When I first saw this tunicate tucked near a rock crevice, I was excited, as I knew this was an unusual find,” reported Jim Auzins, an avid recreational diver and underwater photographer. Jim was diving at Mackenzie Bight, in Saanich inlet just north of Victoria, British Columbia when he photographed an unusual-looking tunicate (a sessile, filter-feeding invertebrate) with what he thought might be fish eggs inside.

He sent several of his photographs to renowned marine taxonomist Andy Lamb, who identified the specimen as an unusually ornate form of glassy tunicate, Ascidia paratropa. Andy also noted the presence of eggs, and felt that this “phenomenon happening in a local tunicate is a significant find.” Andy forwarded the photo to Danny Kent, curator of B.C. Waters and fish propagation expert at the Vancouver Aquarium, who confirmed that the tunicate did indeed contain developing fish eggs.

Underwater shot of tunicate showing un-eyed fish eggs at dive site.

Danny was curious about the species of fish that laid the eggs, and the only way to find out would be to collect some eggs and rear them in a laboratory, where they could be accurately identified. He quickly assembled a dive team and headed for Saanich Inlet. Time was of the essence, as the eggs in the photo looked close to hatching, and Kent wanted to secure a sample before this happened.

Divers in water at Saanich Inlet dive site.

Following Jim’s detailed directions, the team found its way to the location of the tunicates and collected two specimens. In the field, it was difficult to determine if the animals contained fish eggs. Photographs taken on the dive showed indistinct areas that might have been egg masses, but only closer inspection would be definitive.

Collected tunicate in lab. You can see the scale and the eggs inside.

Back at the Aquarium’s Fish Research Laboratory, Danny determined that both tunicate specimens did indeed contain fish eggs and, interestingly, at an earlier developmental stage than those depicted in Jim’s photograph. This meant that the Aquarium team had collected a different set of fish eggs than those observed by Jim, and that strengthened our suspicion that this was a regular behaviour for whatever fish laid the eggs.

Because tunicates are constantly passing water through their siphons for respiration and nutrition, it is suspected that the egg-laying mystery fish is taking advantage of both this constant water flow to keep its eggs oxygenated and the tunicate’s tough outer ‘tunic’ for protection.

Photomicrograph of three fish eggs dissected from main egg mass. The little fish larvae are clearly visible.

Danny set up the specimens in an incubation system in the lab with the intention of hatching, rearing, and identifying the fish that laid the eggs. Hatched fish larvae will be photographed and measured as they proceed through their developmental stages, and genetic analysis of both the eggs and the tunicate is being considered to confirm identification and determine, if possible, if different individuals laid the egg masses discovered (each tunicate contained two distinct egg masses).

Tunicate with eggs in incubating system in the lab.

Scientific progress is often driven by fortuitous discoveries. Thanks to the observations of keen-eyed divers like Jim Auzins, we are a tiny step closer to understanding the complex ocean around us.

Aquablog post by Jeremy Heywood, Vancouver Aquarium Diving and Boating Safety Officer and Justin Lisaingo, Aquarium Biologist


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