Exploring the Deep Reefs of Tubbataha: Scientific Diving with Gombessa

Philippines Underwater
Mar 7
4 min read

In 2024, a new mission of Gombessa Expeditions, led by marine biologist and underwater photographer Laurent Ballesta, took place in the Tubbataha Reefs Natural Park, a remote marine sanctuary located in the heart of the Sulu Sea in the Philippines. Recognized as a UNESCO World Heritage Site, the park protects two large coral atolls and the Jessie Beazley Reef, forming one of the most pristine coral reef ecosystems on Earth.

While the shallow reefs of Tubbataha are well known among divers for their abundance of sharks, turtles, and pelagic fish, the deeper parts of the reef remained largely unexplored. The main objective of this expedition was therefore to investigate the mesophotic zone, a depth range roughly between 60 and 150 meters, where light levels decrease significantly but coral reef ecosystems still persist.

A Technical Exploration of Mesophotic Reefs

Exploring these depths presents major challenges. Such dives require complex breathing gas mixtures, extremely careful dive planning, and long decompression times. To optimize scientific exploration under these conditions, the Gombessa team used an innovative approach known as the STEED method.

This technique involves a controlled descent along a vertical cable equipped with scientific instruments and observation stations. Divers are able to:

observe biological communities at different depths
conduct standardized visual transects
photograph and document organisms
record environmental parameters such as light levels and substrate composition

The advantage of this method is that it allows researchers to study multiple depth layers during a single dive, while maintaining consistent and comparable scientific observations between sites.

A Series of Deep Scientific Dives

During the expedition, the team conducted 13 deep scientific dives across 30 different sites, with observations ranging from 9 meters down to 127 meters.

These dives focused on studying:

reef fish communities
corals and gorgonians
sponges
benthic invertebrates
reef structure and substrate composition

Researchers also used high-definition photographic and video systems, essential tools for documenting species living at these depths and for analyzing ecological communities after the expedition.

Eight Distinct Habitats Across the Depth Gradient

One of the key findings of the expedition was that reef ecosystems change significantly with depth. The team identified eight distinct habitat types, each characterized by specific biological communities.

1. Shallow reefs (10–40 m)

This zone corresponds to typical tropical coral reefs dominated by hard corals (Scleractinians) forming large reef structures. Fish diversity is high, and turtles and predators are commonly observed

2. Upper mesophotic transition (40–60 m)

As light decreases, reef structure begins to change. Coral colonies become more spaced out, and species adapted to lower light conditions begin to appear.

3. Mid-mesophotic zone (60–90 m)

This zone proved to be particularly spectacular. Divers observed vast gorgonian forests, with some colonies reaching more than two meters in height.

These large fan corals create complex three-dimensional structures that serve as habitat for numerous fish and invertebrate species. In many ways, gorgonians act as ecosystem engineers, providing shelter, surfaces for attachment, and feeding grounds.

4. Deep mesophotic zone (90–120 m)

At greater depths, reef-building corals become less common and are replaced by organisms such as:

large sponges
black corals
other filter-feeding organisms adapted to deeper water currents

Although species composition changes, biodiversity remains high and communities become increasingly specialized.

Remarkable Biodiversity

Overall, the expedition documented 108 species of fish within the surveyed areas. Among them:

10 species had never previously been recorded in Tubbataha
several species are typical of deep reefs and rarely observed by divers
some may represent new regional records

The team also documented:

numerous giant gorgonians
large sponges
rare invertebrates associated with deep reef environments

These findings highlight the fact that mesophotic reefs represent a largely unexplored reservoir of marine biodiversity.

A Crucial Ecological Role

One of the most important aspects of the mission concerns the potential role of deep reefs in the resilience of coral reef ecosystems.

Shallow reefs around the world are increasingly threatened by:

ocean warming
coral bleaching
pollution
overfishing

Mesophotic reefs may act as ecological refuges, allowing certain species to survive at depths where environmental conditions are more stable. These deeper populations could potentially repopulate shallow reefs after disturbances.

Although this hypothesis is still under scientific debate, the data collected at Tubbataha contributes valuable insight into how reef ecosystems function across depth gradients.

A Unique Natural Laboratory

Tubbataha Reefs Natural Park provides an exceptional setting for this type of research. Its remote location and strict protection significantly limit human impact.

As a result, scientists are able to observe relatively intact coral reef ecosystems, something that has become increasingly rare in many parts of the world.

The data collected during the Gombessa expedition will now be further analyzed to:

better understand mesophotic community structures
study species distribution across depth ranges
improve conservation strategies for deep reef ecosystems

A New Frontier for Scientific Diving

The Gombessa expedition to Tubbataha highlights how deep coral reefs remain one of the last frontiers of marine exploration. Despite decades of underwater research, much of the ecosystem between 60 and 150 meters is still poorly understood.

Thanks to advances in technical diving, scientific imaging, and observation methods such as STEED, these environments are only beginning to reveal their extraordinary biodiversity.

While shallow reefs continue to amaze divers worldwide, the twilight coral gardens hidden in the depths may well be one of the ocean’s best-kept secrets.