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Aquaculture

Larval Donkey’s ear abalone threatened by climate change

 

 

Source

Tahil, A.S., and Dy, D.T. 2016. Effects of reduced pH on the early larval development of hatchery-reared Donkey’s ear abalone, Haliotis asinina (Linnaeus 1758). Aquaculture doi: 10.1016/j.aquaculture.2016.03.027.

Aba-what now?

Unless you’re like me and read Island of the Blue Dolphins by Scott O’Dell at a formative age, you’ve probably never heard of abalones.  These are a group of edible sea snails (Fig 1).  They’re consumed raw or cooked, and are considered a delicacy in many cultures (Fig 2).  In 2004, abalone meat could cost up to $75 for a restaurant-sized portion (around 4 ounces).  They have calcified shells made of nacre, also known as mother-of-pearl, a valuable substance used for decorative purposes (Fig 3).  While they are naturally found all around the world, especially in the cold waters off the coast of New Zealand, South Africa, Australia, and Japan, abalone farming became popular in the mid-90’s in order to keep up with consumer demand. Farming began in the 1950s and 60s in China and Japan, but now is seen in countries all around the world, including Taiwan, Korea, Ireland, the US, Mexico, Canada, and Australia (to name a few).  Clearly, this is an important group economically and culturally across the world, and it’s being threatened by global climate change.

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Fig 1: A live abalone in its natural habitat. Source: public commons

How climate change hurts marine critters that need to grow shells

Researchers have long thought that climate change might be particularly devastating to marine organisms that use calcium to build shells or other external structures.  In fact, we’ve had a number of Oceanbites posts on the topic (e.g. http://oceanbites.org/ocean-acid-peekaboo-shrimp/). To recap briefly, as carbon dioxide builds up in the atmosphere, the oceans take up a lot of it.  As CO2 dissolves, it reacts with water to form several chemicals that act to lower the pH of seawater.  Calcified marine organisms, like abalone, use a lot of calcium carbonate, which is less likely to form or remain stable in a low-pH system.  Calcium carbonate helps these organisms build important structures like shells, which help them avoid being eaten by predators and protect them from drying out.  Here’s a great video talking about the impact of ocean acidification on calcified marine organisms.  pH is measured on a scale of 0-14; most organisms can only tolerate a narrow range of pH levels.  Seawater pH is expected to drop by .3-.5 units by the year 2100.

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Fig 2: Cooked abalone on the menu! Source: flickr

Studying the effect of lower pH on larval abalone

These authors examined the effect of reduced seawater pH on baby Donkey’s eared abalone (that *is* in fact the species name!) in a hatchery in the Philippines.  They raised larval abalone in boxes with filtered seawater under four treatments (based on that expected reduction of seawater pH by .3-.5 units):

  • pH of surrounding seawater – ambient pH/control group (7.97)
  • Ambient pH reduced by .2 units (7.78)
  • Ambient pH reduced by .4 units (7.60)
  • Ambient pH reduced by .6 units (7.40).

They maintained constant temperature, salinity, and dissolved oxygen levels across these four treatments.  Treatments were maintained for about 6.5 hours, since abalone larvae will hatch after 5-6 hours under normal conditions. They placed fertilized eggs from naturally spawning abalone within the hatchery into those treatments. They measured the following responses:

  • Hatching rate
  • Percentage of normal vs. malformed larval abalone
  • Survival rate
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Fig 3: Iridescent mother-of-pearl comes from the shiny inner layer of calcified shells, which is called nacre. This is abalone nacre. Source: flickr

Results

Hatching rate, percentage of normal larvae hatched, and survival rate all decreased significantly with a decrease in pH (e.g. Fig 4).  Survival rate in the control group was close to 100%. Survival rate was almost 0% in the most extreme treatment, which mimics expected seawater pH conditions about one hundred years from now! Very few of the abalone that hatched in the treatment which reduced ambient pH by .4 units appeared normal under a microscope, and survival rate dropped to about 30% – a drastic difference from the control.

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Fig 4: Survival rate between the different treatments – ambient pH on the left, and progressively more extreme reductions in pH as you move to the right. Source: Tahil & Dy 2016

Conclusions

An average change of .4 units in one hundred years may not sound like a major drop in pH from our human perspective, but it is clearly enough to have drastic evolutionary and developmental effects on these larval abalone.  In the coming years, we can expect to see fewer and fewer surviving abalone (and other calcifying organisms like them), which will have major economic and environmental impacts.  Filter-feeders, like abalone and other shellfish, play a key role in their ecosystems, filtering out and containing nutrients and toxins that are harmful to other marine life.  In fact, the Hudson River Foundation is growing oysters (another calcified filter feeder) to clean up the Hudson River!   Losing abalone and other species like abalone could prove devastating to marine environments.  These organisms may evolve to survive better in a post-climate-change world, but for now it’s clear we need to mitigate the effects of climate change on ocean pH levels if we want to keep them around.

 

Questions?  Comments?  Please sound off below!  I’d love to hear from you :)

 

Dina Navon
I am a doctoral candidate in the Organismic and Evolutionary Biology program at the University of Massachusetts Amherst. I’m interested in how an individual’s genes and the environment in which it grows come together to determine its physical traits. I study a group of closely related freshwater fish called cichlids which live in the African rift lakes like Victoria, Malawi, and Tanganyika.

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