David R. Kobluk

Cryptic faunas in reefs; ecology and geologic importance

Every major marine invertebrate and 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 community to 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 of colonies. 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 of light 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 exposed deeper-water benthic habitats receiving the equivalent irradiance levels and spectral composition. An important implication of this is that deep-water surface-dwelling organism groups may be capable of inhabiting very shallow water environments by exploiting cryptic habitats. The recognition of the partitioning of modern 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 applied 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.

Initial diversification of macroboring ichnofossils and exploitation of the macroboring niche in the lower Paleozoic

The traces of macroboring organisms are known throughout the Phanerozoic, with diversification and exploitation of the macroboring niche paralleling variations in the develop- ment of skeletal metazoa. The oldest macroboring biota is an abundant yet low diversity fauna in hardgrounds and reefs of Lower Cambrian age. Following the extinction of archaeo- cyathids at the end of the Lower Cambrian (and thus the demise of skeletal reefs until the Middle Ordovician), boring organisms appear to be restricted to submarine hardgrounds. With the development of skeletal reefs in the Middle Ordovician the mnacroboring fauna shows a rapid speciation and a dramatic increase in diversity. This same pattern occurs again in the Devonian. This record appears to represent refuge of the fauna in low stress, hardground en- vironments when skeletal reefs were not present and radiation in the high stress environment of the reef when large skeletal metazoa were abundant and diverse.

Storm features on a southern Caribbean fringing coral reef

Although there have been many studies that have documented various effects of storms in coral reefs, to date there have been no systematic studies of the depth distribution of strom features and few discussions of the potential for preservation of evidence of strom activity in reefs. After hurricane Gilbert and tropical strom Joan passed through the Caribbean late in 1988, an opportunity became available to study storm features and to relate them to water depth in the leeward reefs of the southern Caribbean island of Bonaire. In January 1989, sixty indicators of storm impact on the reefs of Bonaire were found over the depth range 1.5 m to 37 m at 5 localities

Pre-Cenozoic fossil record of cryptobionts and their presence in early reefs and mounds

The fossil record of cryptobionts (cryptic organisms) in various kinds of hard substrates spans the Phanerozoic, with the oldest known atpresentfrom Ordovician and early Cambrian reefs and reef mounds. The earliest cryptobiontic assemblages support views that there probably was only very weak polarization of surface-exposed and cryptic communities in the early Paleozoic; by the middle Paleozoic polarization seems to have increased, but did not reach levels comparable to that seen in modern reefs until at least the Mesozoic. There are few accounts of cryptobionts in Cambrian and Ordovician reefs and reef mounds, but some generalizations can be made: 1) early Paleozoic cryptic communities were of low to moderate diversity by the standards of the time; 2) cryptobiontic assemblages generally reflected the makeup of the reef surface assemblages, but there appear to be differences in the importance of some groups within cavities compared to outside; 3) the earliest cryptobionts were micro-organisms or very small metazoa; larger cryptobionts are either uncommon, or are dubious cryptobionts.

Hurricane effects on shallow-water cryptic reef molluscs, Fiji Islands

An assemblage of 343 species of cryptic, shelled molluscs was identified in three large samples from shallow subtidal and intertidal shelter habitats under rubble and corals at Malololailai Fiji in 1983, 1984, and 1985. One species was a polyplacophoran, 273 were gastropods (38 families), and 69 were bivalves (21 families). Cryptic gastropods were more abundant than bivalves, but showed a reduction in abundance relative to bivalves from 1983 to 1985. The abundances of many cryptic molluscs show dramatic adjustments from 1983 to 1985, chiefly due to hurricanes in 1983 and 1985, showing a decrease in equitability with increased physical disturbance. The abundance and diversity of molluscan predators in the crypts means that predation in these habitats may be substantial. The gastropod sample diversity showed the greatest change during the 1984 post-hurricane recovery period. The 1985 hurricanes affected the sample diversity by shifting the gastropod diversity closer to what it was after the 1983 hurricane. The bivalves underwent a similar shift in sample diversity, although larger numbers of individuals in proportionately more species survived the hurricanes. The cryptic bivalves exploited space in new crypts, while maintaining their rate of increase in abundance in the recovery period after the 1983 hurricane and through the two hurricanes in 1985. The gastropods declined in abundance after the 1983 hurricane. They recovered after the 1985 hurricanes by doubling their abundance, showing that they could exploit new resources in crypts. This increase in the gastropod population was not proportional to the increase in available cryptic space. This may mean they were still recovering in August 1985, or they may have been unable to capture their portion of cryptic space in competition with other organisms during recovery. The 1985 hurricanes did not have much effect on the overall molluscan diversity, a possible result of pruning by the 1983 hurricane of molluscs unable to survive storms. Because there was only a short interval between the hurricanes, many molluscs that survived the 1983 event were still in the population, so that the cryptic molluscs probably were better able to deal with the effects of the 1985 hurricanes than they would have been before the 1983 hurricane. The result was that the sample diversity after the 1983 hurricane increased during the recovery period but did not decline later even though the population was devastated by hurricanes in 1985. This lends support to intermediate disturbance models linking increasing or stable diversity with disturbances spaced at intervals allowing recovery.

Southern Caribbean cryptic scleractinian reef corals from Bonaire, N.A.

At 14 sites along the west coast of the southern Caribbean island of Bonaire, Netherlands Antilles, 5045 living scleractinian corals in 1101 reef-growth-framework cavities over the depth range 12 m to 43 m were identified and counted. The sample comprises 28 hermatypic coral taxa, the ahermatype Tubastrea coccinea, and the hydrozoan Millepora alcicornis. Although there is no known fossil record of scleractinian reef corals in cavities, there is a record for the taxa living on the reef surface that also inhabit modern reef cavities. This permits speculation that the modern Caribbean cryptic scleractinian coral assemblage may have roots extending into the Tertiary, and at the family level, possibly into the Mesozoic. The common presence of 8 genera, amounting to 44% of the southern Caribbean cryptic coral assemblage, in reef cavities in both the southern Caribbean and the southwestern Pacific, supports the antiquity of the cryptic coral biota, by suggesting that at least those genera may have already been in cavities before the rise of the Isthmus of Panama.

Depth-related associations of cryptic-habitat bryozoans from the leeward fringing reef of Bonaire, Netherlands Antilles

The distribution of modern reefdwelling bryozoans provides a guide for recognizing and interpreting bryozoan zonation in ancient reefs. Toward that end, cluster analysis and gradient analysis were used to examine depth zonation of bryozoan species occupying cryptic habitats on the leeward fringing reef of Bonaire. Both analytical techniques produced clusters of sample depths representing shallow(1-9 m), intermediate(12-33 m), and deepwater (40-61 m) habitats. Similarly, both grouped species into 4 depthrelated associations, including shallow, intermediate, intermediate-todeep, and deep water associations from cluster analysis, and shallow, shallow-to-intermediate, intermediate-to-deep, and deep water associations from gradient analysis. Results were corroborated by canonical discriminant function analyses. In both analyses, interpretable associations of species were recognizable in spite of intergradational distributions down the reef front; most displayed subtle differences in abundances across a range of depths with only a few species restricted to narrow ranges of depths. Gradient analysis produced the more interpretable results because, unlike cluster analysis, a single ordering of species along the depth-gradient resulted. In contrast, terminal branches or subclusters, determined by cluster analysis, may be rotated at branch bifurcations into a variety of species arrangements, with no objective means of determining which permutation most accurately portrays interspecific relationships along the environmental gradient. Additionally, the ordination of species by gradient analysis produced species associations which contained a higher percentage of the total number of colonies sampled across member species than was the case for cluster-defined species groupings. Because taphonomic processes may strengthen the expression of gradients, similar species associations in fossil reefs have a high probability of being recognized, especially through the use of gradient analytical techniques, and may provide a framework for the establishment of paleobathymetry.