Versatile habitat conditioning by damselfish cultivating turf algae on giant clams

Abstract

Territorial farmer damselfishes are facilitators that selectively farm turf algae in degraded reef patches. The effect of damselfish on algae and corals has been widely studied but the ability of the damselfish to establish farms on other substrates is less understood. In the giant clam ocean nursery in Bolinao, northwestern Philippines, farmer damselfishes, particularly Stegastes nigricans, Dischistodus prosopotaenia, and D. chrysopoecilus, make use of giant clam shells as substrate for turf algae farms. To facilitate growth of the algal crop, damselfishes reduce shading and create more space for cultivation by trimming the overhanging mantles of Tridacna gigas within their colonies. This form of habitat modification occurs within weeks, with affected clams exhibiting mantle lesions and turf algae growth on their shells. These observations suggest that damselfishes can potentially extend their farmed turf algae patches in a manner that, although not typical of natural reef conditions, may be facilitated by the availability of giant clam shells as substrate. The versatility of damselfishes in establishing turf algae farms on a different substrate and their ability to condition the habitat through active modification of other reef biota emphasize the potential of the farmer damselfish to significantly influence reef community structure and biodiversity.