Assessing trophic relationships between shallow-water black corals (Antipatharia) and their symbionts using stable isotopes

Shallow-water antipatharians host many symbiotic species, which spend their adult life with their host and/or use them to have access to food. Here we determine the trophic relationships between four common macrosymbionts observed on/in Cirripathes anguina , Cirrhipathes densiflora and Stichopathes maldivensis in SW Madagascar. These include the myzostomid Eenymeenymyzostoma nigrocorallium , the gobiid fish Bryaninops yongei , and two palaemonid shrimps, Pontonides unciger and Periclimenes sp. The first is an endosymbiont living in the digestive tract, while the others are ectosymbionts. The analyses show that most likely (i) none of the symbionts uses the host as a main food source, (ii) nocturnal plankton represents a main part of the diet of antipatharians while the symbionts feed preferentially on diurnal plankton, (iii) the myzostomid has the narrowest trophic niche, (iv) the two shrimps have distinct trophic niches and feed at lower trophic level than do the other symbionts. Concerning the myzostomids, they had the same δ13C values but had significantly higher δ15N values than the hosts. TEFs (Trophic Enrichment Factors) recorded were Δ13C = 0.28 ± 0.25 ‰ and Δ15N = 0.51 ± 0.37 ‰, but these were not high enough to explain a predator-prey relationship. These worms rely on the coral diet but may also ingest host fluids explaining the slight enrichment in heavier nitrogen isotopes. On the other hand, the ectosymbionts use the coral as a pathway to have access to food from the midwater: they feed from the water passing nearby the black corals, but a kleptoparasitic behaviour cannot be excluded.