Skip to content

UVic sea butterfly study challenges their ocean acidification indicator role

Pteropod shells could be more resilient to, but still face risks from increasingly acidic waters
The top microscopic image shows a pteropod with shell dissolution, the white areas, that were exposed to acidic water while the bottom image shows the usual translucent appearance of the sea snail in non-acidic water. It’s part of what helped a UVic researcher question pteropods being a bioindicator for the effects of ocean acidification. (Courtesy of Matt Miller)

Aboard a Coast Guard research boat off the middle of Vancouver Island, a tiny sea snail is lightly pinned under a microscope with the bristles of a fine-tipped paintbrush to not puncture its three-millimetre shell.

Between the swaying of the Salish Sea, Matt Miller carefully scratches the pteropod’s shell with a small dissecting needle before placing the snail in a solution mirroring the increasingly acidic water from where the creature was pulled.

The University of Victoria PhD student was looking to see if the damaged shell areas dissolved while other spots held strong. The results formed the basis of his new study that casts doubt on whether the creatures dubbed as sea butterflies should serve a key role in understanding the impacts of acidifying oceans.

The burning of fossil fuels and other actions adds carbon dioxide into the atmosphere before the gasses meet with global waters.

“Because we’ve put more CO2 into the atmosphere, it means that more is dissolving into the ocean and when that happens it creates a reaction with seawater and it creates an acid called carbonic acid,” Miller said.

It’s how humans have reduced the PH of the oceans, increasing their acidity by 30 per cent in just 150 years.

Pteropods – microscopic snails that frantically flap little wings as they spend their entire lives swimming the water column – have been the canary in the ocean acidification coal mine for some time due to concerns that more acidic waters would dissolve their shells.

Like many sea creatures, newly hatched pteropod larvae pull seawater into their mantle before using energy to pump calcium and carbon ions from the salty solution into the calcium carbonate that solidifies into their shells.

“It’s wild that essentially these slimy little living tongues are creating parts of their body that are harder than most man-made structures,” Miller said.

But pteropods use a much more soluble, crystal-like calcium carbonate mineral called aragonite. That’s helped back up the claim that pteropods are considered more at risk to acidification, while the zooplankton species being near the food chain’s floor sets them up as a first domino of sorts.

“That really led to being elevated to the status of bioindicator, which is a species we think is more at risk than other species. So we look at them to see; if they’re being affected then potentially other species will be affected later on,” Miller said.

“They have been this sort of poster child for the effects of ocean acidification.”

A pteropod studied by Uvic PhD student Matt Miller. (Courtesy of Matt Miller)
A pteropod studied by Uvic PhD student Matt Miller. (Courtesy of Matt Miller)

But with little study on Pacific Northwest pteropods and heavy academic debate over whether the pteropod periostracum – a thin layer of proteins on the shell – could provide protection to increased acidity, Miller hoped his work could bring more clarity to the conversation.

The results of his experiment matched his hypothesis. The acidic water only dissolved the shells in the spots where he damaged the outer layer, while other areas stayed protected thanks to the periostracum.

“That evidence is really strong that this thin layer of proteins on the outside of the shell does block that acidic water from coming into direct contact with the shell and so it sort of acts like this first line of defence against dissolution.”

For Miller, it’s a good news story that these species thought to be vulnerable – and at risk of being wiped out by more acidic waters – could be more resilient than the scientific community thought.

His work shows shell dissolution may not be an extinction-level threat to pteropods and the species may not be the best choice for the role of bioindicator on the effects of acidification.

But with models showing the oceans could get 150 per cent more acidic by 2100 compared to pre-industrial times, Miller notes the tiny creatures could be impacted as they’ll need to expend more energy to pull the shell-making minerals out of the water as acidification progresses.

“We don’t really know what that might mean for population level or ecosystem effects,” Miller said, adding the tiny snails are an important food source for juvenile pink salmon, a species tied to the west coast’s environment, economy and Indigenous cultures.

READ: Invertebrate inspector decries mowed-down habitats in Galloping Goose trail margins

Jake Romphf

About the Author: Jake Romphf

In early 2021, I made the move from the Great Lakes to Greater Victoria with the aim of experiencing more of the country I report on.
Read more