Ray Biology

The group of cartilaginous fish in the family Mobulidae (Mobulid rays) consists of two genera, Manta and Mobula, with two and nine species respectively^1,2. All mobulid rays have diamond shaped bodies, wing-like pectoral fins used for propulsion, and five pairs of gill slits. They usually inhabit pelagic zones^3,4. Mobulids are often called “devil rays” because of the cephalic fins on the front of their heads that resemble “horns”. The cephalic fins unfurl and help guide water into their mouths, and modified gill features filter zooplankton and small fish, their primary food sources^5-7.

Manta rays

The genus Manta includes the larger Manta birostris (oceanic manta), the smaller Manta alfredi (reef manta), and a possible third species, Manta. cf birostris¹ Both M. birostris and M. alfredi are circumglobal in overall range, and overlap in some locations⁸. M. cf birostris, is likely limited to the Gulf of Mexico and Western Caribbean¹. Manta birostris has a maximum wingspan (disk width, or DW) of seven to nine meters^1,3. Manta alfredi has a maximum 4 to 5 meter disk width⁹, and usually occupies tropical areas.

Mobula Rays

The nine Mobula species range in size from the largest, Mobula mobular, which can reach 5.2 meters DW, to the smallest, Mobula eregoodootenkee, which averages only 1.1 meters DW¹⁰. Mobulas can be found in temperate and tropical waters worldwide. Some Mobula species are range restricted, such as Mobula kuhlii and Mobula eregoodootenkee, found only in the Indian and Western Pacific Oceans respectively. Other species, such as Mobula tarapacana and Mobula thurstoni, are thought to be circumglobal^2,11. Since information on the distribution of this genus is based on sparse records and misidentification is common, the estimated ranges of individual species, and even some species classifications, will likely change in the coming years.

Ray Reproduction

All mobulids are aplacental, viviparous species, meaning that they give birth to fully developed live young^4,7,12, and typically bear only a single pup with each pregnancy^12,13. While the lifespan and age at sexual maturity are not yet known for many mobulid species, long-term studies of M. alfredi populations in various locations indicate a life history incompatible with targeted commercial fishing.

For example, female M. alfredi are believed to reach maturity at 8-10 years⁹, however female M. alfredi in an extensively studied population in the Maldives showed no mating scars and did not become pregnant for a number of years after reaching mature size. These observations indicate that female M. alfredi in some subpopulations may not mate until an age of 15 years or more¹⁴.

M. alfredi near a Mozambique study site and in Maui had a biennial reproductive period with some females pupping in consecutive years¹³, while in the Maldives, the reproductive cycle appears to be significantly slower, with female M. alfredi giving birth on average to only one pup every five years¹⁴. M. alfredi have been confirmed to live at least 30 years¹⁷ and both manta species are believed to live 40 years and possibly longer^9,15.

Brain Size and Intelligence

Recent research has revealed that manta and mobula rays have the highest brain mass to body mass ratio of all elasmobranchs, comparable to some birds and mammals. They exhibit high maneuverability, and increased social and cognitive abilities¹⁶. Divers cite numerous examples of manta rays cooperating and accepting help when entangled in lines, and many report that injured manta rays even seem to seek assistance.