Environmental Factors Affect Manta Rays
October 4, 2012

Moon, Wind, Other Environmental Factors Affect Manta Ray Behavior

Alan McStravick for redOrbit.com — Your Universe Online

The manta ray, or Manta birostris as he is known among his more learned friends, only holds sway in two popular culture references that this writer can recall. As mascot for the Tampa Bay Devilrays and as school headmaster in ℠Finding Nemo´, it would seem his terrestrial visibility was in danger. So too, according to the World Conservation Union (IUCN), a global union of states, governmental agencies and NGO´s that seek to assess the conservation status of species, is his marine existence. The IUCN has the manta ray listed as ℠Near Threatened´.

M. birostris has many common names due to its wide distribution globally. Living in temperate, tropical and subtropical waters, between 35° N and 35° S latitudes, M. bistotris has been called the Atlantic manta, the aforementioned devilray, blanketfish, Prince Alfred´s Ray, diable de mer (French), oni-itomaki-ei (Japanese), and several other names that cross locale and language barriers.

The manta ray can often live up to 20 years of age. One reason for the long lifespan is that due to their size, the list of predators is quite short. Larger sharks, such as the Tiger shark, seem to be the only animals capable of engaging in predatory behavior to the ray.

Long lifespan and limited predation seem an unlikely combination to placing a species in the ℠Near Threatened´ column, but many think the fact that the manta reproduces by ovoviviparity, or birthing only one pup a breeding season, paired with a gestation period that lasts between 1 and 3 years, may play into this designation. Also, while many humans have stopped attempting to harvest the manta for its liver oil and skin (which was used as an abrasive), it is known that the meat of the manta is still considered a delicacy in the Philippines.

In a new study by Fabrice Jaine and her colleagues at the University of Queensland in Australia, it has been determined that certain environmental factors play a role in the behaviors of the manta ray. Social behavior observed by Jaine and her team, along with a whole host of citizen-scientist volunteers, such as when and where and under which conditions the manta might congregate en masse, took a huge leap forward in understanding this week. Their findings have been published in the open access journal PLOS ONE.

Their research focused on the abundance and behavior of manta rays at Lady Elliott Island in the Great Barrier Reef. The purpose behind the study, in addition to increasing the breadth of knowledge on these creatures, was, according to the study authors, important for conservation efforts, “especially in the context of a changing climate and with targeted fisheries increasingly threatening manta ray populations in various parts of the world.”

The ℠citizen-scientist´ volunteers were comprised of local tour operators and SCUBA divers who assisted in monitoring the mantas with a specific attention being paid to three types of behavior: foraging for food, cleaning by smaller fish and cruising. These activities were put in the context of environmental activities that were occurring simultaneously. Though this picture seems to suggest otherwise, the ℠citizen-scientists´ were not in any danger as the manta ray poses very little risk to humans.

What they learned was that the manta rays visit specific sites around the island for specific activities. In addition to visiting specific ℠cleaning stations´, locations where the “client” fish, in this case M. birostris, are cleaned by fish that feed off of parasites that reside on the outside skin, the team found that foraging was a predominant activity for 5 of the 7 observed sites. It was only during foraging that large groups, totaling 80 or more, would congregate. At other sites, the mantas would engage in cruising, an activity that resembles a long, slow flight-like movement.

M. birostris is not all work and no play, however. They have been observed, while near the surface, breaching, or jumping clear of the water and returning with a splash. This activity is often performed in groups with one jumping right after the other. The exact reasoning for this behavior is not fully understood but speculation is that it may play a role in attracting potential mates or is a form of play.

It was determined that the overall number of manta rays at the island was higher in autumn and winter, around the new and full moon, and when wind speeds were lower.

The authors of this study believe that their results could find future application to the understanding of the distribution of manta ray populations around the globe. Additionally, the methods for this study can be extended to other large species for which the ℠citizen-scientist´ model can be relied upon to observe and gather data on these populations.