Adaptations of marine organisms (QCE Marine Science Unit 3)

What this dot point is asking

QCAA wants you to explain how marine organisms are adapted to the abiotic challenges of the ocean, and to classify those adaptations as structural, physiological or behavioural. You should be able to take an abiotic factor (salinity, pressure, temperature, light, oxygen or wave action) and describe a real adaptation that helps an Australian marine organism cope with it.

Three types of adaptation

• Structural adaptations are physical features of the body, such as the streamlined shape of a tuna or the holdfast of a kelp.
• Physiological adaptations are internal processes and chemistry, such as the salt glands of a sea turtle or antifreeze proteins in polar fish.
• Behavioural adaptations are things an organism does, such as a crab sheltering in a crevice at low tide or fish migrating to spawn.

A single organism usually combines all three.

Coping with salinity (osmoregulation)

Seawater is saltier than the body fluids of bony fish, so water tends to leave their bodies by osmosis. Marine bony fish are osmoregulators: they drink seawater, excrete salt through specialised cells in the gills, and produce small amounts of concentrated urine to conserve water. Sharks instead retain urea so their body fluids match seawater, an osmoconforming strategy. Intertidal animals such as crabs face changing salinity as tides and rain mix, and tolerate a wide range.

Coping with pressure, light and oxygen

In the deep sea, pressure is crushing and there is no sunlight. Deep-sea organisms have flexible bodies without gas-filled spaces that would be crushed, slow metabolisms suited to scarce food, and often bioluminescence to attract prey or mates in the dark. In the sunlit surface waters, plankton have features that slow sinking so they stay in the light. Where oxygen is low, such as in muddy sediments, animals may have efficient gills or behaviours that pump water past them.

Coping with temperature and exposure

On the shore, animals are alternately submerged and exposed, facing heat, drying and salinity swings. Limpets and barnacles clamp tight shells shut to hold water at low tide (structural plus behavioural). Mangroves living in warm, salty mud have salt-excreting leaves and aerial roots. On the reef, corals tolerate a narrow warm range, and their bleaching response under heat stress shows the limit of their adaptation, which links directly to the Unit 4 climate material.

Coping with wave action

Where waves pound the shore and reef crest, organisms must avoid being torn loose. Adaptations include the strong byssal threads of mussels, the suckers of sea stars, the cementing of barnacles and oysters, the flexible stems of large algae, and the low, encrusting growth forms of reef-crest corals. These same wave-resistant features explain the zonation patterns seen in the reef and rocky-shore dot points.

Australian examples to use

• Green sea turtle salt glands near the eyes excrete excess salt (physiological).
• Grey mangrove excretes salt through its leaves and has pneumatophores for gas exchange in waterlogged mud (physiological and structural).
• Reef-crest corals grow low and encrusting to resist wave energy (structural).
• Fiddler crabs shelter in burrows at low tide to avoid heat and drying (behavioural).