Patricia A. Hutchings

Biological destruction of coral reefs

The major agents of biological destruction of coral reefs can be divided into grazers, etchers and borers. Each of these groups is reviewed on a world wide basis, together with the mechanisms by which they destroy the coral substrate. Rates of bioerosion attributed to major agents of grazers, etchers and borers are given, together with limitations of some of the measurements. Recent work is highlighting the variability in rates of bioerosion both over time and space. Factors which may be responsible for this variability are discussed. Bioerosion is a major factor influencing reef morphology and the ways in which this is achieved is discussed in some detail. Although the review concentrates mainly on present day reefs, some attempt is made to consider the impact of bioerosion on older reefs.

Bioerosion experiments at Lizard Island, Great Barrier Reef

The rates at which dead coral substrates are modified by bioerosional processes were determined by exposing recently killed corals for up to four years in a variety of reef environments at Lizard Island (northern Great Barrier Reef). Grazers were the major croding agents of these coral substrates and exhibited differences between sites that varied between sampling periods. Subtidal reef slopes and lagoon environments of water depths < 20 m were subjected to higher average rates of grazing erosion (0.30–1.96 kg/m2/y) than shallow depths less than 1 m (0.07–0.26 kg/m2/y). A deep site at 20 m experienced low average rates of grazing (0.08–0.29 kg/m2/y). Boring rates by worms (polychaetes and sipunculans), sponges and molluscs were relatively low and varied between sites, but increased with length of sampling period as larger borers succeeded the initial colonizing small polychaete worms. We hypothesize from these experiments that the extent of boring in reef substrates will be influenced by the interaction between the succession of the boring community and the rate at which the substrate is destroyed by grazing. We suggest that the level of grazing modifies the successional pattern of borers by removing the surface substrate and continually exposing bare substrate that can be colonized by early boring colonists. Thus, constant high levels of grazing may maintain the boring community at an early successional stage and prevent the development of a “mature” boring community. In order to establish large borer populations, reef substrates must be protected from extensive grazing bioerosion. This interaction of grazing and boring has important implications for the way dead coral is preserved in different reef environments.

Spatial and temporal patterns of non-colonial boring organisms (polychaetes, sipunculans and bivalve molluscs) in Porites at Lizard Island, Great Barrier Reef

A study of the spatial and temporal patterns of colonisation by non-colonial boring organisms to dead Porites coral substrate was conducted at Lizard Island, Great Barrier Reef over a 4 year period. These fluctuations were analysed for each group of borers, and most exhibited strong site preferences, with preferred sites being on the windward slope in 10 m and on the reef flat in 1 m. A lagoonal patch reef site exhibited consistently low colonisation. Most groups showed inter-year variations in colonisation with spring/early summer dominating. These variations are discussed in terms of what is known about their life histories. These results together with those of Kiene (in preparation) which document varying rates of bioerosion, at these sites over the same time period, demonstrate that variations in borer colonisation are responsible for the variations in rates of bioerosion calculated. Thus rates of bioerosion by borers will vary significantly between different reef environments.

Phylogenetic relationships within the Terebellomorpha

Terebellomorpha is a clade of predominantly tube-dwelling polychaetes, some of whose species are very abundant and whose habitats range from shallow to very deep waters. The group contains five families (Terebellidae, Ampharetidae, Pectinariidae, Trichobranchidae and Alvinellidae). This study of their inter-relationships uses DNA sequence data from five gene segments. Including outgroups, sequences were available for 15 species for 15 U2 snRNA, 14 for Histone H3, 23 for the D1 expansion region of 28S rDNA, 15 for the D9-10 region of 28S rDNA and 17 for subunit I of cytochrome oxidase. Outgroups included representatives of the polychaete families Cirratulidae, Sabellidae and Siboglinidae, and the clitellate Lumbricus . These sequences include eight GenBank entries for 28S D1 and one for CO1. Generally, and in all analyses restricted to the data collected in this laboratory, but including all of these, Terebellomorpha is monophyletic. Within Terebellomorpha, the single maximum parsimony tree indicates that Alvinellidae (all data from GenBank) belongs to a clade with Terebellidae and some Trichobranchidae, contradicting morphological expectations. Terebellidae is paraphyletic with respect to Trichobranchus , this being associated with the subfamily Thelepodinae. The second trichobranchid genus Terebellides (for which only 28S D1 data is available from GenBank) is topologically very distinct to Trichobranchus . Additional data are needed to establish the familys monophyly. Also within Terebellidae, subfamily Terebellinae is paraphyletic with respect to Polycirrinae, supporting the suggestion that this subfamilys morphological simplicity is derived.

Initial colonization, erosion and accretion of coral substrate

Blocks cut from Porites lutea were laid on the fore reef slope, reef flat and a lagoonal patch reef at Lizard Island, in the Northern Great Barrier Reef, and replicates removed from each environment at intervals of three months over a period of one and a half years. Variations in bioeroders and bioaccretors were noted. Microfaunas are far more numerous than macrofaunas as block colonizers; the principal borers are polychaete worms, whereas encrusters are molluscs, bryozoans, serpulids and solitary corals. The reef slope is more readily colonised by microfauna pioneer communities than are the other areas. All the environments exhibit a change from cirratulids to either sabellids or spionids (polydorids) over the length of the experiment. Accretion occurred on all blocks during the experiment, with significant differences detectable between environments; both reef slope and reef flat blocks showed weight increases of 9–10% whereas blocks from the patch reef showed increases of 15%. Annual erosion rates produced by polychaete worms are 0.694 kg m-2 year-1 (reef front), 0.843 kg m-2 year-1 (reef flat) and 1.788 kg m-2 year-1 (patch reef).Blocks cut from Porites lutea were laid on the fore reef slope, reef flat and a lagoonal patch reef at Lizard Island, in the Northern Great Barrier Reef, and replicates removed from each environment at intervals of three months over a period of one and a half years. Variations in bioeroders and bioaccretors were noted. Microfaunas are far more numerous than macrofaunas as block colonizers; the principal borers are polychaete worms, whereas encrusters are molluscs, bryozoans, serpulids and solitary corals. The reef slope is more readily colonised by microfauna pioneer communities than are the other areas. All the environments exhibit a change from cirratulids to either sabellids or spionids (polydorids) over the length of the experiment. Accretion occurred on all blocks during the experiment, with significant differences detectable between environments; both reef slope and reef flat blocks showed weight increases of 9–10% whereas blocks from the patch reef showed increases of 15%. Annual erosion rates produced by polychaete worms are 0.694 kg m-2 year-1 (reef front), 0.843 kg m-2 year-1 (reef flat) and 1.788 kg m-2 year-1 (patch reef).

Macrobenthic communities in a tropical lagoon (Tahiti, French Polynesia, central Pacific)

Abstract Soft bottom communities were sampled quantitatively in Tahiti lagoon (French Polynesia) at 18 stations in five zones around the island over 1 year. In addition, various environmental parameters (silt/clay fractions, organic content, chlorophyll and phaeopigment content) were sampled at the same stations over 2 years. The temporal and spatial variabilities of the macrobenthic communities are described and related to these environmental parameters. Each zone ran from the fringing reef to the inner flat of the barrier reef. The macrofauna exhibited a high richness (392 taxa) with an average mean biomass of 1.8 g AFDW m−2 (grams ash-free dry weight per square metre). These communities exhibited temporal but not seasonal fluctuations. The biomass of the macrofauna increased from the fringing to the barrier reefs, and the density of individuals was significantly higher on the fringing reefs. Ordination techniques highlighted four groups of stations characterised by distinctive species composition, density and biomass. The first group included stations located on the inner flat of the barrier reef and in the shallow lagoon area and was characterised by highly diverse communities dominated by polychaetes. The second group primarily included stations from the industrial and hotel zones. This group had the lowest diversity and was also dominated by polychaetes, especially the capitellid Dasybranchus sp. 1. The third group was dominated by gastropods and bivalves. The final group of stations was represented only by station 51, in zone 5, which was characterised by mobile soft sediments and wave action and was dominated by the decapod Hippa cf. pacifica. Presumably, these mobile sediments facilitate the development of this benthic community, which does not occur elsewhere in the lagoon. The diversity and biomass of these benthic communities are low compared with most other areas in the South Pacific.

Ecology of spionid polychaetes in the swash zone of exposed beaches in Tahiti (French Polynesia)

Abstract The high-energy black sand beaches around the island of Tahiti show low species richness. Most of the individuals collected in the swash zone belong to two recently described species of polychaete Scolelepis sp. A, and Scolelepis sp. B (Spionidae). Densities of the first species can reach up to 1900 individuals.m −2 and biomasses up to 47.5 g dry weight (gD.W.).m −2 . These high densities and biomasses occur because of their adaptation to this high-energy habitat, where they are restricted to the swash zone on the beach. Scolelepis sp. A appears to be a suspension feeder and feeding occurs only as the wave recedes. This suggests that the species is highly efficient at obtaining nutrients in this oligotrophic environment. Scolelepis sp. B, sampled in one station, exhibits the same patterns as Scolepis sp. A. Only three other species are present in this environment; carnivores or suspension feeders, they occur in low densities ( −2 ).