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Abstract
Sponges, porous filter-feeding organisms consisting of vast canal systems, provide unique substrates for diverse symbiotic organisms. The Spongia (Spongia) sp. massive sponge is obligately inhabited by the host-specific endosymbiotic bivalve Vulsella vulsella, which benefits from this symbiosis by receiving protection from predators. However, whether the host sponge gains any benefit from this association is unclear. Considering that the bivalves exhale filtered water into the sponge body rather than the ambient environment, the sponge is hypothesized to utilize water exhaled by the bivalves to circulate water around its body more efficiently. We tested this hypothesis by observing the sponge aquiferous structure and comparing the pumping rates of sponges and bivalves. Observations of water currents and the sponge aquiferous structure revealed that the sponge had a unique canal system enabling it to inhale water exhaled from bivalves, indicating that the host sponge adapted morphologically to receive water from the bivalves. In addition, the volume of water circulating in the sponge body was dramatically increased by the water exhaled from bivalves. Therefore, this sponge-bivalve association can be regarded as a novel mutualism in which two filter-feeding symbionts promote mutual filtering rates. This symbiotic association should be called a “filtering mutualism”.
Highlights
Sponges are multicellular, porous, filter-feeding organisms with vast canal systems [1], and they provide unique substrates for diverse organisms [2]
Direct observations of dye flow and replicas of the sponge aquiferous system suggest that Spongia (Spongia) sp. has a unique canal system that enables the sponge to utilize the water exhaled from Vulsella vulsella (Fig. 4)
Considering that V. vulsella passively becomes embedded into the sponge due to sponge growth, as the bivalve has no ability to penetrate into the sponge body, the sponge must construct incurrent canal systems to inhale the bivalve’s excurrent water efficiently when encrusting its excurrent region
Summary
Porous, filter-feeding organisms with vast canal systems [1], and they provide unique substrates for diverse organisms [2]. One group of sponge-associated macroorganisms consists of bivalves belonging to genus Vulsella, which are known to inhabit specific sponge species [3,4]. These bivalves are so highly specialized for sponge-embedded life that they have lost the byssus by which bivalves attach to hard substrate; their shells are completely embedded in the sponge and only a short length of ventral commissure communicates with the external environment (Fig. 1). (identified as undescribed species, see details File S1 for details on morphology) utilizes the excurrent water from V. vulsella to circulate water in the whole sponge body via directly observing water currents in the sponge aquiferous system with tracking dye and by comparing pumping rates between the sponge and the bivalve by measuring dye flow We observed water currents in the sponge-bivalve symbiotic system in detail and tested the hypothesis that the host sponge Spongia (Spongia) sp. (identified as undescribed species, see details File S1 for details on morphology) utilizes the excurrent water from V. vulsella to circulate water in the whole sponge body via directly observing water currents in the sponge aquiferous system with tracking dye and by comparing pumping rates between the sponge and the bivalve by measuring dye flow
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