World's largest rays may be diving to extreme depths to build mental maps of vast oceans

An international team tagged 24 oceanic manta rays across three sites to study their deep-diving behavior. The study, published by Frontiers in Marine Science, found dives deeper than 1,200 meters, including a maximum depth of 1,250 meters, with many such dives occurring off New Zealand. After these deep dives, mantas often traveled more than 200 kilometers in subsequent days, implying that the dives serve functions beyond foraging or predator evasion. The researchers propose that by sampling cues such as Earth's magnetic field, oxygen, temperature, and light at depth, mantas may build a mental map that helps them navigate the vast open ocean. The work underscores the need for international conservation efforts and more extensive data on migratory marine species. Source: Frontiers in Marine Science.

Overview

Scientists conducted an international tagging study on the oceanic manta ray, the largest ray species, to investigate whether deep-water dives serve a navigational or informational function. The team tagged 24 individuals at three sites: Raja Ampat in Indonesia, near Tumbes off northern Peru, and near Whangaroa in northern New Zealand, collecting data from 2012 to 2022. Eight of the tags were recovered after months, while 16 transmitted data remotely. In total, the study accumulated 2705 tag-days of observation, revealing 79 days with dives to extreme depths and a maximum depth of 1,250 meters. A notable concentration of extreme dives occurred off New Zealand, with 71 such dives recorded there. "“Deep dives to the deep ocean may help mantas build mental maps of vast, featureless waters,” - Dr. Calvin Beale.

Methods and Data

The research relied on a combination of recovered archival tags and satellite-linked tags to monitor depth and movement. Recovered tags provided high-resolution data every 15 seconds, while non-recovered tags transmitted summary data. A key methodological note is that the dataset, while rich, remains relatively small and based on behavioral snapshots rather than continuous long-term tracks, underscoring the need for larger studies to strengthen the conclusions about the deep-diving function.

Deep-Diving Findings

New Zealand mantas initiated extreme dives soon after entering deeper offshore waters, typically with a stepped descent and little time spent at maximum depth. This pattern suggests that the purpose of dives is not direct foraging, nor a response to predation, but rather the acquisition of environmental information. The maximum depth reached was 1,250 meters, and 71 deeper-than-500-meter dives occurred off New Zealand, highlighting regional differences in behavior that appear linked to depth of the seafloor and the presence of deep-water corridors. "“Tagging 24 manta rays across three sites yielded 2,705 tag-days of data,” - Dr. Calvin Beale.

Interpretation and Implications

The researchers propose that manta rays use cues such as magnetic field strength and gradient, along with changes in oxygen, temperature, and light, to sample the deep environment and build a navigational mental map across hundreds or thousands of kilometers of open ocean. The observed sequence—deep dive, followed by resurfacing and extended movement over the next days—supports the idea that these dives facilitate cross-ocean connectivity rather than strictly local foraging. Understanding this deep-diving behavior could illuminate how migratory species traverse featureless oceans and connect ecosystems separated by vast distances.

Regional Variability and Conservation

Peru and Indonesia showed relatively few extreme dives, potentially due to mantas staying in shallower coastal habitats where deep-water navigation cues are less needed. In contrast, New Zealand's deep offshore habitats provide deeper excursions and the opportunity for critical information gathering. The study emphasizes that migratory species rely on both coastal and offshore habitats and calls for international cooperation in their conservation. It also notes that the deep ocean plays a central role in climate regulation and global fisheries, yet remains poorly understood and under-protected. Limitations include the small sample size and the snapshot nature of the data, prompting calls for larger, long-duration datasets to confirm the proposed function of deep dives.

Limitations and Future Work

While the findings are compelling, the authors acknowledge that using a limited number of tags and non-continuous tracks restricts the ability to definitively prove the function of deep dives. They advocate for expanded tagging programs, longer tracking, and integrated multi-region datasets to corroborate the cognitive-map hypothesis and to guide international conservation strategies for oceanic manta rays and other migratory marine species. "“Conservation requires international cooperation to protect migratory species across coastal and offshore habitats,” - Dr. Calvin Beale.