In situ observations on preferential grazing of seaweeds by some herbivores

Grazing of seaweed tissues by herbivores causes inconsistent crop yields that make commercial seaweed farming a less economically viable venture. In most situations, about 10% of available seaweed biomass is removed by the herbivores. To identify the seaweeds that are preferred by the herbivores, a study was carried out near the experimental seaweed farming site at Krusadai Island (9°14.823′N; 79°12.921′E), southeast coast of India. Abundant populations of grazer fishes, namely Siganus javus (Rabbit fish), Acanthurus sp. (Surgeon fish), Cetoscarus sp. (Parrot fish) and sea urchin Tripneustes sp. were observed near this site. Twenty different seaweed species tested for this study include Caulerpa racemosa, C. taxifolia, Halimeda gracilis, H. macroloba (all Chlorophyceae), Sargassum wightii, Turbinaria conoides, Dictyota dichotoma, Padina boergesenii, Pocockiella vaiegata, (all Phaeophyceae), Hypnea musciformis, H. valentiae, Champia parvula, Acanthophora spicifera, Gelidiella acerosa, Gracilaria crassa, G. edulis, G. dura, G. corticata, Laurencia papillosa and Kappaphycus alvarezii (all Rhodophyceae). Among these, only five species of Rhodophyceae were grazed. G. edulis was the preferred choice of herbivores and 72 ± 17.8% of its biomass (P < 0.03) was grazed from the initial biomass of 5 g fresh wt. The corresponding grazing value for G. dura was 57.4 ± 28% (P < 0.02), for G. corticata, 47.2 ± 27.2% (P < 0.02), for L. papillosa, 41 ± 21% (P < 0.01) and for K. alvarezii, 34 ± 10.4% (P < 0.01) against their original biomass of 5 g fresh wt each. These observations apparently show that grazers prefer coarsely branched, filamentous algae or succulent algae because of the amenability for grazing. Rhodophycean (red) algae were preferred over Pheaophycean (brown) and chlorophycean (green) algae and this could be attributed to palatability of the grazers. Plants gained protection from herbivores, if neighbouring plants are less palatable.