Sponges have what is often considered the most basic body plan of all the multi-cellular animals. They are sedentary as adults, and resemble plants in their simplicity and adherence to the substrate. Because of their simplicity and stillness, they were originally classified as a plant, and were only recognized as an animal in 1765 when their internal water currents were noticed.
Sponges are covered in tiny holes that draw water in. This is accomplished by small beating hairs that bring the water in. As the water is pumped throughout the inside of the animal, it is filtered for food particles. The water is the pumped out, exiting much larger holes.
This simple body plan and feeding technique of sponges has been very successful for a long time, for they have been around for half a billion years. During the Devonian period, well before the Dinosaurs, sponges were one of the most dominant forms of life on Earth. In modern times, they are more obvious in areas of some water flow and nutrient load. Because of the generally clearer and nutrient poor waters of the tropical seas, they are not always as noticeable in the northern Australian waters as they are along the cooler southern coastlines. However, smaller and very attractively coloured species are present around coral reefs.
They vary in form, with some species growing distinctly shaped structures that look like balls, large cups or funnels, or they may simply grow as encrusting layers. Many of the tropical sponges, especially on coral reefs, can be quite colourful.
These are Jellyfish that have eight divided arms hanging from the bell, rather than tentacles.
Cassiopeia ornata, 'Upside-down Jellyfish'.
The largest and usually most obvious genus of coral. Even the same species can grow in many forms, thus they have many common names such as 'Staghorn', 'Table' and 'Plate' Coral.
This the second largest and one of the most obvious genera of coral. Like Acropora, they can grow in many forms, from staghorn to flat plates, sometimes even in the same colony. The colours vary. One of their distinguishing features is the small size of the corallites (the small holes the polyps live in); they are the smallest of any coral group.
The flatworms on coral reefs can be confused with the nudibranch sea slugs, however the latter have obvious fluffy appendages (the Spanish Dancer is an example).
Thysanozoon flavomaculatum, 'Yellow spot Flatworm', 'Gold-flecked Flatworm'.
This group includes the 'true' worms of the soil and sea.
There is a great range of 'worms' on and through the coral reefs, many of which are not related to each other. While most species are not seen, some, like the 'Feather Duster Worms' and 'Christmas Tree Worms', have colourful and noticeable fluffy feeding apparatus.
Like the 'Feather Duster Worms' of the Family Sabellidae, these worms also have two fans extending from the tube, however they differ in that the tube is hard and made from calcium carbonate. When disturbed they withdraw quickly and seal their tube off with the operculum.
Protula bispiralis, 'Magnificent Tube Worm'. Two relatively large spires, with several beautifully delicate whorls. Orange and cream in colour. Tube often extending obviously from reef.
Spirobranchus, 'Christmas Tree Worms'. Small and very common 'fluffy' feeding tentacles that usually grow out of the top of Boulder Coral in groups, and appear in many different colours. The tentacles suck back in quickly when the snorkeller gets too close.
This family contain most of the oysters, including the ones some humans like to eat. (This family does not nclude the commercial 'Pearl Oysters'.)
Lopha spp. 'Cockscomb Oyster', 'Zig Zag Oyster'. Large oyster with extremely angular 'zig zag' toothed inner edge of the valves, providing a "grinning mouth". Because they are sedentary, they are usually covered in many other sessile invertebrates, such as sponges; some encrusting sponges may give the shell a bright red colour. Found on coral reefs in the Indo-West Pacific.
There are at least three species in this genus. Blue-ringed Octopuses infamous for being one of the most venomous animals in the world. It is thought that the toxins from one small individual animal could kill over 25 people in one dose. The list of chemicals in the venom include: 5-hydroxytripamine, hyaluronidase, tyramine, histamine, tryptamine, taurine, acetylcholine, dopamine and tetrodotoxin; it is the latter that is most dangerous. Tetrodotoxin is found in pufferfish, and is reported to be some 100 times more toxic than cyanide. When the victim is bitten, they are partly paralysed, including the diaphragm, and thus unable to breath, and can die within minutes if not treated. There is no antidote.
Hapalochlaena lunulata, 'Greater Blue-Ring Octopus'. Despite the common name, like all Blue-ring Octopuses, this animal is quite small, with short arms, and could fit in the palm of your hand - but don't pick it up! They are found around rubbly coral reefs across northern Australia, New Guinea and tropical Asian waters.
One of the prettiest animals of the coral reef are the Feather-stars. For most of their life, they stand on reefs and catch food as it passes by through the water.
Featherstars usually attach to coral on the edge of drop-offs and/or near areas with some flow of water.
They are capable of 'swimming' freely. There is a huge range of colours and combinations of colours.
As slow moving and large animals, Sea cucumbers are commonly seen around coral reefs. Most species sift through the sand looking for food.
Stichopus, 'Spiny Black'.
Also known as 'Accordion Sea Cucumber'. The are soft and if picked up out of the water, they collapse in the hand like a wet sock. They can retract or lengthen out.
Sea Squirts look superficially similar to Sponges, however, they are about as unrelated as an 'invertebrate' can be.
Cnemidocarpa stolonifera. Small and white, red 'veins'. Endemic to Australia, where it is found around coral reefs in tropics and subtropics, mostly along east coast.
Polycarpa aurata. 'Gilded/Oxheart/Goldmouth/Ink Spot/ Yellow & Purple Ascidian/Sea Squirt'. One of the most obvious and commonly seen sea squirts on coral reefs.
Didemnum molle, 'Green Sea Squirt'. A commonly seen sea squirt on coral reefs throughout the Indo-Pacific. It appears soft and squishy and swishes around in reaction to water movements. Another common sea squirt thay look similar is called Atriolum robustum. Or maybe I have these the wrong way around?
A salp chain.