This article first appeared in Issue 17 of our free digital magazine .
The twilight zone is filled with bizarre, alien-like species that look as if they splash-landed from outer space, and yet there is something that connects us: sexual reproduction. There’s no Tinder in the twilight zone, so how are animals looking for love in a place with little-to-no light?
As NOAA Ocean Exploration biologist , 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 in the deep sea have enabled them to make the most of fleeting physical interactions. That is, unless you’re a . Yeesh…
How do twilight zone species communicate with mates?
Ashley Marranzino (AM): We still don’t know a lot about how most species are mating in the deep sea. It’s likely that animals living in the twilight zone rely on a combination of different signals to find prospective mates. Many animals likely rely on chemical signals emitted by mates. Others, like bioluminescent squids, shrimps, and fishes, also generate light that is likely used to signal to potential mates.
Some species (like myctophids, also called “lanternfishes”) produce their own light in special organs called photophores. The arrangement of photophores is different in each species. In some lanternfish species, there is even a difference in the photophore placement between males and females.
You can imagine that in an environment devoid of sunlight, having a map of different lights on your body would be a great way to signal to another individual if you are of the same species and if you are of a different sex – basically like using flashlights in a dark room to signal if you’re compatible or not.
Is finding a mate difficult?
AM: The twilight zone is the largest habitat on the planet and the animals living there aren’t always clustered together. We tend to think that it is more difficult to find mates in the twilight zone than it is in shallow water environments, like coral reefs, but the animals are continuing to find mates and reproduce so they must have a strategy.
We’re still trying to figure out exactly what triggers 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 instance, some species are broadcast spawners, meaning they release gametes (eggs/sperm) into the water and just hope that an egg will get fertilized by a sperm.
The odds aren’t high, so species that use this strategy usually make a lot of really small eggs/sperm in the hopes that a few will survive. Other species, like some shrimps, have internal fertilization and will carry the fertilized eggs until they are ready to hatch. This increases the chances of survival a bit and species that use this strategy will tend to invest more energy into caring for fewer eggs.
Some species appear to have found ways to maximize their chances of finding mates by evolving forms of hermaphroditism. The lancetfish (Alepisaurus ferox) are simultaneous hermaphrodites, meaning a single individual has mature male and female reproductive organs at the same time. By enabling lancetfish to mate with any other individual in their population (no need to find a male if you’re a female!), this mating strategy may allow lancetfish to maximize their chances of finding a mate in a large, sparsely populated habitat.
Is mating brief or complex?
AM: Mating has very rarely been witnessed in the twilight zone, but like in shallow waters, it is probably a fairly brief affair: a burst of gametes released into the water, which are quickly swept up by currents or gobbled up by other animals. But there are some exceptions.
One of my favorite mating strategies (and arguably the most bizarre) can be found in anglerfishes. Anglerfish are sexually dimorphic, meaning the males and females have different appearances: females are much larger and have the quintessential bioluminescent lure on their forehead we so often think of with these fish.
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Males, on the other hand, are extremely tiny and lack any light-producing organs. But they do have large noses and relatively large eyes, which they use to help find a female mate. Once the male anglerfish has found his mate, he latches onto the side of her body, where he will stay for the rest of his life.
In some species, the male angler fish will entirely fuse to the female mate – their circulatory systems will even join so the male receives nutrients directly from his mate. The parasitic males essentially provide a viable mate for the female whenever she is ready, allowing both to take advantage of their reproductive cycles without having to locate a new mate whenever they are ready to reproduce.
Are there any other reproductive obstacles?
AM: If species are broadcast spawning, there is a pretty high chance that the gametes will be eaten, possibly before eggs have been fertilized. From there, it is still a game of chance. Fertilized eggs are small and a great source of nutrients for other animals, so they have a very low chance of survival.
Even if a fertilized egg manages to beat the odds and survive to hatching, the risks are still omnipresent. Larvae are small and must avoid being eaten until they get larger. They also have to get enough food to grow properly. Many species will spend their larval and juvenile life stages in shallower waters where there are more food sources available. But there are also more predators to avoid in these waters. Even in the face of incredible odds, some individuals will survive to adulthood, allowing them to continue the cycle.
Along with these natural pressures, humans are adding additional obstacles for deep-sea species. Even if they seem alien and remote to us, the animals living in the twilight zone are deeply connected to life on the surface of the ocean. Climate change is altering the ocean even in the twilight zone, increasing temperature, lowering pH levels, and expanding oxygen minimum zones.
All of these changes will have implications for how animals in the twilight zone grow and find food in their most vulnerable larval and juvenile phases of life. Pollution (from land-based sources and drilling activities) also impacts the capacity for animals to reproduce in the twilight zone, exposing these animals to toxins that can lower their reproductive output or increase mortality.
What senses do you need in the twilight zone?
AM: All fishes have six senses. In addition to touch, taste, hearing, vision, and smell, fishes have an added sensory system called the lateral line system that allows them to detect water moving around them – think of it like being able to sense touch at a distance.
While the fishes in the twilight zone share these six senses with their shallow-dwelling relatives, some species have evolved specializations to these sensory systems that allow them to live in a realm with little to no sunlight (only about 1 percent of visible sunlight makes it below 200 meters [656 feet] into the twilight zone and no sunlight reaches below depths of 1,000 meters [3,281 feet]).
What do we still not understand about how deep-sea fishes communicate?
AM: There are a lot of gaps in our understanding of how deep-sea fishes communicate. Unfortunately, research on 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 labs and it is very difficult to observe deep-sea fishes in the wild – the deep-sea is hard to reach, requires expensive technology, and uses tools that typically alter fish behaviors.
Most of what we do know about communication we infer from looking at preserved specimens to hypothesize how the fish could be behaving. For example, we can study the eye of a fish to see what types of rods and cones are present to understand the sensory system’s capability even if we cannot experimentally confirm that by studying animal behavior in a lab.
New technology is being developed to help us gain a better understanding of the twilight zone and the animals that live there, which is really exciting because we still have so much to discover!
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