Lianas (vascular plants) are rooted in the ground and maintain erectness of their stems by making use of other objects and trees for support. The epiphytes such as orchids, “Spanish moss” “old man’s beard”, hanging moss, Usnea and Alectoria grow in the branches of trees, where they are neat light; their roots draw nourishment from the humid air.
All these epiphytes depend upon the trees for support only. They manufacture their own food by photosynthesis. Likewise, ectocommensals or epizoite animals are associated another animal for the purpose of anchorage and protection.
Many small animals, for example, become attached to the outside of larger ones, such as the protozoans Trichodina and Kerona on Hydra, vorticellids (Vorticella, Epistylis) or various other aquatic organisms, branchiobdellid annelids on crayfish, the barnacles which attach themselves to the backs of whales and shells of horse-shoe crabs and so on.
The remora fishes attach themselves to the bellies of sharks, swordfish, tunny, barracuda, or sea turtle by means of a dorsal fin highly modified into a suction disc on the top of the head. They are of small size and are not burdensome to the host. The host benefits the remora, however, for when the host feeds, the scraps of food floating back are swept up by the remora.
Commensals may also be internal. For example, many harmless protozoans occur in the intestinal tract of mammals, including man. The pea crab, Pinnotheres, lives as a commensal in the mantle cavity of certain sea mussels; the crab steals food collected by the host mollusk but does little if any other known injury. The hermit crab normally lives inside gastropod shell.
The pitcher of the pitcher plant found in bogs furnishes a breeding site or home for certain species of midge flies, mosquitoes, and tree toads. Many kinds of micro-organisms, plants and animals (such as small crabs, shrimps, polychacts, ophiuroids, ascidians, etc.) live in the canal system of sponges.
The nest of one species often furnishes shelter and protection for other species as well. Ant nests may contain guest species of various other insects; some birds place their nests close to wasps, bees or ants for the protection offered by these insects. Woodchuck burrows are used also by rabbits, skunks and racoons, especially in winter.