Marine Invertebrates
A large percentage of the life you find on a coral reef is animal. Corals, sea cucumbers, polyps, fanworms, the list is endless. This is something that makes a marine environment quite unique. Freshwater tanks in general are limited to fish and plants, with perhaps some freshwater shrimp or frogs. The variety of life you can keep in a marine tank is seemingly infinite, with colours spanning the rainbow. Your fish will also be happier if they have life around them that would naturally be present. Clownfish, for example, live in anemones in their natural habitat, and the anemone is their protection, and their home. In my own tank I have countless animal species, from polyps to snails, worms and mushrooms. All of these creatures add their own unique quality to an aquarium. Personally my shrimp are one of my favourite creatures, never failing to clean my fish, clean up waste and generally entertain. Invertebrates can be broken down into classes.
Corals: Hard, medium and soft
Worms: Fan worms, bristle worms, tube worms
Polyps: Button Polyps, star polyps
Anemones
Corals
Corals can belong to one of three categories; hard, medium and soft. In general, the hard corals can be more difficult to keep, as they are more sensitive than some of their softer counterparts. Here I can give you a few ideas of corals you can have in your aquarium.
The Staghorn coral (Acropora) This coral can be propagated from fragments of a parent colony, and can grow about half an inch per month if the conditions are right.
Favia coral This coral feeds in low water currents by pushing out hundreds of tiny tentacles.
Honeycomb coral (Goniastrea) This is not one of the more common corals but you can but them. These corals have a brain-like appearance.
Elegance coral (Catalaphyllia) This is one of the more popular stony corals and is quite easy to keep.
Cactus coral (Pavona) These are very delicately formed but are remarkably hardy.
Open Brain coral (Trachyphyllia) Seen in red or green colouring, these corals thrive under proper care and can live for years.
Cup or Pagoda coral ( Tubinaria) This will grow in a well-maintained aquarium. It's made up of hundreds of tubes with fringed edges, which the coral uses to trap plankton.
Heliofungia corals These corals are often mistaken for anemones and are delicate.
Mushroom coral This is a great one to start with. If it doesn't like where it's placed, it will just 'walk' itself to a better spot!
Anchor coral (Euphyllia) This coral gets its name from its tentacles, which are long tubes with anchor-shaped tips. This is not difficult to maintain.
Xenia This is a soft coral, that seems to have hundreds of tiny hands, each pulsating individually in the current.
Star Polyps (Clavularia) These exist in a variety of colours, and are very popular amongst fishkeepers. They're quite easy to keep, and spread rapidly under good care.
Sinularia coral This comes in many shapes and has a tree-like structure.
Organ-Pipe coral (Tubipora) This coral forms a pink shell-like structure similar to organ pipes, with fringed edges protruding from each hole.
Actinodiscus (false coral) These are mushrooms, which come in a variety of shapes and sizes
Bubble coral (Plerogyra) These are very popular in aquariums and thrive with proper care.
FlowerPot coral (Goniopora) This coral is harder to keep, and would be recommended to an experience hobbyist.
Fanworms, Tubeworms
Tubeworms live inside a calcerous tube, growing to about 5mm thick. If threatened, or if it's night-time, the worm will retract its tentacles, using the tube for protection. They're very hardy and require little attention, feeding on microscopic foods in the water. Be aware that tubeworms are food for starfish, so be warned if you have them in the same tank. I have both in my marine tank, and have not had any problems. You can find tubeworms in many colours.
Flatworms
Flatworms are primitive animals. Certain free-living forms (called Turbellarians) are parasites and coral commensals, meaning that the worms benefit from the symbiotic relationship, but the coral neither benefits nor is harmed. Commensals affect the Elegance coral and Plate corals, feeding on the coral's nutrient-rich mucus. Flatworms prefer shade and darkness, and usually only eat from a shaded part of the coral. Some grow to about 18mm, and can camouflage with the coral. Infestations can result in the death of a coral, and are usually caused by high nitrate and phosphorous levels. A full infestation can block out light to a coral, causing it to die. However, lowering the water temperature to about 23C will control the infestation.
Bristleworms
The Fire or Bristle worm (Hermodice)is a known predator of corals. They can grow up to a foot long, and threaten corals, anemones and you, if you don't take care! If the bristles are touched, the worm will 'sting', causing an unpleasant burning sensation. If you find these in your tank, which you will at some point, they can be controlled by adding predators, such as Arrow crabs or Banded Coral shrimp.
Polyps
Star Polyps (Clavularia) prefer to be sheltered from strong water flows, and are often found naturally near land. They tend to grow best on hard substrate or rock. These are recommended for a new hobbyist, as they're extremely hardy. The Green Star Polyp glows an intense green under fluorescent lights, and is quite beautiful. Commonly found in the tropical Pacific Ocean, although hardy, these polyps are quite particular about water flow, not liking too much or too little.
Mushrooms
Many mushrooms are actually false corals. Actinodiscus are known as mushroom polyps, or false corals. They vary in colour, being reddish-blue, multicoloured, blue, or green and brown striped. They're so popular with hobbyists that you may find the more colourful the specimen, the higher the price. Actinodiscus prefer a sheltered habitat, and are found at depths of up to 20 meters. They feed on microscopic organic matter, and don't require a lot of light.
Anemones
Anemones attach themselves to a surface by way of an adhesive 'foot'. From the foot a column extends into a round disc. The anemones' mouth is located in the centre of the disc, surrounded by tentacles, which contain neuro-toxins which is uses to stun its prey and as a defensive mechanism. Clownfish are immune to the anemone's sting, and use it as a home.
