Specific elements of the kelp plant and their associated epi- and endo-fauna

Holdfasts

Stipes

Blades

Tissue

Holdfasts

Jones (1971) listed up to 53 macrofaunal invertebrate species obtained from an individual holdfast, and Moore (1971, 1973a,b) listed and analysed the distribution of numerous meiofaunal and other species, especially amphipods from this habitat. Both these authors have considered the possible effects of pollution on this faunal community but experimental results are lacking. Siltation of the holdfast niche is considered to be a particularly important factor which might be detrimental to the species diversity in the holdfast. Other work on holdfast fauna includes papers by Edwards (1980) and Sheppard et al. (1980). The bulbous holdfasts of Saccorhiza are known to be able to shelter large organisms such as the squat lobsters Galathea, and the clingfish Apletodon (Moore, 1983).

Stipes

The stipes of kelp plants in the upper part of the forest area are often relatively free of epifauna, possibly as a result of the greater effects of wave action and surf on the shallower plants. This is possibly the reason why kelp species found in exposed biotopes (high wave energy) tend to be "cleaner" than those in more sheltered areas, rather than that the species themselves do not form a suitable habitat.

Apart from often supporting an assemblage of smaller algae, kelp stipes frequently carry an array of epifaunal species, the abundance and composition of which varies with kelp species and location. Sponges, hydroids, bryozoans and tunicates may all be common - especially on plants of L. hyperborea in areas of relatively strong currents. Many of the species which inhabit the stipe are colony-forming, but solitary or stolon connected tunicates are also common. The blue-rayed limpet (Helcion pellucidum), which is commonly found on Laminaria species and sometimes on Saccorhiza, feeds on kelp tissue and can weaken holdfasts sufficiently to make them more susceptible to mechanical damage (Kain & Svendsen, 1969). The epiphytic algae on the stipe will, in their turn, play host to an array of meiofaunal species, the abundance and diversity of which will increase with the structural complexity of the epiphyte community (Hicks, 1980).

Blades

In contrast to the stipes, kelp blades are often relatively clear of visible epibiota as the blade tip is constantly abraded and replaced by younger tissue (see section III.B.1.a). However, a number of opportunistic hydroid, bryozoan and other species may settle on the blade. Thus, the blades of Laminaria hyperborea are a preferred settlement site for the bryozoan Membranipora membranacea (Ryland & Stebbing, 1971) and L. saccharina is settled by the serpulid Janua pseudocorrugata (Knight Jones & Knight Jones, 1977). The hydroid Obelia geniculata is also common on the blades of kelp plants (Erwin et al., 1986).

Tissue

A few meiofaunal species may actively burrow into kelp. Benwell (1981) has shown how the nematode Monhystera disjuncta may help weaken the distal areas of kelp where it feeds on decomposition-associated microbiota.

References