Seagrasses

Australia is fortunate in having nearly half the 60 species of the world's seagrasses represented, 14 of which occur on Australian coral reefs. However, in reef waters in Australia, seagrasses may be inconspicuous because they are grazed heavily by invertebrates, fish, turtles and dugongs (Box 16.2, Fig. 16.9). In protected areas they form dense, lush meadows, which harbour a diversity of animal and algal life.

BOX 16.2 SEAGRASS BEDS AND MACROGRAZERS

The larger species of seagrass in the GBR, Zostera muelleri, Cymodocea serrulata, C. rotun-data, Thalassia hemprichii, Enhalus acoroides, and Thalassodendron ciliatum form dense beds near the coast and in sheltered bays offshore on continental islands. These beds are usually formed from several species of seagrass and present a wide diversity of habitats for infauna and inflora. Many epiphytes, epizoans and an extensive microflora occur on the seagrass fronds. Many animals and algae grow on or in the sediment between the sea-grass plants. Organic substances and oxygen released into the sediments from the roots of seagrasses support a specialised habitat of microflora, protists and invertebrate grazers around the roots.

It is therefore not surprising that macrograzers are found in or near seagrass beds. The dugong (Dugong dugon) has received the most attention in Australia, because this marine mammal is almost entirely dependent on seagrass beds, and the seagrasses themselves, for their food. Marine turtles also feed on seagrass beds, although this has been much better documented in the Caribbean that in Australian waters.

Halophila spinulosa

Halophila spinulosa

Halophila ovalis

Thalassia hemprichii

Halophila ovalis

Thalassia hemprichii

Cymodocea serrulata

Figure 16.10 Seagrass species.

Thalassodendron ciliatum

Zostera capricorn^

Thalassodendron ciliatum

Zostera capricorn^

Cymodocea serrulata

Cymodocea rotundata

Halodule uninervis

Cymodocea rotundata

Halodule uninervis

Figure 16.10 Seagrass species.

Seagrasses are not a monophyletic group, being found in five families of monocotyledonous (grass-like) plants. However, they all have a creeping stem (rhizome) and underwater flowering and pollination and are the only vascular plants that are aquatic and inhabit the sea.

Seagrasses grow mainly on soft substrates (silt and sand) in shallow, sheltered marine or estuarine situations. There are two exceptions to this generalisation: (1) Thalassodendron ciliatum, which grows directly on hard substrates (such as dead coral), as it has specialised roots that allow it to become firmly attached to these substrates; (2) many species of Halo-phila that can grow at considerable depths, especially H. capricorni, H. decipiens, H. spinulosa and H. tricos-tata, which can grow down to 60 m. Thus, these latter species of Halophila are often found growing on deep sediments outside coral reefs or in inter-reefal environments on the GBR.

In the main, seagrasses on coral reefs are found in lagoons or in shallow inter-reefal areas, where they support large populations of herbivores. They therefore form a very important source of primary production. In the GBR Zone these same species of seagrass are often found growing on shallow, sheltered sediments in coastal situations, where there are no corals.

Only three species of seagrass, all in the genus Halophila, are present in the Bunker-Capricorn Groups at the southern end of the GBR (H. capricorni, H. decipiens and H. spinulosa). Further north, seagrasses are both more numerous in terms of species (all the species in the Key (14) may be represented in a fairly small area) and they are more conspicuous. Further information is available in Additional reading.

Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Post a comment