The twinkling lights of a starlit night get some humans in the mood for romance, and they seem to have the same effect on certain ostracods — ocean-dwelling crustaceans the size of a sesame seed.
James Morin, professor emeritus in the Department of Ecology and Evolutionary Biology and former director of the Shoals Marine Laboratory, first discovered that Caribbean ostracods use bioluminescence in courtship signaling in 1980. Last fall, Morin and his co-authors described a new genus and species of Caribbean ostracod, Maristella chicoi.
On March 19, the World Register of Marine Species named the creature as one of its top 10 remarkable new species from 2019.
“These displays have been going on for millions of years, but nobody ever saw them because most people who night dive go out with bright lights and on moonlit nights,” Morin said. “There aren’t many people crazy enough to go out and swim around in the dark on moonless nights and watch these things.”
Nicknamed the ‘Star of the Sea’ seed shrimp, the genus name Maristella reflects “the amazing phenomenon of these tiny animals twinkling like little stars in the ocean,” according to the press release from the World Register of Marine Species, a comprehensive taxonomic database of marine organisms. The top 10 species were chosen to reflect the immense diversity found in marine environments and for their notable interest to both researchers and the public.
Maristella chicoi is part of a broader group of ostracods that can be found everywhere in the ocean, from shallows to deep seas and in both tropical and polar waters. In all of these habitats, there are species capable of bioluminescence, primarily serving as a defense mechanism against predators, such as small fish.
Scientists believe bioluminescence protects the ostracods by startling would-be predators with a blinding flash of light and by attracting the attention of even bigger predators to the now-illuminated fish. They also apparently taste terrible, as fish will spit out an ostracod that secretes its bioluminescent mucus ball in a fish’s mouth.
However, only in the Caribbean have certain ostracods – including Maristella chicoi – apparently evolved the ability to also use their bioluminescence in courtship.
In each species, males form swirling trains of light in the water and pulse the same pattern. And different species have distinct train patterns. For females witnessing these displays, the purpose seems to be to not only distinguish one individual from another, but to distinguish the correct species in an ocean full of pulsing creatures.
“If you’ve got thousands of individuals luminescing, there might be five different species in one area. The colors are a tiny bit different between species, but the patterns are really distinctly different,” Morin said. “The total amount of light in the train, the spacing in the train, the light coming from any one pulse, is all very consistent from one male display to another in a species.”
Reproductive pressure on male ostracods is fierce. Females probably mate only once in their lives before returning to the reef to produce multiple broods, whereas males can mate repeatedly.
“When we catch these in our nets, we find dozens of males for every female,” Morin said. “The males are working really hard and we think that’s where the synchronous luminescence comes from. They’re all saying, ‘Me, me, me!’ to the females.”
But surprisingly in all these species, females use bioluminescence only to deter predators, but not in mating.
Very little is known about why this group of Caribbean ostracods, known as a clade, developed the ability to use bioluminescence in their courtship, and why ostracods in the rest of the oceans have not.
“This clade of ostracods is highly diverse,” Morin said. “They’re an interesting group for their own sake, but also as a model system for studying the evolution of bioluminescence and its role in the development of new species generally.”
Krisy Gashler is a freelance writer for the College of Agriculture and Life Sciences.