Detritus in the epilithic algal matrix and its use by coral reef fishes

Abstract

The epilithic algal matrix (EAM) is a ubiquitous component of coral reefs and is the primary grazing surface for many reef fishes. Detritus accounts for at least 10% to 78% of all the organic matter present in the EAM, variation being attributed to hydrodynamic forces such as wave energy and biological elements such as algal morphology. When compared with filamentous algae, the other major source of organic matter in the EAM, protein:energy ratios, C:N ratios and total hydrolysable amino acids all suggest that detritus is of higher nutritional value than the algae. Lipid biomarkers indicate that more than 70% of the detritus is derived from the filamentous algae but the addition of bacteria and microalgae add essential nutrients and improve the nutritional value of the detritus. The detritus is typically of an amorphic form with protein:energy ratios which indicate that it is capable of sustaining fish growth. Detritus within the EAM may be derived from dissolved organic matter, which reduces refractory material, enhancing the palatability and digestibility of detritus relative to filamentous algae. Detritus in the EAM may also come from settling material and fish faeces. Studies that quantified the amount of detritus ingested by fishes have identified at least 24 species from five families that predominantly ingest detritus. These species represent some of the most widespread and abundant EAM feeding fishes on coral reefs. It is estimated that detritivorous fishes account for at least 20% of individuals and 40% of the biomass of an EAM-feeding fish assemblage on the Great Barrier Reef. Comparisons of ingested material with the EAM indicate that many of these species selectively feed on detritus, particularly the small, organic rich particles <125 μm. Furthermore, analysis of lipids in body tissues of blennies and assimilation of nutrients from the alimentary canal of scarids and acanthurids provide strong evidence that detritus is assimilated by coral reef fishes. Consequently, a large percentage of EAM-feeding fishes on coral reefs can unequivocally be classified as detritivores. The ingestion and assimilation of detritus by these fishes represents a significant pathway for transferring energy from within the EAM to secondary consumers, making detritivorous fishes a critically important component of coral reef trophodynamics.