MEPS 302:37-48 (2005)  -  doi:10.3354/meps302037

ABSTRACT: It is generally considered that colonial invertebrates living on the surfaces of macroalgae have a negative influence on the growth rates and survivorship of their substratum. To date no evidence has been provided from natural macroalgal/epifaunal associations to support this view. We investigated the influence of colonial bryozoans and hydrozoans on the growth rate and nitrogen physiology of their substratum, the giant kelp Macrocystis pyrifera, and the impact of seasonal changes of light and the seawater nitrogen concentration on this relationship. Throughout this study there was no evidence of any negative impact of colonization by either epifaunal group on the growth of M. pyrifera. During a period of low concentrations of inorganic nitrogen in seawater, the growth of M. pyrifera fronds heavily colonized by hydroid colonies was higher than less heavily colonized individuals. During this time hydroids could provide on average between 71 and 122% of the nitrogen required for uptake by tagged M. pyrifera fronds at ambient seawater nitrogen concentrations. Evidence suggests that enhanced growth was caused by the provision of ammonium excreted by the hydroid colonies. Evolutionary pressure to limit the impact of colonization by sessile animals on macroalgae is likely to be exerted on both the algal substratum and the attached animal. We suggest that due to their close relationship this pressure may ultimately result in the development of mutualistic partnerships between algae and specialized sessile seaweed-dwelling fauna in natural associations.

KEY WORDS: Mutualism · Kelp · Hydroids · Bryozoans · Epifauna · Nitrogen limitation · Ammonium excretion · Sessile invertebrates