High-resolution imaging sheds new light on a multi-tier symbiotic partnership between a walking solitary coral, a sipunculan, and a bivalve from East Africa

Marine symbioses are integral to the persistence of ecosystem functioning in coral reefs. Solitary corals of the species Heteropsammia cochlea and Heterocyathus aequicostatus have been observed to live in symbiosis with the sipunculan worm Aspidosiphon muelleri muelleri, which inhabits a cavity within the coral, in Zanzibar (Tanzania). The symbiosis of these photosymbiotic corals enables the coral holobiont to move, in fine to coarse unconsolidated substrata, a process termed as walking. This allows the coral to escape sediment cover in turbid conditions which is crucial for these light-dependent species. An additional commensalistic symbiosis of this coral-worm holobiont is found between the Aspidosiphon worm and the cryptoendolithic bivalve Jousseaumiella sp., which resides within the cavity of the coral skeleton. To understand the morphological alterations caused by these symbioses, interspecific relationships, with respect to the carbonate structures between these three organisms, are documented using high-resolution imaging techniques (scanning electron microscopy and mu CT scanning). Documenting multi-layered symbioses can shed light on how morphological plasticity interacts with environmental conditions to contribute to species persistence.

Automated summary

Marine symbioses play a crucial role in the persistence of coral reef ecosystems. The symbiotic relationship between corals and sipunculan worms allows the corals to move and avoid sediment cover, which is essential for their survival. Additionally, the commensalistic symbiosis between the worm and the bivalve contributes to the morphological alterations and species persistence in these organisms.