Climate Change

Ocean Acidification is no Small Matter to Two-Toned Pygmy Squid Reproduction

Paper: Spady, Blake L., Philip L. Munday, and Sue-Ann Watson. “Elevated seawater pCO2 affects reproduction and embryonic development in the pygmy squid, Idiosepius pygmaeus.” Marine environmental research 153 (2020): 104812.

Two-toned pygmy squids are quite small – with adult mantles measuring 20 mm on average. | Photo Source: Flickr, Rickard-Zerpe.

Nestled in seagrass and darting through Indo-Pacific, nearshore waters, two-toned pygmy squids are miniscule cephalopods that represent a much larger problem afflicting our ocean: ocean acidification’s impact on productivity.

The ocean is a critical, natural carbon sink, soaking up excess carbon dioxide like a sponge. Because of unprecedented man-made greenhouse gas emissions, the ocean’s carbon uptake has skyrocketed. Atmospheric carbon dioxide (CO2) dissolved in water (pCO2) binds to water to form carbonic acid, which lowers the ocean’s pH ­– making it acidic.

Another one of climate change’s impacts, ocean warming, has already resulted in well-documented effects on marine productivity. A 2019 report detailed how historical warming has thrown many commercially viable fisheries off balance, shifting where populations aggregate and either reducing or increasing population reproduction. While many species have suffered under ocean warming, such as cod species, some either thrived or remained normal – such as jellies or the Yellow kingfish.

Ocean acidification has similar, documented effects on marine populations. Cephalopods, hypothesized as a class which might thrive in warmer waters, may alternatively suffer from increased pCO2 and consequent ocean acidification.

Click for larger image. The ocean acidification process. Photo Source: Ocean Health Index, University of Maryland.

A not-so-small finding for pygmy squids

Blake L. Spady and his research team examined how increased pCO2 concentrations affect two-toned pygmy squid reproduction and embryonic development. The team collected squid from Cleveland Bay in Queensland, Australia and kept one pair per aquarium in fish breeder baskets. The team held pairs in four experimental conditions: two ambient control conditions, with current ocean pH and pCO2 concentrations, and two elevated experimental conditions following the IPCC’s highest greenhouse gas concentration projection of RCP8.5, with more acidic water and higher pCO2 concentrations. Each pair would breed in these conditions, where the embryos would remain and develop afterwards.

An overview of the four conditions. Notice how pH lowers, which means water acidity increases, when pCO2 concentrations rise. | Photo Source: Spady et al. (2020).

Spady, Munday, and Watson (2020) found that squid in the high pCO2 conditions had mean total clutch sizes, or the total eggs per clutch, almost 40% smaller than squid held in control conditions (about 44 eggs and 74 eggs per clutch respectively). Squid in high pCO2 conditions also laid their eggs 41% closer together compared to squid in the control conditions. Finally, eggs in all conditions swelled during the experiment; however, eggs in high pCO2 conditions grew on average by 32% while eggs in control conditions only grew 24%.

Spady and his team also examined egg conditions after squid pairs reproduced. Immediately after spawning, vitelli (the yolk of an egg) were about 14% smaller in high pCO2 conditions compared to control conditions. When embryos’ retinas and fins became visible, Spady et al. (2020) noted the width of squid heads in high pCO2 conditions were significantly smaller than the squid in control conditions. They also found that when embryos exhausted their yolk and were ready to hatch, squid mantle length ­– the long stretch of body behind a squid’s head – was significantly smaller in high pCO2 conditions compared to control conditions.

These results indicate that elevated pCO2, and consequent acidification, do impact two-toned pygmy squid’s reproductive behavior, output, and embryonic development.  Such effects can alter wild populations, reducing clutch sizes and producing smaller, possibly more vulnerable, embryos and juveniles.

Demonstration of and future possibility for climate adaptation

Spady et al. (2020) explain that the study’s findings could result from adult squid efforts to reallocate energy. While increased pCO2 concentrations do not increase oxygen uptake or metabolism rates in adult two-toned pygmy squid, they do increase squid activity and potentially acid-based regulation – both which would increase energy expenditures. Diminished energy could result in reduced reproductivity. Additionally, how close eggs were laid together and egg swelling could both demonstrate adapting to or coping with higher concentrations of pCO­­2.

A pygmy squid catches its next meal: shrimp. In previous studies, Spady and his research team find that pCO2 concentrations also affect cephalopod predatory behavior, changing how these critters hunt for their food. | Photo Source: Wikimedia Commons, Rickard Zerpe.

Spady and his team also provide an alternate theory. Since two-toned pygmy squid are iteroparous, which means they can reproduce multiple times throughout their lives, they are able to adapt to high energy-costing environmental situations like elevated pCO2 by reducing reproductive output. This is an adaptation that semelparous cephalopods, those that only reproduce once in their lives, do not have, as these species put all their energy into one reproductive output and perish afterwards.

There are a few limitations to Spady et al.’s (2020) study. The scientists only test two generations of two-toned pygmy squids in high pCO2 conditions: the adult breeding pairs and the embryos. Therefore, the team is unclear whether these negative impacts would extend past the studied generations, or if they would eventually diminish through consequent reproduction.

Additionally, cephalopods have incredibly short lifespans – a reason why scientists believe they are fit to adapt to and thrive in warming oceans. Two-toned pygmy squids live an average of 90 days. Paired with the fact that their natural environments experience day-time fluctuations in pCO2, this species of squid may be primed to evolve and adapt to elevated pCO2 and ocean acidification the same way they are thought to be primed for adapting to warmer oceans. Spady et al. (2020) explain that a 90-day life span allows two-toned pygmy squids to complete more than 300 generations by 2100, which allows significant opportunity for adaptation. These negative effects may ultimately be just as short lived as the pygmy squid itself.

Spady et al.’s (2020) study demonstrates that increased pCO2 and ocean acidification’s effects are just as complex as ocean warming. With the IPCC’s latest Special Report on the Ocean and Cryosphere in a Changing Climate, it is clear that the ocean is one our natural resources facing the brunt of climate change. Marine species productivity and reproduction is just one of the ways we can see how the ocean is shifting under the pressures of increased greenhouse gas emissions.

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