Article: Braun CD, Skomal GB, Thorrold SR, Berumen ML (2014) Diving behavior of the reef manta ray links coral reefs with adjacent deep pelagic habitats. PLoS ONE 9(2): e88170. Doi:10.1371/journal.pone.0088170
Many large, pelagic species have been a source of study over the past few years due to their vulnerable status based on human threats, particularly bycatch. The reef manta ray (Manta alfreda) is one such species. With small litters, a long lifespan, and a slow maturation rate, the reef manta ray is vulnerable to extinction. Information on the reef manta ray’s ecology and behavior is needed to better understand and manage a conservation strategy for this marine species. To understand the reef manta ray’s behavior, an understanding of their vertical movements is important for knowing how long and when they are in the upper water column as opposed to the lower water column will help understand their movement and feeding patterns. This article will provide information on the movements and habitat use of the reef manta ray.
During a one-year period, nine reef manta rays were tagged off the coast of Saudi Arabia (Fig. 1). A freediver manually tagged each ray using a sling-spear and each tag was designed to automatically pop off the rays after approximately 100 days. The tags measured several factors including light level, depth, water temperature, and GPS tracking information. Further analysis was taken after the tags came off the ray’s body, allowing the researchers to calculate distance traveled and vertical depth traveled.
Results and Implication
Out of the nine tags placed, two malfunctioned and did not report, and three were successfully recovered after popping of the reef manta rays. The seven tags that recorded data indicated the reef manta rays stayed in the upper 50m of the water column and only occasionally went deeper. The deepest record being 432m. This is similar behavior to other ray species, however, unlike other species, the reef manta rays in this study demonstrated preference for shallower waters during the day and deeper waters during the night. It has been shown that zooplankton (food source for the rays) move from deeper waters to shallower at night, indicating the reef manta ray takes advantage of this transition and positions itself in deeper waters to chow down on the zooplankton as the food shifts upward during the night hours.
It does need to be noted, however, that two reef manta rays were tracked going to deeper waters during midday for a couple hours. One possible explanation for this is reproductive behavior, but more research is needed.
While food source seems to be a good explanation for why the reef manta rays stayed in the shallow reefs during the day and then moved deeper at night, their swimming pattern seemed inconsistent with this. A gliding pattern was noticed when the reef manta rays took their deep dives. Descending slowly, very short bottom time, and then fast ascents were recorded. If the manta rays were indeed searching for food, they would presumably dive down and spend some time in the deep searching for food, but this leveling out dive profile was never seen. The Japanese flounder as well as the whale shark has seen to exhibit a similar “V-shaped” dive profile, slowing descending and quickly ascending. This gliding behavior is an efficient way to conserve energy, relying on the fish’s negative buoyancy while descending. Moving longer distances this way allows the ray to decrease drag and essentially slide through the water, rather than actively swimming.
The reef manta rays behaviors seen in this research provide a new ecological connection between deep and shallow waters and allow us to understand the habitat use and diving behavior of the reef manta ray. Few aspects of the reef manta ray ecology and behavior were known before this study and while more research is needed to completely understand the manta rays dive profile, the data collected in this study is a great starting point. With this new and valuable information conservation measures can be taken in order to help keep the reef manta ray out of extinction.
For my fisheries and aquatic science PhD I am working on how to tank raise urchins and transplant them onto reefs across the Florida Keys in order to help reverse the phase shift from algae dominated back to coral dominated.