One colony, many niches — Modularity in the Mediterranean octocoral holobiont

Mon, 01 Jun 2026 00:00:00 +0000

Mediterranean gorgonians are foundational species of coralligenous reefs, building the three-dimensional habitat that sustains much of the basin’s benthic biodiversity. They are also among the organisms most exposed to the climate crisis: recurrent marine heat waves have driven mass mortalities and steep population declines across the northwestern Mediterranean. Like all corals, gorgonians are holobionts — the animal host together with the bacteria, archaea, protists, and viruses living in association with it. These microbial partners shape host physiology, including the capacity to withstand environmental stress, and their composition can shift under challenging conditions, sometimes differing between colonies of the same species.

Octocoral colonies are modular animals built from many polyps, and in gorgonians those polyps are polymorphic, dividing labour among feeding, reproduction, and the detection of prey and predators. The branching, tree-like growth form gives the upper colony privileged access to resources in the water column, while the base must allocate energy to anchoring and to defending colony integrity against substratum-associated processes. Such physiological trade-offs — energy partitioned among competing functions within a single individual — are known to leave a positional signature in reef-building corals: in calcifying Acropora, branch tips combine a low density of photosymbionts with the highest calcification rates, consistent with carbon fixed in lower branches being translocated upward, and transcriptional studies have found position-dependent expression of genes linked to toxin production and skeleton formation. Whether comparable functional structure exists in non-calcifying, temperate octocorals remains largely unexplored.

This project asks whether the holobiont is itself modular along the colony — whether the prokaryotic microbiome, the protistan (eukaryotic) microbiome, and the host transcriptional response differ systematically between colony regions. We study three Mediterranean octocorals that span contrasting nutritional strategies: Paramuricea clavata and Leptogorgia sarmentosa, heterotrophic suspension feeders, and Eunicella singularis, which uniquely among the three supplements its diet through dinoflagellate photosymbionts. For each species we sample three regions of the colony — apex, centre, and base — and apply a multiomics approach that pairs 16S and 18S rRNA metabarcoding, to resolve the prokaryome and eukaryome, with host transcriptomics, to map functional variation. Our first results characterise the bacterial communities of E. singularis across 27 samples (nine per region); eukaryome and transcriptome analyses are underway.

The work has a direct conservation application. In the Mediterranean, E. singularis and L. sarmentosa are frequently fragmented by fishing gear, and local fishers have begun returning broken fragments to the seafloor to help restore depleted populations. By identifying which part of the colony yields fragments with the gene-expression and microbial profiles most conducive to survival and regrowth, we aim to tell restoration practitioners which fragments to prioritise — turning an accidental by-product of fishing into a more targeted and effective recovery strategy.

Restoration | del Campo Lab - Microbial Ecology and Evolution

One colony, many niches — Modularity in the Mediterranean octocoral holobiont