Facultative Mutualism Between Red Mangroves and Root‐Fouling Sponges in Belizean Mangal

We report results of transplant experiments that examined direct interactions between red mangrove (Rhizophora mangle) roots and two common root—fouling sponges (Tedania ignis and Haliclona implexiformis) on carbonate—based, oligotrophic mangrove cays in Belize, Central America. On these cays, subtidal prop roots of mangroves at water's edge often extend 1—2 m below lowest low water before anchoring in the substrate and host a community of algal and invertebrate epibionts dominated by massive sponges. Live sponges transplanted onto otherwise bare roots increased root growth rate two— to fourfold relative to controls. Roots fouled naturally by these and other massive sponges produce adventitious fine rootlets that ramify throughout sponge tissue; these rootlets structurally resemble underground rootlets that function in nutrient uptake. Sponges transplanted onto bare mangrove roots induced rootlet proliferation within 4 wk. Only live sponges elicited this response, indicating that adventitious rootlet production is not simply a by—product of anoxia or darkness. Sponges transplanted onto bare roots grew 1.4—10 times faster than did sponges grown on polyvinyl chloride (PVC) tubes at identical depths and locations. Relative abundances of $ { 15}$N (expressed as $\delta { 15}$N per mil) and $ { 13}$C ($\delta { 13}$C per mil) in Tedania, Haliclona, an additional sponge, Ulosa ruetzleri, and rootlets, roots, stems, twigs, and leaves of mangrove hosts suggest that mangrove roots obtain dissolved inorganic nitrogen from sponges, and that sponges obtain carbon from mangrove roots. No transfer of N or C was observed in similar analyses of roots fouled by the red alga Acanthophora spicifera. We conclude that where they co—occur, massive sponges and mangroves are facultative mutualists. In mangrove forests, as in other marginal habitats, facilitations may enable increased growth and production of component species.