Manta rays are truly extraordinary creatures, easily distinguishable from other rays due to their unique physical features. Their coloration is particularly fascinating, boasting two distinct patterns. The first pattern, known as the chevron, exhibits a mostly black back and a striking white belly. The second pattern, the black color type, showcases almost entirely black sides. With large diamond-shaped bodies, enhanced by elongated wing-like pectoral fins, gill slits located on their undersides, eyes positioned on the sides of their bodies, and wide mouths, the manta will not be confused with any other animal! As one of the largest ray species, mantas rely heavily on tiny plankton as their primary source of sustenance. Notably, in front of their mouths, manta rays possess two intriguing structures called cephalic lobes, which play a crucial role in channeling water into their mouths during feeding activities.
Manta rays have a widespread global distribution, inhabiting tropical, subtropical, and temperate waters. However, their specific habitats vary based on the species they belong to. Giant manta rays (Manta birostris) are commonly found in offshore and oceanic waters, as well as in productive coastal areas such as estuaries, oceanic inlets, bays, and intercoastal waterways. In contrast, reef mantas (Manta alfredi) tend to reside predominantly in productive coastal environments, particularly reefs and atolls. Distinguishing between these two involves examining not only their preferred habitats but also their coloration patterns. Manta rays typically exhibit pale grey ‘chevron’ shapes on their backs and in reef mantas they form a ‘Y’ shape, while in giant mantas sport a more defined ‘T’-shape. Moreover, a discernible lump at the base of their tail is a notable feature of giant manta rays, attributed to a vestigial remnant of their spine. This distinctive trait is absent in reef manta rays. Every manta ray also has spots on its underbelly that serve for individual identification. However, in reef manta rays, these spots can be found anywhere on their underside, including both the wings and belly while in giant manta rays, the spots are mainly limited to the belly, with no markings under the wings. Recently, reef and giant manta rays were distinguished as separate species based on genetic evidence.
Belonging to the family Mobulidae, their imposing size and captivating beauty make them a significant attraction for tourists. However, this popularity also exposes them to growing threats from the exploitation of their gill rakers, causing concern for their survival. Despite their widespread distribution, their populations are threatened by various factors, including overfishing, habitat degradation, and entanglement in fishing gear. Their slow reproductive rate also makes them susceptible to population declines. With this in mind, research conducted by a collaborative team consisting of scientists from NOAA’s Pacific Islands Fisheries Science Center, the Hawai‘i Association for Marine Education and Research, and the University of Miami, utilized genome scans to explore the genetic distinctions between reef manta rays found on Hawaiʻi Island and Maui Nui. The study recorded only 318 distinct individuals near Hawaiʻi Island since 1979 and 600 individuals in Maui Nui since 2005. The relatively small breeding populations (104 on Hawaiʻi Island and 129 in Maui Nui) increase their vulnerability to individual losses. The groundbreaking genetic study has shed light on the unique and isolated populations of manta rays here, with the findings revealing that these creatures exist in separate island-specific groups, each containing just a few hundred individuals.
In fact, migration rates here are equivalent to one female moving between island groups every 1,300 years and one male only about every 64 years! Supporting the genetic findings, to date, no photo-ID has been matched between the two populations confirming that they do not mix. And by employing genome scans, the researchers were able to approximate the count of adult breeders within each population… which is quite the startling number. The findings revealed that Hawaiʻi Island had a mere 104 adult breeders, while Maui Nui hosted approximately 129 individuals contributing to the breeding population. “These are already small populations and even a smaller proportion are actually breeding,” said Dr. Richard Coleman, an Assistant Professor from the University of Miami and coauthor of the study. “With so few breeders, the loss of a single individual could have long term detrimental consequences to the resilience of Hawai‘i reef manta rays.”
The study also offers the first direct evidence that female reef manta rays reproduce close to their birthplace. Due to this limited movement between islands, and females tending to reproduce near their birthplace, this has resulted in strong genetic isolation among the populations in Hawai’i. “Ultimately, this means that each island population is demographically on its own. So if the manta population in West Hawaiʻi declines, they are unlikely to be repopulated by the neighboring islands,” said Dr. Jonathan Whitney, an ecologist at NOAA’s Pacific Islands Fisheries Science Center and lead author of the study. This restricted migration, coupled with slow migration rates, has made the manta ray populations on these islands self-sustaining. But it also reveals the alarming challenges these isolated manta ray populations face in Hawaiʻi. Their slow growth rate, late maturity, and infrequent reproduction (having only a single pup every 1 to 7 years) make them particularly susceptible to threats from human activities. “We are fortunate that manta rays in Hawaiʻi are protected against being captured or killed, but they are still vulnerable to human-made threats,” said Dr. Mark Deakos, coauthor and Director of the Hawai‘i Association for Marine Education and Research. “These results highlight the vulnerability of these populations and stress the need to ensure we safeguard these precious ocean giants for future generations.”
As a response to these findings, the researchers are taking proactive measures to monitor these small, island-specific manta ray populations and plan to expand their study to gain a better understanding of gene flow within these groups. Additionally, a statewide survey is currently underway, seeking to gather comprehensive data about manta rays across all of Hawaiʻi’s islands. In an effort to involve the community in conservation efforts, researchers encourage people to report sightings of manta rays on any island to the Hawaiʻi Association for Marine Education and Research and the Manta Pacific Research Foundation. Such reports can provide valuable information for ongoing research, enabling scientists to better safeguard these isolated and vulnerable populations.
Read the full article here
