Ecological interactions between the brown seaweed Sargassum muticum and its associated fauna

Abstract

The fauna inhabiting the brown seaweed Sargassum muticum (Yendo) Fensholt was studied at Friday Harbor, Washington, USA over a 2 mo period (July/August, 1978). The fauna lacked diversity, but a few species were found frequently and in great abundance, namely the amphipods Ampithoe mea Gurjanova, Aoroides columbiae Walker, Caprella laeviuscula Mayer and Ischyrocerus anguipes Kryer, and the gastropod Lacuna variegata Carpenter. All of these vagile species were more abundant on the better illuminated distal regions of the plant, where there was an abundant coating of the diatoms Biddulphia aurita (Lyngb.) de Breb and Melosira dubia (C. A. Ag.) Kutz. A comparison between plants bearing different periphyton loads revealed that Aoroides columbiae and C. laeviuscula were far more abundant on heavily infested plants. A similar trend was evident for Ampthoe mea and L. variegata, but was not sufficiently marked to be statistically significant. Plants bearing silt rather than diatoms supported reduced numbers of all the common species of associated animals and especially C. laeviuscula. The results of substratum selection experiments in the laboratory corresponded to the expectations raised by the field observations. Both A. mea and C. laeviuscula were strongly attracted to S. muticum, especially if it was coated with diatoms. Only live diatoms were attractive. C. laeviuscula shunned dead diatoms and also a thin layer of silt. L. variegata proved indifferent to S. muticum, diatoms or silt. The preferences exhibited are discussed in relation to the diet of each species as well as its need for a secure perch in a turbulent environment, and concealment from predators. The grazing rates of each of the three commonest herbivores found on S. muticum were also assessed. Ampithoe mea consumed up to 4.4 mg of S. muticum tissue per individual per day, although nest-building activities greatly added to its depredations. C. laeviuscula fed largely on diatoms and did not harm S. muticum. L. variegata was the most significant of the grazers tested. Full grown specimens consumed 8.1 mg tissue individual-1 d-1, but small specimens consumed more in relation to their body weight, 1.55 mg tissue per mg animal soft tissue-1 d-1. Estimates based on the abundance of grazers on S. muticum, their feeding rates and the growth rates of the plants revealed that in late summer, epiphytic grazers could remove more tissue than is being formed. This was, however, the slow growth period for S. muticum, and earlier in the year its very rapid growth should more than compensate for the effects of epiphytic grazers. In autumn, much of the fauna of S. muticum was lost with the natural defoliation of the plants, which therefore, must be restocked annually, probably from animals inhabiting nearby Zostera marina beds.