Soft coral reefal gardens on unconsolidated substrate

Long description

Subtidal reefal gardens on unconsolidated or intermediate (coffee rock) substrate (shallower than 30 metres depth), typically dominated either by Alcyonarian soft corals and other octocorallians*, or taxa such as sponges, bryozoans, ascidians, hydrozoans or algae. While the short description characterises them as soft coral reefal gardens, equally these ecosystems could be sponge gardens, or dominated by other octocorallians depending on the dominant species present. They may occur as a subdominant of seagrass meadows at lower densities.

Octocorallian is a subclass of the class Anthozoa in the phylum Cnidaria, and include soft corals, gorgonians, sea whips, sea pens, sea fans and octocorals. Like some of the many other anthozoans, Octocorallians are sessile polyp-bearing animals with a mobile larval phase. Octocorallians are distinguished by the eight (i.e. octo) tentacles in each polyp. Most octocorallians do not deposit a rigid calcium carbonate exoskeleton, and therefore tend to attach to reefs rather than contribute to reefal frameworks as per the reef building Scleractinian (hard) corals[4]. Alcyonarian soft corals will grow on the more persistent coffee rock substrates that are alternately covered over by sediments and exposed are more likely to have biota typical of reefal gardens.

Sediment texture may be either gravels or unconsolidated substrates (e.g. coral rubble, duricrust fragments, pebbles etc.), gravelly sands or gravelly muds, with the gravel providing attachment surface for sea fans, sea whips and sponges. Gravel substrates may also be calcareous i.e. shellgrit.

Coffee rock is a soft, dark-brown coloured friable rock consisting of sand grains weakly cemented together by organic matter (mostly plant-based). See type 25 for further information.

Algae present may include erect calcareous algae e.g. Halimeda spp.. When these ecosystems occur close to the coastline, they often occur in high Energy magnitude areas experiencing tidal or current Energy source e.g. steep slope of channels experiencing rapid flushing (species list for Little Woody Island's sponge gardens available[3]). For more information on taxa of these ecosystems, see the Structural macrobiota attribute field. Where Sediment texture grades into sands, muddy sands or sandy muds, these ecosystems may become sparser and grade into subtidal seagrass dominated ecosystems, sandy or muddy ecosystems. Where coral rubble is colonised by hard corals, these ecosystems grade into non-branching hard coral ecosystems not on consolidated substrates.

A typical Alcyonarian soft coral found in these gardens is from the genus Dendronepthya, which captures planktonic prey and lacks the endosymbiotic dinoflagellates (e.g. zooxanthellae) that live in corals and photosynthesise[5].

Similar ecosystems are known to occur on coffee rock reefs (i.e. intermediate consolidation, see type 101).

Special values

High species diversity, provide structure for mobile fauna, notably providing refuges for fish migration where these areas are surrounded by expanses of unconsolidated ecosystems.

Diagnostic attributes

Inundation 'Subtidal'

Consolidation 'Unconsolidated', 'Intermediate', 'Unknown'

Structural macrobiota 'Soft coral – octocorallians'

Qualifiers

Although unmapped, modified Naturalness of coffee rock reefs is relevant as anchoring on these substrates can damage the substrate and soft corals (see Jensen's video). They can also be subject to disturbances such as trawling.

Distribution

May occur on subtidal coffee rock reefs (see type 101) often on the lee of large sand systems and extensive in southern Queensland from Fraser Island southward. Very little inventory exists for these ecosystems. See seagrass types 51, 50, 49, 53, 52, 45, 46, 47, 48. They can also occur on gravels see types 106 and 107.

Dendronephthya australis soft coral gardens exist in Port Stephens, New South Wales[2]

The following relates to distribution of this ecosystem type within the Central Queensland mapping area:

• Includes areas of Port Curtis and Little Woody Island's sponge gardens in the Great Sandy Strait
• Soft corals grow on coffee rock reefs in eastern Hervey Bay, and in the Great Sandy Strait Ramsar area at at Fig Tree, Seary’s Ledge and other locations
• Coffee rock reefs with subtidal soft coral are generally on low-moderate energy shorelines, such as the Fraser Island side of the Great Sandy Strait and in Platypus Bay. A good example of a soft coral garden on coffee rock is Seary’s Ledge (see Jensen's video).

Comments

Other relevant attributes include Terrain morphology, Benthic depth (this type includes shallow and deep water less than 30 metres deep) and Terrain slope for example subtidal deep depressions (swales) and channels between ridges (dunes) are known to have high diversity biota resembling that of reefal gardens, found on shelly gravels off Rooney’s Point. Energy source and Energy magnitude are relevant as these deep reefal gardens experience high seabed stress (current stress) as evident in modelling[1][6].

Additional Information

Soft Corals of the Great Barrier Reef - eAtlas

Tin Can Bay (Cooloola) marine biodiversity assessment: species list and 4K underwater video footage

Classification - corals of the world

References

1. ^ CSIRO (2018), eReefs CSIRO Hydrodynamic model (Temperature, Salinity, Wind, Current) Summaries (AIMS). [online] Available at: https://eatlas.org.au/data/uuid/375c9dc9-70f8-4d97-a3e9-c013896a81e5 [Accessed 5 April 2019].
2. ^ Davis, T, Harasti, D & Smith, S (1 January 2015), 'Extension of Dendronephthya australis soft corals in tidal current flows', Marine Biology. [online], pp. 2155-2159. Available at: https://epubs.scu.edu.au/esm_pubs/2780.
3. ^ Devantier, LD (2010), Reef-building corals of Hervey Bay, South-East Queensland. [online] Available at: https://www.researchgate.net/publication/285100297_DeVantier_2010_Reef-building_Corals_of_Hervey_Bay_South-east_Queensland.
4. ^ Fabricius, K (2010), 'Octocorallia', in Encyclopedia of Modern Coral Reefs, pp. Chapter-35.
5. ^ Fabricius, KKE & Alderslade, PP (1 June 2001), Soft Corals and Sea Fans: A comprehensive guide to the tropical shallow water genera of the central-west Pacific, the Indian Ocean and the Red Sea. [online], Australian Institute of Marine Science (AIMS). Available at: https://www.researchgate.net/publication/258516656_Soft_Corals_and_Sea_Fans_A_comprehensive_guide_to_the_tropical_shallow_water_genera_of_the_central-west_Pacific_the_Indian_Ocean_and_the_Red_Sea [Accessed 24 March 2019].
6. ^ Mason, L (2000), BSTRESS_GRID (Grid of Bottom Stress). Produced by James Cook University for the Great Barrier Reef Marine Park Authority, Great Barrier Reef Marine Park Authority, Townsville, Queensland.

Last updated: 19 July 2019