Exploring the Hidden Forests of the Deep – A Q&A with Eugenia Thomas

In this special feature for the Knowledge Hub, Eugenia Thomas, a MPhil candidate at the University of the West Indies, St. Augustine Campus, explores one of our ocean’s most fascinating ecosystems: deep-sea wood falls.

These submerged fragments of forest create havens for life on the seafloor, fueling biodiversity and connecting distant habitats. Drawing from her pioneering research in the Caribbean, Eugenia reveals how wood falls reveal information on deep-sea connectivity, resilience, and the vital bond between land and ocean.

What are wood falls and why do they matter?

Wood falls are terrestrial wood parcels on the surface of the seafloor that create a source of food, habitat and substrate for a range of organisms. Terrestrial wood, often displaced by storms, earthquakes or hurricanes, is transported to coastal areas by rivers and streams. Upon arrival to the ocean, it drifts via currents and becomes saturated with seawater, ultimately causing it to sink to the seafloor.

Wood falls are vital deep-sea ecosystems as they provide a source of organic carbon to the deep-sea. They support a wide range of species, from specialized wood-boring bivalves to microbial organisms, each with their own ecological niche that sustains and maintains these highly productive communities.

Why is studying deep-sea wood falls important? 

The deep sea is largely dependent on food from the surface in the form of marine snow and organic falls. The release of organic carbon and sulphur-reducing bacteria found within wood falls makes them biodiversity hotspots for the deep ocean. By studying wood falls we can explore how each functional group present at different successional stages helps contribute to deep-sea resilience and overall productivity.

Wood falls support not only life living on these wood surfaces, but they provide an ecological pathway and overall connectivity to other habitats such as cold seeps and vents. Understanding wood falls is therefore important for understanding deep-sea ecosystems and informing effective conservation and management efforts.

What role do wood falls play within deep-sea ecosystems? 

Deep-sea wood falls go through three key stages – specialist, opportunistic and sulphophilic.

Within the first ‘specialist’ stage, wood falls are colonized by specialist wood-boring bivalves known as Xylophagaidae. Often referred to as keystone species, they use their ridged shells to bore into the wood, breaking it down into cellulose and lignin and, in the process, disperse wood chips and organic carbon to the surrounding seafloor.

This progresses into the second successional stage – ‘opportunistic’, where the wood chips and organic carbon attract many predatory species, such as shrimps, crabs, sea urchins and fish, typically lasting up to two years. Additionally, the boreholes produced by the Xylophaga create a space and shelter for these species.

Within the 3^rd stage – ‘sulphophilic’, the high organic load causes a spike in respiration rates which then produces high sulphide levels and low oxygen levels. This attracts chemoautotrophic bacteria as well as other prominent chemosynthetic fauna such as bivalve molluscs – Mytilidae (Idas and Adipicola) and Bathymodiolinae, as well as siboglinid worms including Lamellibrachia barhami and Sclerolinum contortum.

What do wood falls tell us about connectivity, biodiversity and ecological processes in the deep sea? 

Wood falls are connected to other large organic falls, such as whale falls, as well as to other chemosynthetic habitats and serve as evolutionary “stepping stones” across the deep-sea floor.

Both wood and whale falls have similar sulphophilic stages and share similar species, such as the polychaete Nereis anoculis and the limpet Paracocculina cervae. Molecular phylogeny evidence shows that the gills of the bivalve molluscs in the Bathymodiolinae group, first found thriving on wood falls, host chemosynthetic symbionts, similar to those found at hydrothermal vents and cold seeps, a link known as the “wooden steps to deep-sea vents” hypothesis.

Through their successional stages wood falls create oases of life, from wood-boring fauna (Xylophagaidae and Teredinidae (shipworms)), to predators (squat lobsters, fish, starfish and sea spiders) and chemosynthetic fauna such as Lucinidae and Mytilidae mussels and microbial communities – revealing the deep-sea’s remarkable interconnectedness.

How do you study something so rare and hard to find in the deep ocean?

Wood falls are short-lived habitats not typically seen on every ROV dive, so studying them can be difficult. However, because of their terrestrial nature, wood falls are more commonly found along wooded coastal areas, rivers, maritime shipping routes and tropical and temperate forested areas. Areas known to have wooden shipwrecks can also serve in studying and advancing our knowledge on deep-sea wood falls.

Regions that are more prone to natural disasters such as storms, hurricanes and volcanic eruptions are also promising study areas as these events quickly transport trees and logs into the ocean, increasing the chances of finding these rare habitats.

What inspired your research?

The deep ocean is filled with many unknowns. Here in Trinidad and Tobago, I often look out at the ocean and wonder, “What exists there? I would like to know”. The fact that the deep ocean is home to many undiscovered species sparks my curiosity. The more technology advances, the more we are capable of exploring the deep sea.  We humans get a glimpse of what exists in the deep from ROV imagery, blog posts about the deep sea, etc. From corals, to sea cucumbers, vampire squid and the breathtaking and beautiful bloody-belly comb jelly, the deep sea is filled with amazing creatures. What is even more intriguing is materials, like forested trees, that are not of marine origin but still immensely contributing to life in the deep sea. This is why I am fascinated about wood falls and other organic falls.

For my MPhil thesis, my first chapter investigates the biodiversity of deep-sea wood falls in the Caribbean Island of Montserrat. Given that the coastline of this island is wooded and the island has experienced volcanic eruptions, it is highly likely that many wood falls exist, yet data remains sparse. My research seeks to bridge that gap, and so far we have located more than 10 naturally occurring wood falls on the sea floor! Upon examination in the laboratory, incredible fauna such as specialist species (e.g. Xylophaga abyssorum, Copulabyssia cf. riosi, and Ophiambix devaneyi) and chemosynthetic species (e.g. Idas sp. and Lucinidae sp.) were identified.

I also have the best supervisors, Dr. Diva Amon, Professor Judith Gobin, Dr. La Daana Kanhai,  who introduced me to the deep sea and inspire me daily. They are building capacity for deep sea science within the Caribbean, a region that is underexplored due to many technical and financial challenges. Together, we are building deep-sea knowledge here in the Caribbean region and fueling deep-ocean stewardship for many generations to come.

Read more about Eugenia’s research in her recent co-authored article “The Hidden Forests Below: A Review of the Ecology and Evolution of Wood Falls on the Deep Seafloor” published in Marine Biology here.