Biodiversity series, Paper no. 2
Department of the Environment, Sport and Territories, 1994
5. Biodiversity in the sea
Australia's marine environment spans the warm tropical waters off northern Australia only 1000 kilometres from the equator, to the cool temperate coasts off southern Tasmania some 3700 kilometres further south. From east to west, Australian coasts span a distance of almost 4000 kilometres from Cape Byron to Shark Bay. This vast geographic range is reflected in a diversity of major marine habitats.
Extremely rich and specialised coral reefs are well developed off the east coast of northern Australia, and on a smaller scale off Western Australia and Lord Howe Island. Extensive sand and mud-bottomed habitats exist over much of the continental shelf, which, relative to other regions of similar total area, is comparatively small and bordered by a steep continental slope. Some of the world's largest seagrass habitats occur in Australian waters, in the mid-latitudes of both sides of the continent. Temperate rocky reef habitats can be found in areas further south, as can the habitats of giant kelp forests off the coasts of Tasmania and Victoria. In the major cold water habitats off southern Australia, the open sea floors and the near-shore pelagic zone (surface and middle depths) are the sources of most of the fish eaten by Australians.
Biodiversity in the sea - summary
Australia's fish fauna is one of the richest in the world (Table 6). This is due to the range of climatic conditions, including warm currents that flow down the east and west coasts, and the presence of the world's largest coral reef. The fish fauna is strikingly regional with a tropical northern zone; eastern and western warm temperate zones; and a small southern cool temperate zone.
The northern tropical zone has most species diversity but the majority of species are distributed widely in the Indian and Western Pacific Oceans. The temperate zones have less species diversity but a majority of the species are unique to the areas (endemic). Interestingly, because breeding does not usually occur between eastern and western members of a species, many 'species pairs' have developed, with one original species diverging to become two separate species. Some fish families are particularly diverse in Australian waters, for example pipefishes and seahorses, leatherjackets and anglerfishes. There is also an extremely rich fauna of sharks, chimaeras and rays, including 97 species that may be new to science.
Some marine species that are not endemic to Australia find security in Australian waters and are found here in greater numbers than in other parts of their range, for example a number of turtle species and the dugong.
Many ancient marine animals live on the southern marine platform of Australia, which has been relatively stable for some 40 million years. The direct ancestral lineage of many invertebrate species found on the platform today can be traced back in the deposits to ancient times (for example some bivalves, cowries and volutes). Other species are relicts of groups that have become extinct elsewhere.
5.1.1 Regional features of the marine fauna
The most striking feature of the Australian fish fauna is that it possesses very strong regionality. Distinct tropical, warm, and cool temperate zones, with broad areas of intermixing, are identifiable. Recent information indicates that, at least for fish, it is more accurate to describe four elements, a northern tropical zone, and three distinct provinces in the southern part of Australia: the eastern and western warm temperate, and the cool temperate, fish faunas 205. Broadly speaking, northern Australia is part of the tropical Indo-West Pacific faunal region. It has an extremely high species diversity and the majority of these species are distributed widely in the tropical Indian and Western Pacific Oceans. There is, however, a growing awareness that endemicity in some invertebrate groups may be higher than originally thought, as molecular techniques reveal morphologically indistinguishable (cryptic) species 183 184.
In contrast to tropical Australia, the marine fauna of temperate southern Australia is characterised by lower diversity and very high species endemism. Of the estimated 600 inshore fish species in this zone, about 85 per cent are endemics, and 11 per cent are shared with New Zealand 178. Endemism at the genus level is about 38 per cent (of an estimated 290 genera). A high number of endemic invertebrates are also found in this region 43, with 90 per cent of echinoderms and probably over 95 per cent of molluscs restricted to these waters 178. Endemism in other marine invertebrate groups is difficult to estimate because of the paucity of information 189.
The fish faunas of the Australian continental shelf and slope both demonstrate well-defined regional structure. Hutchins71 has shown that three near-shore reef faunas, conforming to tropical, subtropical and warm temperate provinces, exist off west and southwestern Australia. Of the 728 fish species recorded, 67 per cent are tropical, 24 per cent are warm temperate, 5 per cent are subtropical, and the remaining 4 per cent have uncertain distributions. Similar trends are evident within the trawl fish communities of the continental slope off Western Australia 205.
A noteworthy aspect of the temperate fauna is the occurrence in many genera of eastern and western species pairs that do not interbreed because of geographic isolation (allopatric species, see Figure 5). There are some 18 of these pairs of sister species and in a number of cases, a species previously considered to be widespread has been shown to consist of distinct eastern and western species. Examples include Pelates octolineatus (western species) and Pelates sexlineatus (eastern species) of the grunter family, Terapontidae, and Vicentia species (western species) and Vincentia conspersa (eastern species) of the cardinal fish family Apogonidae 178. As a result, warm temperate fish faunas of the Pacific and Indian Oceans are distinct from each other.
A small cool temperate province, off Tasmania, Victoria and eastern South Australia, can also be identified in fishes and some invertebrates 205. The generic composition is similar to that of warm temperate provinces but differs significantly in species composition.
Where populations originally of one species seperate, they may eventually become genetically distinct and then isolated. In some cases, these distinct species may have overlapping distributions.
Source: Redrawn in Hughes (1985) from an original by Dee Huxley
5.1.2 Australia's rich and endemic marine environment
Australia has one of the largest fish faunas in the world, due to the wide latitudinal range of Australian marine habitats, the presence of the world's largest coral reef and the radiation of a number of temperate fish families. The warm East Australia Current and Leeuwin Current, flowing down the east and west coasts respectively, also seasonally bring tropical species into more southerly latitudes. The richness of the Australian fish fauna is indicated in Table 6 below.
In some eight families of fish which have more than ten species, more than half the total world species occur in Australian waters. Of the 16 world species in the family Cheilodactylidae, for example, 13 are found in Australian waters and nine of these are endemic. The five marine fish families Brachionichthydae, Pataecidae, Enoplosidae, Gnathanacanthidae and Dinolestidae are endemic to Australia and a number of these families are monotypic.
Several families of fish are noted for their explosive radiations in Australia. They include pipefishes and seahorses (Syngnathidae), leatherjackets (Monacanthidae) and anglerfishes (Antennariidae). The Australian pipefish fauna is regarded as uniquely diverse. Nearly 80 per cent of the 47 genera in the Indo-Pacific region are found in Australia and 14 of these genera (38 per cent) are endemic. Ten of these genera are monotypic 178. The Monacanthidae is distributed widely in tropical and temperate seas with approximately 100 species in about 30 to 35 genera. Well over half of the species and nearly all of the genera are represented in Australian seas. The 20 species and 11 genera that are confined mainly to southern Australia by far exceed the numbers found in any other region of similar latitude 178. Anglerfishes are generally confined to tropical seas except in Australia where seven temperate endemic genera occur. The family is represented in Australia by 23 species belonging to 12 genera; nine of these are primarily confined to temperate seas of southern Australia and the remaining ten species are mainly tropical reef dwellers 114.
|Area||Total fish species|
|Australia 2||4000 +|
|US and Canada||2268 +|
|Species beyond 200 metres|
|Eastern Atlantic and Mediterranean||1248|
|England and NW Europe
Australia also has an extremely rich chondrichthyan (sharks, chimaeras or ghost sharks and rays) fish fauna. It is estimated that at least 296 species, comprising 166 sharks, 117 rays and 13 chimaeras, live in Australian waters. This includes 97 species not identifiable from the current literature, of which many appear to be new to science. The richness of this fauna is apparent through comparisons with other areas: 182 species have been recorded from southern Africa; 174 from the Japanese Archipelago; and 130 from the eastern North Atlantic and Mediterranean Seas 89.
Levels of endemism in the Australian shark and ray fauna are significant. More than half of the entire chondrichthyan fauna, some 54 per cent, is endemic to Australia. Of the shark fauna, 48 per cent of the species are endemic, with most of the endemic species being found in tropical or warm temperate areas. More than half of the Australian chimaeras appear to be regional endemics, and some 73 per cent of Australian rays are endemic. In addition, two ray species are most likely restricted to freshwater and another two found only in estuarine waters 89.
The central Indo-Pacific region of northern Australia and Southeast Asia supports by far the richest invertebrate fauna associated with coral reefs in the world. These areas have more than 100 coral genera and some 500 coral species, compared with only 25 genera in the Western Atlantic, and about 60 species for the Caribbean. While the central Indo-Pacific region as a whole has essentially a common fauna, Australia's importance as part of it has additional significance because of the level of degradation in the region's equatorial reefs 217. This coral centre has possibly been a reservoir of species since the late Tertiary.
A recent orange roughy survey around the sea mounts off southern Tasmania indicates that the deep water coralline environment in this region may possess a large number of endemic and as yet undescribed species of fish and invertebrates. Three samples containing just 12 fish specimens yielded two undescribed deepwater cods (Paralaemonemona, family Moridae), and two undescribed lings (family Bythitidae) 202.
The waters off Australia's southern coastline provide habitats for one of the world's largest marine floras. Of the 4100 species of red algae known in the world, at least 1100 occur in these waters, and 75 per cent of these are endemic 219. In addition, all but four of the 24 genera of crustose coralline algae are found in these waters 219.
5.1.3 Ancient and relict components of Pangaean and Gondwanan origin
A microorganism (cyanobacterium) found in actively growing stromatolites at Shark Bay, Western Australia, represents one of the longest continual biological lineages known (1900 million years) while other fossil stromatolites from the Pilbara, Western Australia, are about 3500 million years old (see Section 2).
Literature on the Gondwanan marine flora is sparse. This is mainly because algae, particularly macrophytes, have soft tissues and therefore leave a poor fossil record. Southern Australia has been separated from other land masses since the Palaeocene or Eocene and it is thought that the algal flora, which may include Gondwanan elements, has therefore remained or evolved in relative isolation 180.
Australia's southern marine platform, one of the largest in the world, is of considerable evolutionary significance. Due to its tectonic stability since the late Eocene (approximately 40 million years ago), sequences deposited have been little disturbed compared with many other parts of the world. Although the record is not complete, because there appear to have been long periods of nondeposition due to sea-level variation, it nevertheless provides a unique glimpse of the direct ancestral lineages for many invertebrate species found there today 211.
Many ancient marine animals, considered to be 'living fossils', occur on the southern marine platform. These include the bivalve genus Bassina (family Veneridae), which has a continuous fossil record from the Oligocene, and Neotrigonia, the only extant member of the family Trigoniidae, which was widespread and abundant in the Mesozoic 43 53 188. Many of the endemic genera, such as the cowries Notocypraea and the volutes Ternivoluta, have fossil lineages dating back to the early Tertiary and may be survivors from an ancient palaeoaustral fauna. Other endemic genera, such as the cowries Zoila, the bivalves Miltha (family Lucinidae) and gastropods of the genus Diastoma (family Diastomatidae) are relicts of once widespread Tethyan groups that have become extinct elsewhere 178.
5.1.4 The refuge value of the sea
A number of Australia's marine species, although found elsewhere, find the greatest security in Australian waters. These include the green turtle (Chelonia mydas), hawksbill turtle (Eretmochelys imbricata) and loggerhead turtle (Caretta caretta), now rare except in Australian waters. The dugong (Dugong dugon), which is suffering from population decline in many parts of its range, is found in greater numbers in Australian waters than anywhere else in the world 155 98.
Special marine areas
Ecosystem diversity is just as important in the sea as it is on land or for freshwater environments, and Australia has some remarkable marine habitats. The most well-known and spectacular of these is the Great Barrier Reef, covering 350 000 square kilometres off the tropical northeastern coastline. The reef has an incredibly rich biodiversity including about 400 species of corals, 1500 species of fish, six species of turtles, some 4000 species of molluscs and 23 species of mammals. Some 215 species of birds also occur in the Reef area.
Other important marine habitats are the giant kelp forests of Tasmania and Victoria and the seagrass meadows found in many of our coastal waters. Kelp is a large seaweed, which forms dense canopies up to 30 metres above the seabed. There are two species in Australia. These 'forests' provide a refuge for many species of fish (including the leafy sea dragon) and invertebrates. Seagrass is a marine flowering plant and 30 of the world's 58 species are found in Australia (and 11 of 12 genera). Western Australian seagrass meadows, which collectively cover as much area as all the rainforests of Australia, are the most diverse in the world. The 'meadows' are home to many animals, from worms and whelks to octopuses and crabs, and provide a nursery for many of their young. They also play an important role in nutrient cycling and the food web of inshore coastal areas.
Mangroves, which have already been discussed in 3.4, also provide valuable habitat for many marine species.
5.2.1 Great Barrier Reef
The Great Barrier Reef, covering an area of 350 000 square kilometres, is by far the largest coral reef system in the world and is an area of extraordinary biological diversity. The Reef comprises more than 2900 individual reefs which range in size from less than one hectare to more than 100 square kilometres, and in shape from flat platform reefs to elongate ribbon reefs. It includes 545 fringing reefs which have developed on some mainland coasts and around many high islands. Within Reef waters there are 940 islands, 618 of which are high or continental islands, and 87 are vegetated sand cays.
A great diversity of corals, some 400 species in 60 genera, are present on the Reef. The majority belong to the order Scleractinia which have calcareous skeletons. The remainder are the members of the subclass Octocorallia (soft corals, sea whips and sea fans) and members of the class Hydrozoa. Many corals have a variety of growth forms that relate to the changes in water levels and exposure of the location in which they develop. Some examples of different genera and their characteristic growth forms are: Acropora and Pocillopora which are characteristically branching forms; Platygyra and Leptoria, which are generally massive or brain shaped; Montipora and Turbinaria which are generally plate-like; and Fungia which are unattached mushroom shaped corals made up of a single polyp.
The Reef supports a large variety of other animal life, including approximately 1500 species of fish. These range from: species such as marlin (Makaira indica) and mackerel (Scomberomorus species) which move through the Reef area (pelagic species); to species such as coral trout (Plectropomus species) and sweetlip (Lethrinus species) which spend most of their lives in and around the reefs (demersal species); to small territorial fishes, such as butterfly fish (family Chaetodontidae) and damsel fish (family Pomacentridae), which spend most of their adult lives in a restricted and strongly defended territory of a few square metres of reef. Some 4000 species of molluscs have so far been collected from the Reef, and these include trochus shells, cowrie shells and the giant clams of the family Tridacnidae. Twenty-three species of marine mammals can be found in the Reef area, and the cays and continental islands of the area support 215 species of birds 38.
In addition, six species of turtles can be found in the Great Barrier Reef area: the green (Chelonia mydas), leatherback (Dermochelys coriacea), flatback (Natator depressus), loggerhead (Caretta caretta) and hawksbill (Eretmochelys imbricata) as well as the rare visitor, the Pacific ridley (Lepidochelys olivacea). Areas of sand around the Reef are nesting sites of world importance for the green and loggerhead turtles. Raine Island, for example, has the largest nesting populations of green turtles in the world and is Australia's most significant seabird rookery 38.
5.2.2 Giant kelp forests
Source: J. Sanderson, Marine Environment Systems
The two species of kelp that form the giant kelp forests in Tasmania and Victoria are Macrocystis pyrifera (south, east and western Tasmania), and M. angustifolia (northern Tasmania and Victoria). The fronds of these species can grow up to 30 centimetres a day, forming a dense canopy up to 30 metres above the seabed.
Knowledge of the ecology of giant kelp forests is still incomplete, but it is believed that the upper canopy formed by Macrocystis species increases habitat complexity, and provides a refuge and possible nursery area for many fish and invertebrate species, including lobster (Jasus species) and abalone (Haliotis species). It has also been suggested that the giant kelps are important in developing a detritus-based food chain.
Among the many animals that find shelter in Australia's kelp forests is the attractive leafy sea dragon (Phycodurus eques), which feeds on amphipods within these kelp communities. Amphipods feed on such elements as the red algae growing in the shade of the kelp. The leafy sea dragon is remarkably well camouflaged, and resembles a floating piece of torn off kelp.
Even though the full importance of the giant kelp forests to local marine ecology is not yet understood, anecdotal evidence links the decline of several species of fish and invertebrates from Tasmanian coasts with reduced kelp populations. The decrease in Tasmanian Macrocystis stocks may, for example, be responsible for the disappearance of stripy trumpeter and real bastard trumpeter (Latridopsis) from Tasmania's east coast, as well as for decreased stocks of abalone and sea urchins 213.
5.2.3 Seagrass meadows
Seagrass meadows are formed by marine flowering plants which are believed to have evolved over 100 million years ago. They make an important contribution to coastal productivity and stability in many parts of the world 83.
Australia is of global significance for the diversity of its seagrasses with 30 of the 58 species found worldwide occurring here, and 11 of the world's 12 genera represented 85. Some 18 species are endemic to Australia, and 16 of these are temperate species 88. A few more Australian species are yet to be described 203.
Seagrasses are found in many Australian coastal waters, from the tropics to the Tasmanian coasts. They usually grow on mud, sand or coral sand, from the intertidal zone to not more than 50 metres in depth, but mostly at 2-10 metres. Their distribution is dependent mostly on light, but also upon temperature and exposure to wave action.
The largest seagrass meadow which is at Shark Bay, covers over 4000 square kilometres, and comprises 12 species. It has been estimated that seagrass meadows in Western Australia, with 26 species, cover the same area as rainforest in the whole of Australia 81. These areas have been considered as the most diverse assemblages of seagrasses in the world.
A variety of animals are found in seagrass meadows, including whelks, worms, octopus, squid, prawns and crabs. Much of their life depends on diatoms and filamentous seaweeds which are attached to the seagrass and smaller animals. Seagrasses provide nurseries for some of these invertebrates, including the western rock lobster (Panulirus cygnus), the blue swimmer crab (Portunus pelagicus), and some 20 species of penaeid prawns 13. They are also the habitat of the world's largest living gastropod, the trumpet shell Syrinx aruanus. Sixty-six species of algal and 40 species of animal epiphytes have been found on the seagrasses of Shark Bay 164, and more than 50 species of epiphytic algae on three seagrass species in Botany and Jervis Bays 101. It has been estimated that on Amphibolus seagrasses around all of Australia there are more than 100 epiphytic species 41. The epiphytes are extensively grazed by small organisms such as amphipods and molluscs as well as by fish. Seagrasses are important to many fish, not only for food and shelter but also because they are significant nursery areas for species such as bream, garfish and snapper. Some 41 species of fish have been found in the Cockburn Sound seagrass meadows in Western Australia alone and most of the fish are restricted to this environment for at least part of their life cycle 141. The green sea turtle eats leaves of tropical seagrasses, and the vulnerable dugong is almost entirely dependent on seagrass for food. About 10 000 dugongs, or 10 per cent of the world's remaining population, live in Shark Bay 2.
Seagrasses play an important role in the nutrient cycling and the food web of inshore coastal areas. Leaf litter from seagrass communities together with seaweeds accumulates on the beach to heights of two metres along parts of the south Western Australian coastline during winter, and these accumulations form the basis of a community of microbes, detritivores such as small crustacea, and carnivores. Small pieces of organic matter and plankton in the water provide food for filter feeders.