MEPS 544:281-286 (2016)  -  DOI: https://doi.org/10.3354/meps11605

ABSTRACT: Symbiotic interactions in the marine environment have long been represented by mutualisms between photosymbionts and benthic marine invertebrates like corals and sponges. Although ‘upside-down’ epibenthic jellyfish in the genus Cassiopea also derive a substantial metabolic benefit from abundant communities of the dinoflagellate symbiont Symbiodinium, comparatively little is known about the efficiency of carbon (C) and nitrogen (N) assimilation within the Cassiopea holobiont. Using standardized 6 h incubations with ¹³C- and ¹⁵N- enriched compounds, we assessed symbiont C and N assimilation in both oral arm and bell tissue of C. xamachana under light and dark conditions. Carbon fixation was light dependent and highest in the photosymbiont-rich oral arm tissue. In contrast, ¹⁵NO₃ assimilation was light independent in both tissue types and was highest in bell tissue that was sparsely colonized by photosymbionts. This, coupled with higher bell tissue ¹⁵N enrichment under dark conditions, implicates non-photosynthetic microbes in Cassiopea N metabolism. This zonation of microbial activity may allow C. xamachana to simultaneously fix C and assimilate ambient or porewater N released during Cassiopea pumping activity. Although C. xamachana may utilize symbiont-derived N, lower ¹⁵N enrichment relative to C fixation suggests that Cassiopea may also rely on exogenous sources of N for growth. This study provides initial evidence that the efficiency of symbiont metabolism within Cassiopea jellyfish is comparable to, or exceeds, that of other common benthic marine invertebrates, supporting the contention that Cassiopea have an important role in the productivity and nutrient dynamics within their local environment.

KEY WORDS: Jellyfish · Symbiosis · Photosymbionts · Nutrient dynamics · Stable isotopes · Carbon fixation · ¹³C and ¹⁵N tracers