Ocean acidification and baby “squidlife” crises?

An egg mop of D. pealeii, found in shallow waters. Each egg capsule contains between 50-200 eggs. Ceridwen, (2009), geograph.org.uk. Bottom: An adult D. pealeii; the species is semelparous, meaning it only reproduces once before dying. It is a critical organism to the ecology of the North Atlantic Ocean. Clyde F.E. Roper, tolweb.org.

It’s approaching summertime in the northern hemsiphere. Dip beneath the waves rolling over the continental shelf of the northwestern Atlantic and you’ll notice something new along the sea floor in addition to sand and rocks: clusters of gelatinous tubes, forming something resembling mop heads. Upon closer inspection you’d find dozens of eggs within each tube or sac, and (depending on the stage of development) hundreds of eyes staring out into the blue. Meet the longfin inshore squid, Doryteuthis pealeii, as it gets ready to hatch from its anchored egg capsule and take its place, center stage, in the coastal habitat.

You may have never given squids much thought, beyond their suitability as a calamari appetizer, but these animals are tremendously important within the marine ecosystem. Not only are they food for us, they are tasty meals for other large marine organisms, as well as voracious predators of smaller prey. Because of this central role of squid in the environment, scientists have grown concerned over how environmental stressors will affect these species, especially throughout different life stages. Of particular concern has been stress induced by ocean acidification (OA).

While the increasing CO2 concentration in the atmosphere contributes to rising global temperatures, it also impacts the acidity of the ocean. After being absorbed in ocean waters, CO2 will go through a series of chemical reactions; the end result is extra hydrogen ions being released (which increases acidity) and carbonate ions (the compound needed for shell and coral skeleton formation) becoming less abundant. So how could soft-bodied cephalopods with no external shells or skeletons be at risk from OA-induced stress?

Just because squids have no external shells, doesn’t mean they do not have hard parts that could be affected. Specifically, squid have statoliths—tiny calcium carbonate structures that are integral to helping the animal maintain balance and perceive its acceleration through the water. For juvenile and adult squid capable of moving away from more acidic waters/tolerating acidic conditions over a short period of time, this might not be as large a problem—but what about for those babies, developing in those anchored mops? These were questions that led Casey Zakroff and a small team of researchers at Woods Hole Oceanographic Institution to conduct a series of trials across a summer breeding season.

Just how was this done?

First, wild D. pealeii collected in Vineyard Sound off the Massachusetts coast were brought back to breed and lay their egg capsules in controlled laboratory conditions. Egg capsules were then moved to a culture system, where they could develop in separated tanks under various acidity treatments. In particular, Zakroff was interested in determining if hatchlings would show signs of OA-induced problems at any acidity, or if a “tipping point” existed where any coping strategy they might have would no longer be enough.

During each hatching day, baby squid were randomly sampled and specific measurements taken—and since these organisms were the size of grains of rice, microscopes and cameras were essential! Of particular interest were the hatching times post-laying, mantle lengths (indicating an individual’s size [see Fig. 2]), the yolk sac volumes (indicating energy stores available until the squid would have needed to feed on its own), and the state of the statoliths (indicating if acidity was altering mineral formation). Mantle lengths could be measured on live individuals, while yolk sac and statolith measurements required preserving specimens for later imaging and dissection.

Juggling eight armfuls of data!

Instead of clear trends across all these measured aspects, Zakroff soon saw variability in the response of the squid to the increasing acidity. Hatching times delayed with increasing acidity, but also across the season, with eggs laid during the later trials hatching an average of two days later than those of earlier trials. Likewise, all trials revealed decreasing mantle length with increasing acidity, but not all acidity levels within a trial showed significantly different declines. Yolk sac volume seemed to be tied to decreasing mantle length in some trials, but not in others—and in some cases actually showed an increase in response to higher acidity! As for the statoliths, their decline in surface area across trials and acidities was similar to the trends displayed by the yolk sac volumes. In some trials the decrease appeared to match increasing acidity, while other trials had statoliths extracted that were hardly any different across treatments.

Andrea Schlunk, Oceanbites, 7 May 2019. Press release.

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