Reverse development in the ctenophore Mnemiopsis leidyi

[This post is co-written by Joan-Josep Soto Angel and Pawel Burkhardt.]

What is this?

The video depicts the process of reverse development in the ctenophore Mnemiopsis leidyi over several weeks. Adult and larval M. leidyi are anatomically different: lobate adults have lobes and auricles that are not yet developed in the larval stage. Cydippid larvae have a rounded body and tentacles that get reabsorbed during the lobate adult stage. The timelapse video shows an adult comb jelly slowly transitioning to a larval form over time, with lobes and auricles disappearing, and tentacles being regained. This is a process called reverse development.

Where can this be found?

This footage was obtained in our Ctenophore Facility at the University of Bergen, under controlled laboratory conditions, and following the same individual over time. Whether or not these comb jellies are equally capable of doing this in the ocean still remains a mystery, but the potential is definitely there!

Read more about our Ctenophore Facility at Michael Sars Centre here: https://www.uib.no/en/michaelsarscentre/122963/ctenophora-facility

How was this taken?

The timelapse is made out of 24 individual pictures, each taken every 2-3 days, and shows the animal in the same position for comparison purposes. We used a Canon 5D Mark IV coupled to a Canon MP-E 65mm f/2.8 1-5x Macro Photo, usually known as an extreme macro lens. As the animals are very transparent, and the details are difficult to observe, we used a black background and added an extra light source from the side using a Canon speedlite strobe.

What causes M. leidyi to reverse develop?

Reverse development in M. leidyi is triggered by improved environmental conditions after a period of stress. Stress was simulated by either removing the lobes (lobectomy) or by prolonged starvation. Mnemiopsis leidyi is able to efficiently regenerate any missing body part, as well as shrinking considerably when starved. However, when adequately fed after shrinking to a size of just a few millimetres, instead of growing back the lobes, they grew tentacles typical of the larval stage.

Why should people care about this?

So far, reverse development was thought to be restricted to a few cnidarian species and one cestode. Our study is the first to report the occurrence of this peculiar feature in ctenophores, suggesting that reverse development may be more widespread than previously thought. The occurrence of reverse development in a lineage that originated prior to cnidarians can help to better understand central aspects of life cycle plasticity and evolution in early animals. The ability to rejuvenate in harsh conditions also provides further research opportunities for ecological studies aiming to explain, among others, the high invasive success of this comb jelly. Our study highlights Mnemiopsis leidyi as a potential model species to study life cycle plasticity, aging and rejuvenation.

How would you explain this to an 8-year-old?

Aging is a one-way route for a great majority of animals. However, there are a few that seem to be able to escape the fate of getting old. An adult transforming into a baby was only known for a species of jellyfish, usually called the immortal jellyfish (its scientific name is Turritopsis dorhnii). We found this capacity in an entirely new group of animals: the comb jellies. When Mnemiopsis leidyi (also known by its common name as sea walnut) becomes an adult, it grows two lobes and four pointy finger-like structures called auricles that they use for feeding. Baby sea walnuts do not have these body parts. Instead, they use two long tentacles to trap their prey and direct it to the mouth. The tentacles are lost once these animals reach their final adult form. We discovered that adult sea walnuts can rejuvenate when they eat properly after going through a period of stress.

Where can people find more about it?

You can find more information about our finding in the link here: https://www.pnas.org/doi/10.1073/pnas.2411499121

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