ROVing the Reef

In tropical and subtropical regions, mesophotic coral ecosystems (MCEs), which are crucial light-dependent ecosystems that exist at depths of 30 to 150 meters, harbor a diverse array of marine organisms, including corals, sponges, and algae. These ecosystems serve as crucial habitats for a variety of marine life and contribute to overall marine biodiversity.

While shallow coral reefs grapple with substantial threats like pollution, MCEs remain relatively less affected by these stressors. As such, they provide a potential refuge for marine organisms and play a significant ecological role.

Despite their importance, MCEs remain poorly understood. Comprehensive surveys are urgently needed to deepen our understanding of their basic biology, taxonomy, distribution, and ecological connectivity.

Traditionally, these surveys have been carried out by scuba divers with expert-level knowledge of MCEs. However, this approach does not scale well, and becomes impossible when ecosystems at greater depths must be explored. Underwater robots have emerged as another avenue to collect more data, however, they are commonly equipped with high-resolution cameras as their primary data collection instrument. Unfortunately, appearances alone are of limited value in this area of research.

To enable the collection of the richer data that is sorely needed, a new approach must be found. A research team at the Okinawa Institute of Science and Technology Graduate University recently put forth a potential solution that may help to fill that gap. They experimented with an underwater mini-remote operated vehicle that is capable of extensively surveying MCEs. Because it can be precisely controlled, the vehicle can collect seawater samples in very close proximity to the coral.

It is well known that these corals continuously secrete mucus into sea, and that mucus contains environmental DNA (eDNA) samples. Using the submersible, large numbers of these samples can quickly be acquired from an extensive geographical range, before being returned to the surface where they can be analyzed in a lab. This information can then reveal the specific types of corals that are present in each area.

The researchers tested out their plan by collecting samples from 24 different sites at 20 to 80 meters in depth using the underwater vehicle. In each case, seawater was collected about one to two meters above the top of the coral. After coral-specific eDNA amplifications were performed, the results were assessed. For 23 of the 24 samples, it was demonstrated that the genera of the coral could be determined using no more than 0.5 liters of seawater. There was also a clear distinction observed between the types of corals found in the shallower and deeper waters.

While future technical upgrades are planned by the team, the remote-controlled underwater vehicle has already proven its usefulness in helping researchers to better understand MCEs. With future enhancements, the quality of the data should only get better.