The Plutei: An Unexpected Journey

Hailey Conrad, Rutgers University

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Me, using a microscope with a camera lucida coupled to a rotary encoder and a digitizing pad to record landmarks for the plutei (Photo credit: Carly Lovas)

In previous blog posts I have documented my project studying the impact of ocean acidification on the larval skeletal growth of Atlantic purple sea urchin, Arbacia punctulata, from Woods Hole, Massachusetts. After breeding my sea urchin parents and rearing their larvae, I was able to record the coordinates of landmarks at different points on each larvae’s anatomy. Using those coordinates I calculated the distance between each point and was able to get the lengths of different skeletal segments (Fig. 1). During the actual experiment I had a lot of difficulty getting my larvae to survive- due to a variety of factors, some still unknown, I ended up collecting far less data overall than I wanted to. I wasn’t sure if I would get any meaningful results out of the data at all, and had mentally prepared myself for that to happen. However, at the very last minute, I got statistically significant results, that confirmed my hypotheses!

By comparing the average skeletal length of the larvae from specific parents reared in either current atmospheric CO2 levels (~400 ppm), or 2.5 times current atmospheric CO2 levels (1000 ppm) I was able to see the effect that being raised in higher CO2 had on the size and development of the larvae. I found that being reared in higher CO2 conditions had a negative impact on overall larval skeletal growth, as show in Fig. 2 below. In addition, I found that being reared in higher CO2 conditions caused larval to grow more bilaterally asymmetrical (Fig. 3). Interestingly, these results were only found in the post oral arms, not the anterolateral arms. After analyzing those results I also answered the main guiding question of my research- is there heritable genetic variation in these Arbacia‘s skeletal growth under higher CO2 conditions? And according to preliminary results, there is, at least when it comes to post oral arm length.

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Fig 1. A plutei with landmarks indicated and skeletal segments highlight.

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Fig. 2 Total skeletal length for larval reared in the two different carbon dioxide levels, as compared to the line y=x

 

 

 

 

 

 

 

 

 

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Fig. 3 Total skeletal evenness for larvae under two different CO2 conditions as compared to the line y=x. Large values indicate greater asymmetry.

But what do these results mean? The larval experience reduced skeletal size, and increased asymmetry under conditions of increased ocean acidification. The larvae use their cilia-coated arms to suspension feed and propel themselves. Reduced larval skeletal size directly impacts larval survival by reducing their ability to reach food. The larval development period may also increase to that larvae have time to grow to their full size, which means they spend more time vulnerable to predators who feed on larvae. Several NOAA climate change projections calculate that atmospheric CO2 will be 950 ppm by the year 2100, so any negative consequences of reduced skeletal growth and increased asymmetry we saw happening at 1000 ppm will occur in the near-future. However- heritable genetic variation in response to increased CO2 concentrations mean that Arbacia have the capacity to evolve in response to ocean acidification. Before you start celebrating- future studies are needed to determine the frequency of these resistive traits within the population overall to see if there are enough individuals with these traits for evolution to realistically happen. Now, my mentor can compare this data set to a previously collected data set from Arbacia in Charleston to see if there is variation between populations.

A special shout out to my incredibly supportive mentor Dr. Robert Podolsky! I’d also like to thank the National Science Foundation for providing funding for this REU program, and all of the staff at Grice Marine Lab who made this program possible.

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