Flight of the manta
Manta rays sure are beautiful. Sleek, graceful, and fairly docile congregations these once feared ‘devilfish’ are now attracting tourists from all over the world. You can’t blame them – just look at this footage from a night dive off Hawaii http://ow.ly/tpZou
. But despite all this interest – and the recent listing of the genus Manta
to CITES – the Convention on International Trade in Endangered Species – there is still a lot we don’t know about them. But a new piece of research by Camrin Braun of +King Abdullah University of Science and Technology
/ +Woods Hole Oceanographic Institution
and colleagues sheds a little more light on one species these enigmatic creatures.
Manta rays – the Mobulidae – are actually made up of two genera. In the Mobula
genus there are 9 species and in the Manta
genus just 2. Camrin and his team focused their work on the reef manta (Manta alfredi)
just off the coast of Al Lith, Saudi Arabia (south-central Red Sea). The team noted that some progress has been made in learning a little more about the movements of these rays, but there is one glaring hole. Most research has focused on how they move horizontally through the water, but the ocean isn’t a one-dimensional habitat – it has depth as well as breadth. The reef manta is classified as vulnerable to extinction, it has earned its spot on the Bonn Convention (Convention on Migratory Species) and as I mentioned earlier, a CITES listed species. Understanding the spatial ecology of these creatures – how and why they use different parts of the ocean – is really important for implementing effective conservation measures and managing human interactions with this at risk species.
A trained freediver tagged 9 of the Red Sea reef mantas with Pop-up Satellite Archival Tags (PSAT) to collect movement data on the rays. PSATs are really handy pieces of technology. Once attached, they can store all sorts of data such as GPS location, time and date, temperature, depth, and salinity levels. The beauty of PSATs is that you don’t have to recover the tag to access the data. After a set amount of time, the tags pop off the animal, surface, and upload the data to your computer via satellite. Very handy for species that move around a lot and are tricky to find. In this study the tags were set to record depth, temperature, and light levels every 10 – 15 seconds. Two of the tags also transmitted GPS data when possible (for this bit to happen, the individual needs to be at the surface long enough for satellites to get a fix). The tags collected data for up to 180 days, after-which it was time for the team to do some analysis and see where the rays went to.
As you would expect, each individual differed slightly it its movements, but there appeared to be some solid patterns. During the day they moved nearer to the surface and during the night headed down deeper, waters. This is a little odd because the rays are filter feeders; zooplankton is their prey. Zooplankton undergo what is known as a diel vertical migration, moving to the surface waters during the night and the depths during the day. Pointing to studies looking at other filter-feeders, the researchers hypothesize that the rays may be feeding on the plankton as they move up into shallower waters, as well as plankton coming off the reef benthos at night. The mantas do stray away from coastal reef areas into more offshore regions too. Here, there seemed to be a correlation between the moon and how deep they went. During full moon phases, the mantas seemed to favour deeper waters, most likely in response to the movements of the zooplankton that wanted to go deeper to hide in the darkness.
So just how deep do the reef mantas go? A previous study indicated 100 meters, but it this research one ray went to a whopping 432 meters. Deep dives were rare though and it didn’t appear that the rays was feeding below 200 meters – their bottom time was just a bit too short for that. The researchers aren’t entirely sure what the rays were doing down there either, but based on other species and the physiology of the rays it might be an energy efficient way of moving. As they ascend, the hypothesis goes, the rays body shape and negative buoyancy allows they to gain substantial lift resulting in a rather effortless glide through the ocean. Well, who doesn’t like to relax.
This paper is published in the open access journal PLoS ONE
. If you fancy having a read of it, you can find it here http://dx.doi.org/10.1371/journal.pone.0088170
Image: Take by Andrea Marshall. This image was the +National Geographic
Photo of the Day on 14 July 2013. Andrea took the photo off Nusa Penida, near Bali. She has done a short write-up on how this amazing image came about – check it out here http://ow.ly/tq58e #science #sciencesunday #marinescience #mantaray #spatialecology #behaviouralecology