5 min readKochiAug 12, 2021 05:59 PM IST
First published on: Aug 12, 2021 at 05:59 PM IST
In the 2003 blockbuster Finding Nemo, a spotted eagle ray is shown as Nemo’s class teacher carrying various tiny fish on its back. It was a classic depiction of rays as they are often seen in association with hitchhiker fish species. A recent study followed two manta ray species in the Maldives for over a 30-year period (1987–2019). Based on more than 72,000 sightings, the team found 13 species closely associated with reef manta rays and oceanic manta rays.
In an interview with indianexpress.com, lead author Aimee E. Nicholson-Jack explained more about manta rays and the recent findings. She is a researcher at The Manta Trust, UK and the paper was published last month in PLOS ONE.
Excerpts from the interview:
Can you describe the two manta ray species studied?
Reef manta rays (Mobula alfredi) have a semi-circumglobal distribution, while oceanic manta rays (M. birostris) have a circumglobal distribution; both in tropical and subtropical waters. Reef manta rays spend their lives resident to certain reefs and atolls in shallow or coastal areas. Unlike the local patterns of residency exhibited by reef manta rays, oceanic manta rays spend their lives hanging out in the open ocean or in close proximity to deep water.
The study noted that sharksucker remora (Echeneis naucrates) and giant remora (Remora remora) were the most common “hitchhiker” species. Can you explain the association?
Remoras attach themselves to their host using a modified dorsal fin that acts as a suction cup. The remora receives protection from predation, enhanced foraging opportunities, increased encounters with conspecifics, and locomotor efficiency. The benefits to the remoras are clear, but there is still debate about whether the manta ray receives a significant return in benefits. The remora may forage on the ectoparasites of the manta ray but there are certainly several negative impacts that the manta ray receives from the association.
Remoras may reduce the swimming efficiency of manta rays through extra drag created by giving remoras a ride. Also, skin abrasions and sores were created by the remora’s suction discs. Juvenile remoras have been found to force themselves into the gill sits and pelvic region of their host. One species of remora, the little remora (Remora albescens), can spend its whole life inside the mouth of a manta ray. This must be very uncomfortable for the host, especially because it can erect a spiny fin to prevent it from being squashed when the manta ray closes its mouth.
Why is it difficult to study rays and their associations?
Our understanding of these marine symbionts remains limited due to the logistical challenges associated with studying complex associations in mobile organisms over large spatial scales.
Reef manta rays can grow to have a wingspan of up to four metres and oceanic manta rays can grow to have a wingspan up to seven metres (so pretty BIG!).
What are the future studies planned?
Our study really serves as a basis for a deeper understanding of the associations that occur between manta rays and their hitchhiker’s. There is so much more to learn about these associations. Further research of hitchhikers in different manta ray populations is warranted to evaluate whether the associations found within the Maldives apply to other locations.
Investigation into the dynamics of the manta ray and remora interaction and where this relationship fits into the symbiotic continuum (mutualism, commensalism, parasitism) is definitely worthy of future research i.e., how much extra energy it ‘cost’ the manta rays to ‘carry’ these remoras.
Future research could also focus on how the relationship varies between species and life-stages of the remora.
Fun fact: Female manta rays have a gestation period of just over one year and give birth to a live pup, which measures on average 1.5-1.8 metres from wing-tip to wing-tip at birth. However, a birth in the wild has never been documented! Many gaps remain in our knowledge of the life history strategies and reproductive behaviour of these animals.
Reef manta ray and associated sharksucker remora (Aimee Nicholson-Jack) 
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