of interconnected chiton burrows extend between 6 and 10 cm deep and contain one C. lata for every six openings, with the majority of animals (66%) ranging from 16 to 30 mm in length. Gut contents of the chiton consist predominantly of P. pachydermum (51%), followed by bacterial detritus (30%), Cyanophyta (13%), Bacillariophyta (3%), and fleshy microalgae (3%). The close SEM (scanning electron microscope) match between radular morphology of C, lata and grazing scars on the thallus surface of P. pachydermum shows how the chiton regularly feeds on the coralline alga without causing mortality. Virtually all P. pachydermum in the vicinity of C. lata burrows contain radular tract scars of °10 @mm in depth, whereas the photosynthetic meristematic, and reproductive tissues of coralline lie below 20 @mm. P. pachydermum under intense chiton grazing is photosynthetically competent with 0.1 mg C fixed°g—1 organic dry mass°—1, which is not significantly different from ungrazed material and within the range of rates for other crustoe coralline algae. The result is continuous net accretion at a mean rate of 2.3 mm/yr.