Competition & commensalism

Sea anemone (Stichodactyla sp.) Sea anemone (Stichodactyla sp.), often the home for the Anemone crab (Neopetrolisthes maculatus), Wild Guide to Moreton Bay Rose Barnacle (Tesseropora rosea) Rose Barnacle (Tesseropora rosea), Wild Guide to Moreton Bay

All marine animals form partnerships at some stage of their lives. There are four categories of partnerships classed according to the benefits each partner receives. Symbiosis is where two species cannot survive apart from each other. Mutualism is where two species benefit from living together but can also survive apart. Commensalism is where one partner uses the food supplied by the other (the host) without affecting the host in any way. Parasitism is where one partner obtains nutrients from the host, who gains no benefit at all and may be adversely affected by the parasite. These relationships are very common. Many animals fixed to the seabed have other creatures living on and inside them, using them as ‘hotels’ (and not necessarily as parasites), and there are some very colourful and famous partnerships, such as clown fish living in sea anemones.

Most of the seafloor consists of sandy or muddy plains with plenty of room for everything that wants to live there, but in some like coral reefs and rocky shores, where conditions are ideal for life, space is at a premium with sometimes fierce competition for resources (space, light, best exposure to water currents, food supplies). These communities are highly dynamic, with certain groups of species or phyla dominating others depending on local conditions and external factors including human disturbance. This dynamic cycle of advance and retreat includes physical smothering and shading (such that the faster growing species can overgrow slow ones) and the use of potent cocktails of chemicals (such that the most toxic creatures counter-attack the less toxic ones) to defend their patch. Sponges, corals and ascidians have evolved an arsenal of chemical weapons to defend their territories, attack their neighbours, and deter their predators. Some of these chemicals are also of great interest to the pharmaceutical industry which is continually searching for new drugs to aid human health.
