Role In Bioerosion Research Articles

Polyplacophoran Feeding Traces on Mediterranean Pliocene Sirenian Bones: Insights on the Role of Grazing Bioeroders in Shallow-Marine Vertebrate Falls

Chitons (Polyplacophora) include some of the most conspicuous bioeroders of the present-day shallow seas. Abundant palaeontological evidence for the feeding activity of ancient chitons is preserved in the form of radular traces that are usually found on invertebrate shells and hardgrounds. We report on widespread grazing traces occurring on partial skeletons of the extinct sirenian Metaxytherium subapenninum from the Lower Pliocene (Zanclean) of Arcille (Grosseto Province, Tuscany, Italy). These distinctive ichnofossils are described under the ichnotaxonomic name Osteocallis leonardii isp. nov. and interpreted as reflecting substrate scraping by polyplacophorans. A scrutiny of palaeontological literature reveals that similar traces occur on fossil vertebrates as old as the Upper Cretaceous, suggesting that bone has served as a substrate for chiton feeding for more than 66 million years. Whether these bone modifications reflect algal grazing, carrion scavenging or bone consumption remains unsure, but the first hypothesis appears to be the most parsimonious, as well as the most likely in light of the available actualistic data. As the role of bioerosion in controlling fossilization can hardly be overestimated, further research investigating how grazing organisms contribute to the biostratinomic processes affecting bone promises to disclose new information on how some marine vertebrates manage to become fossils.

Functional links on coral reefs: Urchins and triggerfishes, a cautionary tale

Urchins are ubiquitous components of coral reefs ecosystems, with significant roles in bioerosion and herbivory. By controlling urchin densities, triggerfishes have been identified as keystone predators. However, the functional linkages between urchins and triggerfishes, in terms of distributional patterns and concomitant effects on ecosystem processes, are not well understood, especially in relatively unexploited systems. To address this we censused urchins and triggerfishes on two cross-shelf surveys on the Great Barrier Reef (GBR) at the same times and locations. We also evaluated the role of urchins in bioerosion. Although urchin abundance and triggerfish biomass varied by 80% and nearly 900% across sites, respectively, this variability was driven primarily by shelf position with no evidence of top-down control on urchins by triggerfishes. Low urchin abundances meant urchins only played a minor role in bioerosion. We highlight the potential variability in functional links, and contributions to ecosystem processes, among regions.

Echinoid community structure and rates of herbivory and bioerosion on exposed and sheltered reefs

Echinoid–habitat relations are complex and bi-directional. Echinoid community structure is affected by the habitat structural and environmental conditions; while at the same time, echinoids may also act as ‘reef engineers’, able to alter marine environments on a wide geographic scale. In particular, echinoids play a major role in bioerosion and herbivory on coral reefs. Through feeding, echinoids reduce algal cover, enabling settlement and coral growth. However, at the same time, they also remove large parts of the reef hard substrata, gradually leading to reef degradation. Here, we compared coral and macroalgal abundance, echinoid community structure and species-specific rates of echinoid herbivory and bioerosion on reefs subjected to different intensities of oceanic exposure. Spatio-temporal variations in coral and macroalgal cover were monitored, and populations of the four most abundant echinoid species on the coral reefs of Zanzibar – Diadema setosum (Leske), Diadema savignyi (Michelin), Echinometra mathaei (de Blainville) and Echinothrix diadema (Linnaeus) – were compared between the Island's eastern exposed reefs and western sheltered ones. To account for the effect of management in the context of reef exposure, we included marine protected areas (MPAs) of both types of reef categories (i.e. sheltered and exposed) in our comparison. Coral and macroalgal cover presented a conspicuous contrasting pattern across exposed and sheltered sites. While coral dominance and lack of macroalgae were prominent on sheltered reefs, an opposite trend of low coral cover and moderate–high macroalgal cover were found on exposed reefs. Bioerosion was also significantly higher on exposed reefs than on sheltered ones (4.2–13 and 1.2–3.9kg CaCO3m−2year−1, respectively). The highest rates, recorded on Pongwe, with almost 7kg CaCO3m−2year−1, are among the highest echinoid bioerosion rates known to date. Management had a substantial effect on habitat and echinoid community structure, as coral cover was significantly higher, macroalgal cover lower, and echinoid densities generally reduced on MPAs regardless of exposure intensity. Our findings suggest that exposed reefs are susceptible to markedly higher degrees of echinoid bioerosion; however, adequate management measures can significantly reduce these rates, consequently altering the reef's trajectory for degradation.

Sea urchins, macroalgae and coral reef decline: a functional evaluation of an intact reef system, Ningaloo, Western Australia

The number of relatively untouched coral reefs worldwide is rapidly decreasing. Nevertheless, one coral reef ecosystem remains relatively intact: the largest west-continental reef ecosystem in the world, Ningaloo Reef in Western Australia. This study investigated the status of 2 potential bio-indicators for coral reef decline, macroalgae and sea urchin densities, on this reef. Sur- prisingly, both were abundant, with the presence of extensive macroalgal beds in the sandy lagoon and a sea urchin-dominated reef slope. The algal distribution on Ningaloo reflected marked cross- shelf variation in the composition of fish functional groups, with only the back reef and the reef slope exhibiting high grazing rates (completely scraped every 43 and 59 d, respectively). Estimated bioero- sion rates by fishes ranged between 1 and 2.3 kg m -2 yr -1 . Echinoids only played a significant role in bioerosion on the reef slopes owing to their high abundance in that habitat (>12 individuals m -2 ). Here, estimated echinoid erosion equalled that of the most abundant excavating parrotfish, Chloru- rus sordidus. High echinoid and macroalgal abundances on this relatively intact reef system highlight the need for caution when using these metrics for evaluating reef ecosystem condition.

The long–term survival of bone: the role of bioerosion

Fossil bones (N = 350) spanning more than 350 million years, and covering a wide range of depositional environments, were studied to compare the distribution of microbial destruction features in fossil bones with previously published data sets of bones of archaeological age. The distribution of bioerosion in fossil bones is very different from that found in bone from archaeological sites. Fossil bones typically show little or no bioerosion. Under normal conditions, if a bone is to survive into the fossil record, then rapid bioerosion must be prevented (or halted). This conclusion suggests that early post mortem processes,such as the mode of death, influence the potential of any bone to survive into deep time.

Bioeroding foraminifera: a review

Twenty species of foraminifera, mainly from turbulent, warm, shallow-water environments, are known to make cavities in hard substrates. They range from Jurassic (Callovian) to Recent in age. Several hypotheses (feeding, protection, and test-building) have been proposed to explain this mode of life and the processes involved in boring. This review indicates further studies on the biology and physiology of these foraminifera should evaluate their role in bioerosion of hard substrates and the subsequent production of grain-sized sediment that may affect the biotopes of sediment dwellers.

Role of Bioerosion in Mass-Wasting of Pleistocene Outcrops on Georgia Coast: ABSTRACT

Outcrops of semiconsolidated Pleistocene sands, exposed by meandering tidal rivers and creeks in the Georgia estuaries, are undergoing rapid disaggregation by a variety of plants and animals. Bioeroders include large and small mammals, wasps, crabs, isopods, and pelecypods. In this area, bioerosion is responsible for retreat of bluffs as high as 15 m and contributes significantly to mass-wasting and subsequent erosion by currents and waves. Blocks initially break away from the bluffs along fractures created by plant roots. As the blocks move downslope they serve as substrates for numerous supralittoral, littoral, and sublittoral invertebrate organisms which burrow, bore, and scrape the blocks as well as the bluff itself. Different organisms are restricted to distinct vertical zones and the degree of bioerosion increases to a maximum at the low-water line where boulders are riddled with borings, mainly by the isopod Sphaeroma destructor. Multiple isopod borings are in turn further eroded to form large burrows and galleries occupied by crabs. Rates of excavation and colonization of exposed surfaces are rapid, and obvious changes occur in periods of less than 2 weeks. End_of_Article - Last_Page 424------------