Rays and bony fishes such as flatheads dig through sandy sediments to hunt for food 📷 Kade Mills
Because the sediment in sandy habitats is fairly unstable and is shifted by water currents plant life such as seaweeds and seagrasses are unable to establish themselves. As few plants grow in sandy habitats, much of the food present comes in from the sea and land. Dissolved nutrients in the water, drift algae and dead animals are brought in by the sea, while insects and plant litter come from the land.
The instability and lack of shelter also mean that few large vertebrate animals live here permanently. While a species of giant marine worm found in sandy habitats can grow up to 2 metres long, most animals tend to be small enough to burrow into the sand or live in the tiny spaces between sand grains.
The stark appearance of sandy habitats is deceptive. The ripples lining a sandy seafloor may look bare and empty, but they can be rich and plentiful in animals. A scattering of large rocks, shells and rubble provides anchorage for plants and non-mobile animals like sponges. Healthy sands and sediments usually have small animals living on or under the surface including worms, snails, crustaceans, and bivalves. These are important food items for many fish, rays, and sharks that forage over the sand. Other more permanent residents of this habitat such as crabs and flounders use their sandy coloured bodies as camouflage to blend into the soft sea floor.
Many species have adapted to living in sandy habitats with great success, on and under the surface of the sand. Other microscopic animals have even colonised individual sand grains. A square metre of sand can yield several thousand organisms and hundreds of species. Water currents and waves sculpt sands along the bottom into soft peaks and troughs like miniature submerged mountains. Variation in the size and type of sand grains, the force of currents and waves and depth all combine to create different habitats which support different species of animals.
Many worms are present in Victoria’s sandflats. These include thin narrow ribbon worms, which slide between the sand grains, and segmented bristle worms which roam around on top of the sand in search of prey. The mucus secreted by many worms and similar looking sea cucumbers (who are in fact echinoderms, like seastars) helps bind and stabilise the sand. Other worms stay permanently fixed in tubes in the sand with only their mouthparts extending into the water to capture food. Many fish love to make a meal of the tubes and pipi siphons which emerge from the sand. These sand dwelling animals are also collected for bait by fishers.
Many small fish such as sardines and sprats dart about in the water above the sand and intermingle with hardyheads and silversides, toadfish and King George Whiting (Sillaginodes punctatus). Sand Whiting (Sillago ciliata) forage on the bottom, feasting on worms, small crabs and ghost shrimps. Flatheads (Platycephalus spp) are common and voracious predators, eating small fish and a wide variety of crustaceans such as prawns. Many fish species in sandy habitats, including Sand Whiting and flatheads, are important to recreational and commercial fishers.
Sandy habitats in the cool waters of Victoria are home to unique and bizarre species, many of which have not even been named. Sheltered inlets with fine sand and many nutrients, such as the intertidal population near the Werribee treatment plant, can support communities of 300,000 organisms per square metre. Ninety Mile Beach, to the west of Lakes Entrance, is Victoria’s most extensive stretch of sandy seafloor habitat, and home to spider crabs, sea anemones, sand skaters and stingrays. Up to 800 species have been recorded in a 10 square metre area on the sandy seafloor from Lake Tyers to Cape Conran, and one square metre of sand from the subtidal area off Ninety Mile Beach can contain 6,000 individuals.
Few people are aware of the importance of soft sediment habitats in nutrient cycling in the marine environment. Microscopic algae growing on sediments play an important role in controlling nutrients. In Port Phillip Bay, for example, mats of minute algae take up nitrogen produced from the Werribee treatment plant and other sources to create carbon, a rich food source for many small animals, and keep the water clean. This delicate system could be permanently damaged by decreasing light levels, for instance through dredging, or by overloading the amount of nitrogen put into the Bay.
Sand can range from coarse, shelly rubble to fine silty sediment. Grain size is a good indicator of wave energy and an important determinant of whether animals can burrow in or not. Large grains are deposited by stronger currents whereas fine-grained sediments indicate quieter water. Coarse sediments are more unstable and difficult to burrow into. They tend to have more animals living on top of the sand or buried deep down. Fine silty sands in sheltered inlets are more compact and therefore provide a more stable environment and contain more animals (and sometimes plants) in the upper layer than coarse sand.
Shrimps, prawns, juvenile crabs, gobies and young flounder may use sheltered sandy areas as nursery grounds and eat large numbers of the small invertebrates living in the sand. Delicate animals such pink and orange sea pens are found in deeper waters scattered amongst stalked sea tulip (ascidians), sponges and feather-like hydroids.