Cryptic Faunas in Reefs: Ecology and Geologic Importance

Every major marine invertebratend vertebrate group (except the mammals) uses cavities (cryptic habitats, herein referred to as crypts) in reefs as a domicile, for temporary shelter, or as a source of food. Some crypts can expand in size, allowing the cryptobiontic communityo display all successional stages; pioneering assemblages can thereby maintain themselves by capturing new substrate as it appears. In habitats that have a fixed size (e.g., under boulders), the initial advantage, upon the opening of the habitat, goes to rapid-establishment and rapid-growth organisms (such as solitary forms), which lose dominance as the system matures. More overgrowth-competitive, but often slower-growing organisms (such as colonial forms) then take hold. An additional control that may help explain the dominance of colonial over solitary cryptobiontic forms is the greater longevity ofcolonies. In shallow-water crypts, particularly under mobile rubble, physical disturbance may be the most important mechanism by which community diversity is maintained. In deeper-water crypts, and in some well-protected shallow water crypts, diversity may be determined mostly by predation and competitive networking. There is a relationship between the depth of the entrance to a cavity and its size that governs the difference between the spectral composition oflight entering the cavity and the spectral composition of light that reaches the furthest recesses. The thickness of water filling large cavities makes the irradiance and spectral conditions within the cavity mimic those of much deeper-water exposed benthic environments. This may help explain why on a broad scale, cryptobiontic communities often resemble communities found in exposedeeper-water benthic habitats receiving the equivalent irradiance levels and spectral composition. An important implication of this is that in the use of generalized reef models based upon organism distribution. If cryptobiontic assemblages reflect, at least in part, deeper-water surface-dwelling communities, then there is a need to separate cryptobiontic from non-cryptobiontic assemblages in studies of ancient reef systems. The current paucity of reports of ancient reef cryptobionts appears to be the result of their having been overlooked or included inadvertently within surface-dwelling assemblages, rather than their absence or rarity in ancient reefs. This creates an impression that cryptobionts were rare in ancient reef systems, especially in the middle and early Paleozoic. There is evidence, however, that cryptobionts were significant in early reefs and mounds, and that they were a part of the total reef biota for as long as there have been skeletal reefs and mounds.deep-water surface-dwelling organism groups may be capable of inhabiting very shallow water environments byexploiting cryptic habitats. The recognition ofthe partitioning ofmodern reefs into opensurface (non-cryptic) and cryptic realms at any given water depth has important implications for the interpretation of community structure in ancient reefs and for confidence levels to be applie