The Biogeography of the Austra

C H A P T E R

7

Benthic Shelf and Slope Habitats

7.1 THE CONTINENTAL SHELF

On the inner shelf to depths of around 50 m, communities of seagrasses,thaloid algae, and surface-dwelling microorganisms may be an importantsource of primary production. There may also be particulate organic fooddelivered from the land via river discharge. But on the middle and outershelf, below the photic zone and beyond much influence of terrestrial eco-systems, primary producers are largely absent, and benthic communitiesare essentially assemblages of secondary producers and their predators,relying on delivery of organic material from the photic zone of the ocean.In middle and outer shelf benthic habitats, suspension feeding and detri-tus gathering predominate. Nevertheless, biodiversity in deeper benthicshelf habitatsmay be very high, especially in areas like the outer shelfmar-gins of the North West Shelf where there is upwelling from the deep sea.

Soft substrate communities vary enormously in their composition accor-ding to the nature of the substrate. Mud, grades of sand, and gravel offerdiverse opportunities and constraints for infaunal living. Suspension-feedinganddeposit-feedingpolychaetes, bivalvedmolluscs, echinoderms,and small crustaceans dominate these communities, along with theirpredators. In keeping with their diverse adaptations to varied sedimenttype and sea floor water movements, soft substrate infaunal communitiesare characteristically extremely patchy in their spatial distribution.

Hard substrate fauna are predominantly sessile invertebrates such assponges, alcyonarians, and bryozoans, all filter-feeders that live attachedto the firm substrate. These assemblages support diverse assemblages ofpredatory and commensal invertebrates and provide important habitatfor deep-sea demersal fishes.

The demersal fish fauna of theNorthWest Shelf and its continental slopeis moderately well known as a result of fishery stock surveys1–6 and studiesof fish species distribution and stock variability.7–9 Benthic invertebratesof the continental shelf in the region are less well known. The Swedish

259lian North West Shelf © 2013 Elsevier Inc. All rights reserved.

260 7. BENTHIC SHELF AND SLOPE HABITATS

Mjoberg Expedition to Australia (1910–1913)10 and H.L. Clark (1929 and1932)11made collections of benthic invertebrates dredged on the inner shelfof the Canning Bioregion. Since then staff of the Western AustralianMuseum have participated in various trawling operations, for example,CSIRO scampi resource surveys aboard the vessels Courageous and Soela.1,2

As a result of these collecting ventures, museum collections have providedsource material for taxonomic studies on invertebrates that have revealedthe high biodiversity of the benthic shelf fauna, but this large componentof the North West Shelf marine fauna remains imperfectly known.

An indication of the poor state of knowledge about the NorthWest Shelfbenthic shelf invertebrate fauna may be seen in a summary of availableinformation on the ecology of the Browse Basin by Geoscience Australiawhich searched the online Ocean Biogeographic Information Systemdatabase and found records of just 53 species of benthic molluscs(R. Przeslawski, personal communication, December 2009). In a review ofkey ecological features of the northwest marine region, a Geosciencereport12 noted that “one of the challenges in determining the ecologicalimportance of the proposed KEFs is the absence of comprehensive biologi-cal datasets.”

There have been just a few systematic studies of benthic invertebratecommunities on theNorthWest Shelf. An intensive survey of benthicmol-luscs of the inner shelf in the Dampier Archipelago has been carried out.13

The report includes a taxonomic list of the molluscs sampled, informationon their diverse habitats, and a discussion of species assemblages and bio-geographic affinities. This is the only published account of this kind.

Fry et al.14 carried out a dredging and video transect survey of innershelf benthic habitats in the Canning Bioregion15 aimed at defining thedistribution and extent of the major benthic habitat types in that part ofthe Kimberley rather than species distribution patterns.

Themost comprehensive benthic survey in the region has been done byGeoscience Australia in the eastern part of Joseph Bonaparte Gulf that hascharacterized the diverse benthic habitats of the Sahul Shelf and producedlarge collections of invertebrates that await taxonomic studies.15,16

7.1.1 Soft Substrate Communities

The sediments of the North West Shelf are described in Chapter 2.1.4.With the exception of areas of terrestrial sediment in the nearshore zoneof the inner shelf in the Kimberley and central Pilbara bioregions, shelfsediments of the region are primarily biogenic carbonates, generally condu-cive to development of diverse infaunal invertebrate communities. Acrossthe width of the shelf, there are distinct zones of sediment type (Figure 2.9),and great variation in the species composition of soft substrate communitiesmay be expected.

2617.1 THE CONTINENTAL SHELF

7.1.2 Hard Substrate Benthic Shelf Communities

In Section 2.1.4.1, note was made of the prevalence of exposed rockpavement on the inner shelf of the Pilbara Bioregion. Also, in the Kimber-ley Bioregion and on the Sahul Shelf, there are extensive areas of base rockexposed on topographic highs and in scour channels on the sea floor of theinner shelf. It is known that there are significant areas of hard substratealong terraces of the middle and outer shelf, most notably the LGM shore-line terrace along the 120 m contour that stretches intermittently from atleast the Montebello Islands to the Sahul Banks (Section 2.1.6). Industryvideo surveys in these areas indicated the presence of dense, species-richepifaunal assemblages, predominantly sponges and alcyonarians. Diverssearching for specimen shells at depths to 60 m in the vicinity of theMuiron Islands also report dense “sponge gardens” that dominate thesea floor (Figure 7.1).

Existing information on the sponges of northern Australia has been col-lated, revealing a species-rich sponge fauna in the region, although mostof the records are from coral reef communities, and the epifaunal speciesof benthic shelf habitats have received little attention to date.

The widespread epifaunal, filter-feeding community in benthic shelfhabitats of the North West Shelf remains almost undescribed. Surveysto determine its taxonomic composition and spatial distribution areneeded. This community appears to be especially significant along the

FIGURE 7.1 “Sponge garden” habitat: an example of the epifaunal filter-feeding commu-nity, dominated by sponges and alcyonarians, that is, prolific on rocky substrata on the innershelf of northern Western Australia; 21 m, off Point Murat, Exmouth Gulf, Ningaloo MarinePark. Photo: Peter Clarkson.

262 7. BENTHIC SHELF AND SLOPE HABITATS

shelf margin, supported by the shelf-edge upwelling and mixing of ITFand coastal water, representing an important, extensive, and diverse ele-ment in the biodiversity of the Australian marine fauna.

7.2 CONTINENTAL SLOPE (BATHYAL)

The slope, terraces, and plateaux from depths of 200 to 2000 m arereferred to as the bathyal or deep-sea zone. The bathyal zone and the abys-sal and hadal zones below it are referred to as the deep sea. In much of thehydrocarbon resource area of the North West Shelf, the sea bed lies in thisdepth zone.

Sediments of the North West Shelf slope are carbonates, generally siltysands composed of skeletal remains of pelagic foraminifera. Ripple marksindicate bottom currents, but apart from these, the topography of the sea-bed is regular and featureless except for the high biohermic banks such asthe Rowley Shoals and Scott and SeringapatamReefs on the slope terraces.Typically, the physical conditions of these benthic bathyal habitats arerelatively uniform and constant compared to the benthic shelf. However,reviews of the world’s deep-sea benthos17 show that this ecosystem ispatchy but with surprisingly high biomass in places and with high levelsof species-richness that surpass that of many terrestrial and other marinesystems that are commonly regarded as supporting high biodiversity.

The most abundant and species-rich component of the bathyal fauna issmall infaunal invertebrates, predominantly polychaetes, nematodes, fora-minifers, crustaceans, and bivalvedmolluscs.18 Thesemay be in dense com-munities, feeding on detrital organic material sinking to the sea floor fromthe water column. There are also infaunal pogonophoran and sipunculidworms and many epifaunal species including echinoderms, sponges, andcoelenterates. Supported by this trophic base of secondary producers areassemblages of predatory crustaceans, gastropod molluscs, and fishes.

The deep-sea benthic bathyal fauna of Australia, especially that of theNorth West Shelf, is little known.19 A review of key ecological features inthe Northwest Marine Region12 provides a summary of knowledge of thedeep-sea environment and identified databases of relevant informationand specimens from the region. Two systematic accounts of benthic slopeinvertebrates in the region deal with ophiuroids20 and azooxanthellatecorals21 and include references to species that inhabit this depth zone.

There is some information on demersal fishes of the continental slopeincluding the results of trawling conducted by CSIRO. In the 1970s, ascampi trawling industry operated on the Rowley Terrace. There havebeen several inspections of the sea floor, using ROV technology, and somegrab sampling on behalf of the petroleum and gas industry operating inthe region. For the most part, the reports of these surveys are not publiclyaccessible. Nevertheless, some fish and invertebrate specimens from these

2637.2 CONTINENTAL SLOPE (BATHYAL)

surveys and from the commercial scampi trawlers have been deposited inthe collections of the Western Australian Museum. In particular, there aresamples of benthic molluscs, notably from the scampi grounds in thevicinity of the Rowley Shoals at depths from 300 to 450 m and from grabsamples taken at the Pluto gas field off the Montebello Islands.22

Of particular note are specimens of gastropods (e.g., the pleurotomaridPerotrochus westralis and unidentified species of the trochid genusCalliostoma) that feed on organic detritus gathered from the sedimentsurface, deposit-feeding bivalves of the genera Verticordia, Poromya, andAmygdalum, and many species of large predatory gastropods of thefamilies Olividae, Volutidae, Muricidae, Conidae, and Turridae. Thesegastropods are mostly vermivores. Their variety and numbers indicatethe presence in these bathyal habitats of a dense and diverse infaunalcommunity of small detrital-feeding invertebrates. A selection of the shellsof these predatory gastropods is illustrated in Figure 7.2.

FIGURE 7.2 A selection of large predatory gastropods of the bathyal zone trawledat depths of 300-660m on the Rowley Terrace, on the continental slope of theNorthWest Shelf.(A) Mipus vicdani (Muricidae), (B) Comitas sp. (Turridae), (C) Pinguigemmula philippensis(Turridae), (D) Conus teramachii (Conidae); (E) Thatcheria mirabilis (Turridae), (F) Teramachia

dalli (Volutidae), (G) Conus ichinoseana (Conidae), (H) Bathytoma atractoides (Turridae),(I) Teramachia johnsoni (Volutidae), (J) Gemmula unedo (Turridae).

264 7. BENTHIC SHELF AND SLOPE HABITATS

From the biogeographic perspective, these molluscs appear to be rep-resentative of taxa that are widespread bathyal species in the Indo-WestPacific realm, although this may be a simplistic assumption based on littleinformation. Some of the large gastropod species are described fromcomparable habitats in the Western Pacific. They have no biogeographicaffinity with the benthic molluscan fauna of the adjacent shelf and repre-sent a deep-sea fauna that probably has had a quite different evolutionaryhistory to that of the shelf fauna. They are likely to respond to disturbancein different ways. The bathyal fauna along the margins of the North WestShelf is a significant element of the Australian marine biota that appearslikely to be rich in species and patchily dense in biomass but which isvirtually undescribed.

References

1. Sainsbury KJ, Kailola PJ, Leyland GG. Continental shelf fishes of Northern and NorthWesternAustralia: an illustrated guide. London: John Wiley and Sons; 1985.

2. Sainsbury KJ. Assessment and management of the demersal fishery on the continentalshelf of north western Australia. In: Polovina JJ, Ralston S, editors. Tropical snappersand groupers, biology and fisheries management. Boulder, USA: Westview Press; 1987.p. 465–503.

3. Nowara G, Newman S. A history of foreign fishing activities and fishery-independentsurveys of the demersal finfish resources in the Kimberley region of Western Australia.Department of Fisheries, Report no. 125; 2001.

4. Newman SJ, Dunk IJ. Growth, age validation, mortality, and other population charac-teristics of the red emporer snapper, Lutjanus sebae (Cuvier, 1828) off the Kimberley coastof north-western Australia. Estuarine Coastal Shelf Sci 2002;55:67–80.

5. Newman S, Evans D. Demersal finfish resource assessment survey of North West Slopeof Western Australia. FRDC Report, no. 1998/152, Department of fisheries, North Beach,Western Australia; 2002.

6. Last P, Yearsley G, Gledhill D, Gomon M, Rees T, White, W. Validation of nationaldemersal fish datasets for the regionalisation of the Australian continental slope andouter shelf. Report to the National Oceans Office, Sept., 2004, CSIRO Marine Research,Hobart; 2005.

7. Holliday D, Beckley LE, Weller E, Sutton AL. Natural variability of macro-plankton andlarval fishes off the Kimberley, north-western Australia: Preliminary findings. J R Soc

West Aust 2011;94(2):181–94.8. Hutchins JB. Biogeography of the nearshore marine fish fauna of the Kimberley,Western

Australia, In: Seret B, Sire J-Y, editors. Proceedings of the Fifth Indo-Pacific FishConference, Noumea 1997; 1999. p. 99–108.

9. Travers MJ, Potter IC, Clarke KR, Newman SJ, Hutchins JB. The inshore fish faunas oversoft substrates and reefs on the tropical west coast of Australia differ and change withlatitude and bioregion. J Biogeogr 2010;37(1):148–69.

10. Mortensen TH, editors. Results of Dr. E. Mjoberg Swedish Expedition to Australia,1910–1913; 1918.

11. Clark HL. The echinoderm fauna of Australia. Publ Carnegie Inst 1946;566:1–567.12. Falkner I, Whiteway T, Przeslawski R, Heap AD. Review of ten key ecological features

(KEFs) in the northwestmarine region. GeoscienceAustralia Record 2009/13. GeoscienceAustralia, Canberra; 2009.

2657.2 CONTINENTAL SLOPE (BATHYAL)

13. Taylor JD, Glover EA. Diversity and distribution of subtidal benthic molluscs from theDampier Archipelago, Western Australia; results of the 1999 dredge survey (DA2/99).Rec West Aust Museum 2004;(Suppl. 66):247–91.

14. Fry G, Heyward A, Wassenberg T, Colquhoun J, Pitcher R, Smith G, et al. Benthic habitatsurveys of potential LNGhub locations in the Kimberley region. A joint CSIRO andAIMSreport for the Western Australian Marine Science Institution; 2008. 131 pp.

15. HeapAD, Przeslawski R, Radke LC, Trafford J, Battershill C. Seabed environments of theEastern Joseph Bonaparte Gulf, Northern Australia: SOL4934—post survey report.Geoscience Australia Record 2010/09; 2010.

16. AndersonTJ, Nichol S, Radke LC,HeapAD, Battershill C,HughesMG, et al. Seabed envi-ronments of the Eastern Joseph Bonaparte Gulf, NorthernAustralia: GA0325/SOL5117—post survey report. Geoscience Australia Record 2011/08; 2011.

17. Thistle D. The deep-sea floor: an overview. In: Tyler PA, editor. Ecosystems of the deep

oceans. Elsevier; 2003 [chapter 2].18. Gage JD, Tyler PA. Deep-sea biology: a natural history of organisms at the deep-sea floor.

Cambridge, UK: Cambridge University Press; 1991, 504 pp.19. Ponder WF, Hutchings P, Chapman R. Overview of the conservation of Australian

marine invertebrates. A report for Environment Australia, Sydney; 2002.20. O’Hara T. Bioregionalisation of Australian waters using brittle stars (Echinoderrmata:

Ohiuroidea), a major group of marine benthic invertebrates. Department of the environ-ment, water, heritage and arts (Australia); 2008.

21. Cairns SD. Azooxanthellate Scleractinia (Cnidaria: Anthozoa) of Western Australia. RecWest Aust Museum 1998;18:361–417.

22. Fromont J, Moore G, Titelius MA, Jones DS, Slack-Smith SM. Report on the processingand identification of deep water marine specimens from Pluto field. Western AustralianMuseum; 2006.