Scientists are cataloging a variety of marine fish eggs

This article was originally featured on Hakai Magazine, an online publication on science and society in coastal ecosystems. Read more stories like this at

They come in a seemingly endless variety of shapes, sizes and colours, and marine fish are easy to tell apart. Unfortunately, the same cannot be said for their eggs.

The eggs of marine fish are so difficult to tell apart that even aquarium curators like Kylie Lev, who works at the California Academy of Sciences Steinhart Aquarium, often have no idea which fish created the eggs floating around the aquarium’s multispecies exhibits. The inability to identify these embryos is a major obstacle to building a more sustainable aquarium trade, says Lev.

Although many aquariums have breeding pairs on display, the majority of tropical marine fish in public aquariums come from the wild, says Lev. “We can offset some of that by raising fish,” she says.

Although the number of fish species being bred in captivity is increasing all the time, aquarists and private breeders have only figured out how to grow a small fraction of the marine fish species commonly seen in aquariums, such as clownfish and blue kelp. Tens of millions of fish are removed from coral reefs and other sensitive marine habitats each year to stock private and public aquariums. Although scientists have yet to determine the full ecological impact of the aquarium trade, what they do know doesn’t look good.

That is why Lev and her colleagues have been working with other aquariums for the past year and created an open source catalog of marine fish eggs.

Using this catalog, launched late last year, aquarists can easily identify the stray eggs in their saltwater exhibits, allowing them to focus their limited breeding resources on species they have the best chance of rearing successfully. The catalog already contains nearly 50 different species, including—thanks to Lev—the highly sought-after peppermint angelfish, a shovel-shaped fish with a sugar cane color that costs thousands of dollars. Peppermint angelfish are considered the holy grail of ornamental fish.

Although no one has figured out how to breed peppermints in captivity, knowing what their eggs look like could help aquarists breed them in the future – a feat that will save time, money and a huge amount of effort. The breeding pair Steinhart Aquarium currently has on display was collected during an expedition to Moorea in French Polynesia, where divers had to use special equipment to reach the fish 90 meters below the sea surface.

Aquarians at the New England Aquarium and researchers from Roger Williams University in Rhode Island hatched the idea for a catalog more than a decade ago. The idea was on hold until 2021, when a grant from the Association of Zoos and Aquariums allowed them to develop the catalog to what it is today.

To create the catalog, aquarists like Lev collected fish eggs from the aquarium’s marine exhibits, photographed them under a microscope, and then sent them to a laboratory for DNA barcoding. Although most eggs look identical to the naked eye, their differences become more apparent under a microscope. The size of the oil globule, a fatty deposit found in the yolk of fish eggs, varies from species to species, as does the color of the tissue that surrounds it. While most eggs are clear, some eggs are tinted yellow or pink. Some even have spots.

The catalog provides new insights into the early life stages of many marine fish species and creates opportunities for aquarists to raise fish that have never been reared in captivity.

“I think it’s a great initiative,” says Joanna Murray, a marine ecologist at the UK government’s Center for Environment, Fisheries and Aquaculture Science.

Murray, who was not involved in the creation of the catalogue, hopes that new farming protocols developed in the wake of the catalogue’s publication will be shared with the countries where the fish come from. “I think sharing this (information) with source countries can have a really positive impact on the long-term sustainability of the trade,” she says.

Right now, six public aquariums across the United States contribute to the catalog, and new fish are added every few months.

“I think it’s something to celebrate,” says Lev. “It takes a huge amount of people to make this work.”

This article first appeared in Hakai Magazine and is republished here with permission.

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