Abstract
BackgroundOn coral reefs, damselfish defend their territories from invading herbivores and maintain algal turfs, from which they harvest filamentous algae. In southern Japan, intensive weeding of indigestible algae by Stegastes nigricans results in overgrowth by one filamentous alga, Polysiphonia sp. 1. Because this alga is highly susceptible to grazing and is competitively inferior to other algae, it survives only within the protective territories of this fish species, suggesting an obligate mutualism between damselfish and their cultivated alga. The wide distribution of damselfish species through the Indo-Central Pacific raises the question of whether this species-specific mutualism is maintained throughout the geographic range of the fish. To address this question, from all 18 damselfish species we conducted comprehensive surveys of algal flora within their territories throughout the Indo-West Pacific, and identified species of Polysiphonia using morphological examination and gene sequencing data.ResultsSeveral species of the genus Polysiphonia were observed as a major crop in territories throughout the geographic range of S. nigricans. Polysiphonia sp. 1 occurred only in territories of S. nigricans in central areas of the Indo-Pacific. However, its occurrence was low from the Great Barrier Reef and Mauritius. In contrast, other indigenous Polysiphonia species, which formed a clade with Polysiphonia sp. 1, occurred in the territories of fishes from Egypt, Kenya, and the Maldives. The other Polysiphonia species in the clade only inhabited damselfish territories and were never found elsewhere.ConclusionsCultivation mutualism between the damselfish S. nigricans and algae of Polysiphonia was maintained throughout the Indo-West Pacific, although algal crop species and the mode of cultivation (e.g., presence/absence of selective weeding, the species composition of algal turfs) varied among localities. This finding implies that damselfish utilize indigenous Polysiphonia species in newly colonized habitats in different ways, and therefore the algal composition and means of cultivation have diverged.
Highlights
On coral reefs, damselfish defend their territories from invading herbivores and maintain algal turfs, from which they harvest filamentous algae
Multiple species of Polysiphonia coexisted in the territories of S. nigricans and Hemiglyphidodon plagiometopon in some localities
Two other Polysiphonia species were abundant and almost exclusively inhabited S. nigricans territories from the Maldives and Kenya. These results suggest that S. nigricans provides these Polysiphonia species with their sole or primary habitat, and harvests these filamentous algae as a staple food in all localities from the Indo-West Pacific
Summary
Damselfish defend their territories from invading herbivores and maintain algal turfs, from which they harvest filamentous algae. Damselfishes individually defend territories from invading vertebrate/invertebrate grazers and maintain algal turfs, from which they obtain all of their food [9,10]. These algal turfs are rich in biomass [11,12,13,14], are highly productive [12,15], and are dominated by delicate, filamentous rhodophyta, especially members of the genus Polysiphonia [14,16,17]. The fish and the alga are highly dependent on one another
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