Michael Stachowitsch

Patterns and processes of shell fragmentation in modern and ancient marine environments

Shell fragments are important components of many Recent and fossil marine benthic ecosystems and can provide crucial information on past and present environmental conditions. Interpreting such fragments requires integrated knowledge in various fields and the information potential is therefore rarely optimally utilized. This paper uses the definition of a fragment as being a piece of shell having less than 90% of its original form. It then outlines the potential characteristics, pathways, and fates that shells and their fragments can have. Fragmentation is a key factor shaping the shelly part of death assemblages, but it is difficult to interpret because it can be broadly caused by ecological, biostratinomic or diagenetic processes and also depends on shell strength. Strength, in turn, reflects multifunctionality during ontogeny and depends on a complex set of skeletal and taphonomic factors. Therefore, no particular shell parameter clearly determines strength, but thickness, microstructure type and degree of organic matrix have the strongest influence on pre- and post-mortality strength. Size measurements are usually less important for shell strength than thickness, although ecologically complex size refuges from predation do exist. Similarly, shell shape, sculptural features and specific aperture types (in gastropods) provide various defence strategies rather than increasing strength per se. Key ecological factors of fragmentation include predation due to crushing, peeling, along with mistaken predation, selfinflicted damage during predation and burrowing, and more physical aspects such as impacts by stones. Modern studies must consider damage by benthic commercial fisheries or dredging by scientific vessels. Key biostratinomic factors include transportinduced abrasion (littoral zone of surf-washed beaches), bioerosion and dissolution (mainly sublittoral environments). Diagenetically, fragmentation during compaction mainly occurs when shells are in direct contact with one another or with coarser grains; taphonomic features (e.g., drillholes) have only minor influence. A key step in interpreting fragments is to categorize breakage into repaired versus unrepaired, diagnostic versus nondiagnostic, and severe versus localized damage. Quantifying the above categories can then provide useful information on breakage patterns and underlying processes. Understanding the many characteristics of shells and their fragments is a significant interpretive tool in maximizing the information gain in palaeoecological and taphofacies analyses. D 2003 Elsevier Science B.V. All rights reserved.

On the ecology of a North Adriatic benthic community: Distribution, standing crop and composition of the macrobenthos

The boundaries as well as the macro-epibenthic biomass distribution and composition of a North Adriatic benthic community, dominated by the brittle star Ophiothrix quinquemaculata (D.Ch.), the sponges Reniera ssp., and the ascidians Microcosmus ssp. were determined. Transects totalling more than 80 km (i.e., a recorded area of approximately 150 000 m2) were taken by means of a combined TV- and photocamera sled. The observations, TV-recordings, and photos, together with 392 diver-collected quantitative samples were evaluated. The biomass values were used to establish isobenths. Within the community, the mean biomass, measured as wet weight, amounted to 370 (±73) g/m2 with maxima of more than 1000 g/m2. 64% of the biomass was due to the designating group Ophiothrix-Reniera-Micrcosmus, 87.5% were represented by filter- and suspension-feeding species alone. The mean biomass in the peripheral areas was evaluated at 166 (±62) g/m2. Biomass distribution and composition is examined, and the ecological function and meaning of the observed patterns is discussed.

Effect of hypoxia and anoxia on invertebrate behaviour: ecological perspectives from species to community level

Coastal hypoxia and anoxia have become a global key stressor to marine ecosystems, with almost 500 dead zones recorded worldwide. By triggering cascading effects from the individual organism to the community-and ecosystem level, oxygen depletions threaten marine biodiversity and can alter ecosystem structure and function. By integrating both physiological function and ecological processes, animal behaviour is ideal for assessing the stress state of benthic macrofauna to low dissolved oxygen. The initial response of organisms can serve as an early warning signal, while the successive behavioural reactions of key species indicate hypoxia levels and help assess community degradation. Here we document the behavioural responses of a representative spectrum of benthic macrofauna in the natural setting in the Northern Adriatic Sea (Mediterranean). We experimentally induced small-scale anoxia with a benthic chamber in 24m depth to overcome the difficulties in predicting the onset of hypoxia, which often hinders full documentation in the field. The behavioural reactions were documented with a time-lapse camera. Oxygen depletion elicited significant and repeatable changes in general (visibility, locomotion, body movement and posture, location) and species-specific reactions in virtually all organisms (302 individuals from 32 species and 2 species groups). Most atypical (stress) behaviours were associated with specific oxygen thresholds: arm-tipping in the ophiuroid Ophiothrix quinquemaculata, for example, with the onset of mild hypoxia (< 2mLO(2) L-1), the emergence of polychaetes on the sediment surface with moderate hypoxia (< 1mLO(2) L-1), the emergence of the infaunal sea urchin Schizaster canaliferus on the sediment with severe hypoxia (< 0.5mLO(2) L-1) and heavy body rotations in sea anemones with anoxia. Other species changed their activity patterns, for example the circadian rhythm in the hermit crab Paguristes eremita or the bioherm-associated crab Pisidia longimana. Intra-and interspecific reactions were weakened or changed: decapods ceased defensive and territorial behaviour, and predator-prey interactions and relationships shifted. This nuanced scale of resolution is a useful tool to interpret present benthic community status (behaviour) and past mortalities (community composition, e.g. survival of tolerant species). This information on the sensitivity (onset of stress response), tolerance (mortality, survival), and characteristics (i. e. life habit, functional role) of key species also helps predict potential future changes in benthic structure and ecosystem functioning. This integrated approach can transport complex ecological processes to the public and decision-makers and help define specific monitoring, assessment and conservation plans

Movement, activity pattern, and rôle of a hermit crab population in a sublittoral epifaunal community

Abstract The movements and activity of hermit crabs in a sublittoral soft-bottom community in the North Adriatic Sea (Gulf of Trieste) were monitored by means of observations by a SCUBA diver and time-lapse photography. Tagging experiments showed that the hermit crabs exhibit a pattern of movement which causes them to remain in a defined area for extended periods. During a two-year period, the radius of activity of a crab is estimated to be under 15 m. This leads to a high recovery rate of tagged shells. The average speed of a hermit crab was 2.1 m/h and the average distance travelled per day was 21.6 m as calculated from the analysis of the time-lapse films. This distance is an order of magnitude larger than that estimated from the tagging experiments (1.1 m/day). The activity of the crabs was restricted to the sediment surface between multi-species clumps. The approximate area of the sediment surface traversed by a crab is 0.4 m 2 /day. Many inter- and intraspecific encounters were observed, particularly among hermit crabs and the brittle stars Ophiothrix quinquemaculata (D.Ch.) and Ophiura lacertosa (Pennant). The daily activity pattern of the crabs is related to the light and dark phases on the sediment surface. The high density and conspicuous activity of the hermit crabs point to the important role of the pagurids in the investigated community.

Research on intact marine ecosystems: a lost era.

It is proposed that a new, fifth era should be added to the four historical phases of marine research identified by Rupert Riedl, specifically an era devoted to studying and ameliorating disturbed marine ecosystems. In an age of global environmental deterioration, many marine ecosystems and organisms are high on the list of threatened entities. This poor status prompts research that would otherwise have been unnecessary and hinders research that would normally have been conducted. I argue that research into intact marine ecosystems is becoming increasingly difficult, and that most of our future insights into marine habitats will stem from knowledge gained by examining various disfunctions of those systems rather than their functions. The new era will therefore differ from past research in its underlying aim, the range of topics studied, the selection and funding of those topics, the validity of its conclusions, and in its urgency. Sea turtles and cetaceans are cited as case studies at the organismic level, shallow-water benthic communities, including coral reefs, at the ecosystem level.

Offshore oil platforms and fouling communities in the southern Arabian Gulf (Abu Dhabi)

This study examined the fouling organisms on the legs of offshore oil platforms at two sites in the southern Arabian Gulf (offshore Abu Dhabi, United Arab Emirates). 100% of the metal structures was colonized by encrusting organisms. Both the number of individuals and the total biomass tended to decrease with depth. The total weight of dead shells always exceeded that of living organisms. Sessile filter feeders dominated the biomass, whereas small mobile forms had the largest number of individuals. The biomass at the deeper platform (22 m) was dominated by bivalves, barnacles and bryozoans, while polychaetes and amphipods had the greatest number of individuals. Biomass values here ranged from 1 g/0.1 m2 at 20 m to 147 g/0.1 m2 at 5 m; the corresponding individual numbers were 266 (20 m) and 11,814 indiv./0.1 m2 (5 m). The results at the shallower platform (11 m) differed in several respects: barnacles clearly dominated over bivalves, and sponges exceeded byrozoans, while total individual numbers fell due to a decline in polychaete dominance. Biomass values here ranged from 84 g/0.1 m2 at 10 m to 153 g/0.1 m2 at 0 m; the corresponding individual numbers were 695 (10 m) and 3,125 indiv./0.1 m2 (0 m). The potential role of such fouling communities on artificial structures in the Gulf is discussed.

THE HERMIT CRAB MICROBIOCOENOSIS -THE ROLE OF MOBILE SECONDARY HARD BOTTOM ELEMENTS IN A NORTH ADRIATIC BENTHIC COMMUNITY*

ABSTRACT A soft bottom benthic community at a depth of 23 m in the North Adriatic Sea (Gulf of Triest) was found to contain a high density of hermit crabs (1.88 individuals/m 2 ). This population is composed mainly of the species Paguristes oculatus which inhabits the shells of the gastropod species Aporrhais pes-pelecani, Murex brandaris and M.trunculus . Over 50 species of epifaunal as well as boring invertebrates were found to be associated with the hermit crab occupied shells. Both species of Murex as well as Aporrhais pes-pelecani were frequently found at the base of established multi-species clumps which are characteristic for this bottom and account for the large epifaunal biomass (370 g/m 2 ). The hermit crab microcosm is compared to these clumps in an attempt to determine whether the gastropod shell of the hermit crab forms the basis of the observed sedentary associations. In situ experiments as well as diver taken samples show that as opposed to living gastropods, empty shells and artificial substrates, hermit crab occupancy affords a unique opportunity for the development of a fauna and flora which is capable of survival and further growth once deposited by the hermit crab.

Benthic communities: eutrophication's “memory mode”

Oxygen deficiency in shallow water regions is a key component in the eutrophication phenomenon. Benthic mortalities can therefore serve as one criterion for the degree of coastal marine eutrophication. The present study documents two mortality events (1983, 1988) and the recovery process in a highly developed benthic community in the Gulf of Trieste. Mortality in the more extensive 1983 event proceeded very rapidly, with more than 90% of the macroepifauna biomass being lost within only four days. At the same time, the recolonization process was very slow. Two parameters for community reestablishment were chosen: total biomass and percent composition of the formerly designating species. Compared with premortality values of 390 g wet wt./m2 and 66% contribution by designating species, the corresponding values during the recovery process were low: 1984 (60 g:1%), 1985 (192 g:10%), 1986 (142 g:11%), and 1987 (61 g:39%). The distinct biomass drop in 1987 is attributed to intensive dredging for the bivalve Pecten jacobaeus. A small-scale anoxia in 1988 again led to almost total mortality in the area investigated (2 g:0%). The 1989 values were 18 g:0%. Benthic communities may not only function as a natural eutrophication control; changes on three levels (behavior, survival, recolonization) can also be regarded as a long-term memory of disturbance events. This underscores the importance of benthic studies in the investigation of eutrophication. A hypothetical scheme is presented demonstrating how even a small eutrophication-induced acceleration in the rate of oxygen depletion below the pycnocline could severely affect the benthic community and thus destabilize the entire coastal ecosystem.

Epifauna-dominated benthic shelf assemblages: Lessons from the modern Adriatic Sea

Abstract It has recently been proposed that the northern Adriatic shelf is a living laboratory in which to test the causes of the evolutionary shift from Paleozoic-like, stationary suspension feeders on the sediment surface to modern, infauna-dominated assemblages. The suggestion is that todays “Paleozoic” ecosystems, composed of a modern fauna, are a regular feature in environments of low nutrient levels and predation intensity. We, however, argue that a high-biomass epifauna is not restricted to oligotrophic settings in the northern Adriatic Sea and that predation intensities are instead at Mediterranean levels, which are neither pre-Cenozoic nor similar to those at high latitudes. Environmental requirements of modern, suspension-feeding epifauna do not support the low-nutrient hypothesis, and we suggest that this striking epifauna depends on the presence of stable hard substrata on the seafloor, is very sensitive to sediment input by flood events and storm-induced sediment resuspension, and is related to seasonally high productivity. Elevation above the sediment-water interface has the advantage of feeding from higher-tier levels and helps these organisms to survive hypoxia, which is a typical seasonal feature of the Adriatic shelf and of many ancient epeiric seas. We hypothesize therefore that the gradual disappearance of large, epicontinental seas, along with their low sedimentation rates and frequent bottom-water hypoxia during the Mesozoic, supported the replacement of the archaic epifauna by modern, bivalve-dominated infaunas.

Snorkelling and trampling in shallow-water fringing reefs: Risk assessment and proposed management strategy

Shallow reefs (reef flats <1.5 m) in the northern Red Sea are impacted by growing tourism that includes swimmers, snorkellers and reef walkers but have largely been neglected in past studies. We selected a fringing reef along the lagoon of Dahab (Sinai, Egypt) as a model for a management strategy. Point-intercept line transects were used to determine substrate composition, coral community and condition, and the coral damage index (CDI) was applied. Approximately 84% of the coral colonies showed signs of damage such as breakage, partial mortality or algal overgrowth, especially affecting the most frequent coral genus Acropora. Questionnaires were used to determine the visitors’ socio-economic background and personal attitudes regarding snorkelling, SCUBA-diving and interest in visiting a prospective snorkelling trail. Experiencing nature (97%) was by far the strongest motivation, and interest in further education about reef ecology and skill training was high. Less experienced snorkellers and divers – the target group for further education and skill training – were those most prepared to financially support such a trail. We therefore recommend a guided underwater snorkelling trail and restricting recreational use to a less sensitive ‘ecotourism zone’ while protecting the shallow reef flat. Artificial structures can complete the trail and offer the opportunity to snorkel over deeper areas at unfavourable tide or wind conditions. This approach provides a strategy for the management and conservation of shallow-water reefs, which are facing increasing human impact here and elsewhere.