Many reef dwellers hitch rides on other animals. This can be for protection, to get to new feeding areas, to steal food from their host, or any combination of the three.
The most commonly seen hitchhiker is possibly the Emperor Shrimp, Xenopontonia imperator. They are usually found in pairs on various sea cucumber, and nudibranch species.Their hosts are usually fairly mobile, so bringing the shrimps in contact with new food sources. At the first sign of danger they slip to underside of sea cucumbers, or into the gill areas of nudibranchs.
Xenopontonia imperator riding on Nembrotha chamberlaini
Another traveller often found on certain sea cucumbers is the crab Lisssocarcinus orbicularis. Which presses itself into the surface of the sea cucumber almost hiding.
Lisssocarcinus orbicularis
Sea urchins and starfish tend to host shrimps, particularly Periclimenes soror in SE Asia. This tiny species lives in family groups on the undersides of their transport. They rely also on a certain amount of camoflage for protection in that they often match the background colour of the host.
Periclimenes soror
A more bizarre hitchhiker are sessile comb jellyfish! Coeloplana astricola seems to prefer starfish as hosts.
Coeloplana astricolaSea urchins with long spines provide a certain amount of physical protection. In SE Asia, long-spined sea urchins (Diadema) are often host to groups of small jet black cardinal fish, which seem only to be found in this association. Occasionally the Urchin Clingfish, Diademichthys lineatus, seek the protection of the long spines. They don't reply on the urchin for protection. The shrimp Stegopontonia commensalis Is however a permanent resident on long spined urchins.
Stegopontonia commensalis
Going a step further, some hitchhikers live on hosts that have highly poisonous spines, so they get free transport to new feeding areas, and active protection. Colemans Shrimps and Zebra Crabs are the best known guests on Fire Urchins (Asthenosoma varium).
Periclimenes colemani - large female & small male
These are just some examples of what is lost when a reef dies.
