How do you find a partner in the twilight zone?

This article first appeared in issue 17 of our free digital magazine CURIOUS.

The Twilight Zone is filled with bizarre, alien species that look like they landed from space, and yet there is something that connects us: sexual reproduction. There is no Tinder in the Twilight Zone, so how do animals search for love in a place with little to no light?

As NOAA Ocean Exploration biologist Ashley Marranzino, who has a background in deep-sea fish and sensory biology, told us, there are many unanswered questions, but what we do know is that many of the species’ adaptations to life have occurred in the deep sea . allowed them to make the most of fleeting physical interactions. That is, unless you’re a male anglerfish. Yes…

How do twilight zone species communicate with partners?

Ashley Marranzino (AM): We still don’t know much about how most species mate in the deep sea. It is likely that animals living in the twilight zone rely on a combination of different signals to find potential mates. Many animals likely rely on chemical signals emitted by partners. Others, such as bioluminescent squid, shrimp and fish, also generate light that is likely used to send signals to potential mates.

Some species (such as myctofids, also called ‘lanternfish’) produce their own light in special bioluminescent organs called photophores. The arrangement of photophores is different in each species. In some species of lanternfish, there is even a difference in the arrangement of the photophores between males and females.

You can imagine that in an environment without sunlight, having a card with different lights on your body would be a great way to signal to another individual whether you belong to the same species and whether you are of a different gender – ​​basically just like using flashlights in a dark room to indicate whether you are compatible or not.

Is finding a partner difficult?

BEN: The Twilight Zone is the largest habitat on Earth and the animals that live there are not always together. We tend to think that it is harder to find mates in the twilight zone than in shallow water environments such as coral reefs, but the animals continue to find mates and reproduce, so they must have a strategy.

We’re still trying to figure out what exactly drives different species to mate, how often they reproduce, and how many offspring, or gametes, they produce. Many of the general strategies are similar to what we see in shallow water species. For example, some species are broadcast spawners, meaning they release gametes (eggs/sperm) into the water and just hope that an egg is fertilized by a sperm.

The odds aren’t great, so species that use this strategy usually make lots of very small eggs/sperm in the hope that a few will survive. Other species, such as some shrimp, have internal fertilization and will carry the fertilized eggs with them until they are ready to hatch. This increases the chances of survival slightly and species that adopt this strategy will tend to put more energy into caring for fewer eggs.

Some species appear to have found ways to maximize their chances of finding a mate by developing forms of hermaphroditism. The lancetfish (Alepisaurus ferox) are simultaneous hermaphrodites, meaning that a single individual has mature male and female reproductive organs at the same time. By allowing lancetfish to mate with any other individual in their population (you don’t have to find a male if you’re a female!), this mating strategy can allow lancetfish to improve their chances of finding a mate in a large, sparsely populated maximize habitat.

Is the mating short or complex?

BEN: Mating has been observed very rarely in the twilight zone, but as in shallow waters it is probably a fairly brief affair: a burst of gametes released into the water, which are quickly swept away by the current or swallowed by other animals. But there are some exceptions.

One of my favorite mating strategies (and perhaps the most bizarre) is found in anglerfish. Anglerfish are sexually dimorphic, meaning the males and females look different: females are much larger and have the typical bioluminescent lure on their foreheads that we so often think of in these fish.

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Males, on the other hand, are extremely small and have no light-producing organs. But they do have large noses and relatively large eyes, which they use to find a female mate. Once the male anglerfish finds his mate, he clings to the side of her body, where he will remain for the rest of his life.

In some species, the male anglerfish will fuse completely with the female partner; their circulatory system will even connect so that the male receives nutrients directly from his partner. The parasitic males essentially provide a viable mate for the female when she is ready, allowing both to benefit from their reproductive cycles without having to find a new mate when they are ready to reproduce.

Are there any other reproductive obstacles?

BEN: If species are sent out during spawning, there is a fairly good chance that the gametes will be eaten, possibly before the eggs are fertilized. From there it’s still a game of chance. Fertilized eggs are small and a great source of nutrients for other animals, giving them a very low survival rate.

Even if a fertilized egg manages to beat the odds and survive until hatching, the risks are still ubiquitous. Larvae are small and must avoid being eaten until they grow larger. They must also be given enough food to grow properly. Many species spend their larval and juvenile life stages in shallower water where more food sources are available. But there are also more predators to avoid in these waters. Even against incredible odds, some individuals will survive into adulthood, allowing them to continue the cycle.

In addition to these natural pressures, humans add additional obstacles for deep-sea species. Even though they seem strange and far away to us, the animals that live in the twilight zone are deeply connected to life on the surface of the ocean. Climate change is even turning the ocean into the twilight zone, causing temperatures to rise, pH levels to drop and oxygen minimum zones to widen.

All of these changes will impact how twilight zone animals grow and find food in their most vulnerable larval and juvenile life stages. Pollution (from land-based sources and drilling activities) also affects the ability of animals to reproduce in the twilight zone, exposing these animals to toxins that can reduce their reproductive potential or increase mortality.

What senses do you need in the twilight zone?

BEN: All fish have six senses. In addition to touch, taste, hearing, sight and smell, fish have an additional sensory system called the lateral line system that allows them to detect water moving around them. Think of it as if you can sense touch at a distance.

While twilight zone fish share these six senses with their shallow relatives, some species have evolved specializations in these sensory systems, allowing them to live in a realm with little to no sunlight (only about 1 percent of visible sunlight allows) . under 200 meters [656 feet] in the twilight zone and no sunlight reaches a depth of 1,000 meters [3,281 feet]).

What do we still not understand about how deep-sea fish communicate?

BEN: There are many gaps in our understanding of how deep-sea fish communicate. Unfortunately, research into communication and behavior requires observing live animals, either in a laboratory or in the wild. It is not possible to keep these species alive in laboratories and it is very difficult to observe deep-sea fish in the wild – the deep sea is difficult to reach, requires expensive technology and uses tools that usually change the behavior of fish.

We infer most of what we know about communication by looking at preserved specimens and hypothesizing how the fish might behave. For example, we can study the eye of a fish to see what types of rods and cones are present to understand the capabilities of the sensory system, even if we cannot confirm this experimentally by studying animal behavior in a laboratory.

New technology is being developed to help us better understand the Twilight Zone and the animals that live there. That is very exciting, because we still have so much to discover!

CURIOUS magazine is a digital magazine from IFLScience with interviews, experts, deep dives, fun facts, news, book excerpts and much more. Issue 20 is out now.

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