Since October 2013, we have received reports of an explosion of egg masses from barnacle-eating nudibranch (Onchidoris bilamellata) off local coastlines.

Sea slugs, or nudibranchs (“new-dee-branks”), lay their eggs in flat, coiled ribbons attached to rocks on the sea floor. Depending on the species, hundreds to thousands of eggs are protected in capsules nestled in a mucous matrix. 

This increase in egg mass sightings has left divers to wonder if this is an unusual event. But despite the numbers observed since last fall, Dr. Jeff Marliave, vice president of marine science at the Vancouver Aquarium, says,There is no evidence that there has been any long-time delay in this occurrence.”

A barnacle-eating nudibranch. Photo: Lee Newman

A barnacle-eating nudibranch. Photo: Lee Newman

He goes on to say that this type of “abundant reproduction is typical of many of our marine invertebrates. It is a regular and expected type of thing that is of interest when we witness the type of extreme in abundance that can occur, as with sea stars in recent years.”

According to the book Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast, barnacle-eating nudibranch egg masses are most reported from October to December and February to June.

Donna Gibbs, an Aquarium research diver and taxonomist, pulled data from Pacific Marine Life Surveys Inc. that supports these time frames in local waters (mostly B.C. and a little of Washington State). It shows that in 28 years, barnacle-eating nudibranch egg sightings were reported most from December to February – or right about now.

So what can we take way from this information?

It can certainly seem like an explosion of eggs when it’s spawning season, especially when the animal in question gathers with others of its species and lays millions of them at a time. But the data (and the scientists) tell us that this particular explosion is naturally-occurring and thus, normal. Sorry to disappoint.

Written by Karen Horak, writer-editor, content and digital experience at the Vancouver Aquarium.

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