Over the millennia, perpetual geological sea level changes have continuously shaped coral reef environments. Coral reef biodiversity, considered at its peak within the Coral Triangle, is most notable throughout the Philippine archipelago largely due to it having among the highest number of islands (more than 7,100) per geographical area. The origin of such diversity remains the subject of much controversy and investigation generating hypotheses of speciation and migration from within or outside the region. Nevertheless, central to determining reef biodiversity and biogeographical distributions are species responses to environmental change, particularly as human impacts accelerate and/or exacerbate natural processes.
Coral reefs in the ‘twilight zone’ between 40 and 200 m are termed ‘mesophotic coral ecosystems’ (MCEs). MCEs are classified as conservation priority ecosystems (e.g. IUCN) and posited to act as refugia against environmental disturbances. Until recent technological advances, such mesophotic reefs were largely inaccessible or overlooked, being too precarious for conventional SCUBA and too shallow to justify the cost of a deep-sea submersible. However, exploration using advanced closed-circuit rebreather technology has enabled us to penetrate the mysteries of these little-studied reefs. Here, as part of a collaborative research team supported by the Seaver Institute, our mission as the Bishop Museum rebreather team was twofold: to test-pilot the Poseidon next generation rebreather; and to discover, document and characterize mesophotic reefs within the Batangas region of the Philippines.
Descending to depths unexplored, away from the bustle of the shallows, is not without risks. As inquisitive explorers we are caught in peace and exhilaration between the enticing attenuation of the deep slope and the mesophotic reef before us. To me these reefs are akin to being transported back in time. Not only do MCEs harbour a myriad of new species, behaviours and interactions just waiting to be discovered, they also hold the keys to questions of persistence, polyphyly, and the evolution of structure–function relationships over geological time. What fauna and even flora are present, how do they survive, how are they beneficially connected to the surrounding habitats if at all, and are such patterns duplicated in other areas?
With the exception of reefs within Hawai’i which are often dominated by members of the zooxanthellate genus Leptoseris, low-light habitats throughout the Pacific are typically characterized by gorgonian sea fans, soft corals, sponges and black corals, the majority of which are unknown to science. The Philippines are no exception; on this trip alone we have discovered at least seven new fish and 20 new gorgonian species at depth, the latter providing substrate for fish to lay their eggs. Our diving experience tells us that it is at around 80 m depth that the temperature drops, a shelf emerges, and where such characteristic ‘twilight’ assemblages begin to be found. Dynamic sea-level changes produce extreme variations in shallow water habitats of up to 80–100 m, with these deeper reefs remaining relatively unchanged, and less than 50% species overlap with shallow taxa. Therefore, gorgonian corals and reef fish are key taxonomic groups within deep-reefs with some taxa spanning considerable depths (e.g. 5–2,000 m).
At depths greater than 100m we work against time, determined by the ratio between the saturation of our mortal tissue with dissolved gas and the safety gas that we carry. Yet no part of what we encounter is lost; all digital imagery is synchronized with the rebreathers’ advanced electronics, capturing time, depth and specimen information. I continue to collect gorgonians as we ascend—intrigued by their dynamic patterns of diversity with depth. The swim bladders of sampled fish are vented with needles as we ascend (the notorious ‘Pyle’ stop). At the surface, specimens are catalogued and preserved, whilst digital imagery is systematically documented for immediate dissemination in the web-based archive Explorers Log. The discovery of the twilight zone is not exclusive— everyone can immediately enjoy!
Are shallow reefs being seeded by deep reefs? Are certain taxa habitat specialists or bathymetric migrators? These questions remain to be elucidated, but when they are, key evolutionary mechanisms facilitating survival over geological time will undoubtedly be unveiled. So even though as humans we are challenged to save habitats from their ongoing destruction, ‘twilight’ reefs of refugia may likely replenish many of their inhabitants.
Dr. Sonia Rowley (soniajrowley@ gmail.com) is an invertebrate zoologist at the Bernice Pauahi Bishop Museum, and the University of Hawai’i, USA.
Funding and collaborations: Bernice Pauahi Bishop Museum, Hawai’i, USA; Hawai’i Institute of Marine Biology, Hawai’i, USA California Academy of Sciences, California, USA; Seaver Institute, California, US