Epibionts are organisms that grow attached to the surface of a living host. They are made up of plants (epiphytes) and sessile animals (epizoans) and in the marine environment will attach themselves to any firm, exposed surface.
Seagrass blades make a perfect host for epibionts which can often be seen in the form of brown and white scaling (epizoans, such as diatoms, bacteria, the larvae of numerous types of organisms, bryozoans, hydroids, etc) and/or as a furry covering of algae (epiphytes).
There are many benefits for epibionts from this living arrangement: the advantage of being able to colonise an unused surface in a highly populated environment, being able to attach to a flexible host higher in the water column bringing a better supply of nutrients without danger from the increased turbulence, being exposed to more light in an exposed habitat allowing better growth, more nutrients can be made available from the host organism, and in some cases the advantage of being protected by the host’s defences.
There are also disadvantages for the epibionts: the potential instability of the host due to physical disturbances, changes in the host’s life cycle, competition for nutrients with the host, or being eaten by a predator of the host organism. However, even with these disadvantages, the epibionts usually benefit more from this relationship than their hosts.
In the case of seagrass, the disadvantages of being the host to epibionts can be disastrous. Rigid epizoans (e.g. Bryozoa) reduce the flexibility of the leaves causing them to become brittle and to break off in high turbulence. Epibionts can also increase weight causing a reduction in buoyancy. They can increase surface friction resulting in the host plant becoming dislocated from the substrate in high turbulence. Seagrass blades can be damaged from grazers of the epibionts. One of the most drastic effects can be from shading of the leaf surface therefore reducing the plant’s ability to photosynthesise.
There are some advantages to the seagrass host: nutrients can be passed indirectly through the water column to the seagrass from epibiont outputs, at low levels of epibiont cover seagrass leaves may be protected from sunburn when high and dry.
The abundance of epibionts on seagrass depends on a number of factors: the leaf turnover rate, water temperature, availability of nutrients, salinity levels, current, velocity, depth and light availability.