Samuel Daughenbaugh, DePauw University
The Approach: In my previous post, I described a group of chemical additives called phthalates and their potential impact on the development of sea urchin larvae. The plastic industry uses several phthalates that vary in chemical structure and toxicity levels. One way phthalates differ in structure is by their size. I am studying the effects of three phthalates with different molecule sizes — DMP (small), DBP (medium), and DEHP (large) — on mortality (lethal effect) and larval skeletal growth (sublethal effect).
My first major challenge was to dissolve the chemicals in seawater. As hydrophobic liquids, phthalates only mix with water molecules at very low concentrations; larger types (longer side chains) are less soluble. By dissolving each chemical in acetone, I am able to get DMP into seawater at 1000 parts per million (ppm), or 0.01%, and DBP and DEHP at 1 ppm. I am testing 5 concentrations of each chemical in addition to an acetone control (no phthalate), and a seawater control (no phthalate or acetone).
Once the chemicals are in solution, I spawn male and female sea urchins via electric voltage and collect their sperm and eggs. Then, I fertilized the eggs and introduce them to experimental jars where they then begin to develop into larvae. Small paddles stir the water to increase the oxygen level and keep the larvae suspended. After growing the larvae for two days, a period before they start to depend on food, I transfer them into small tubes, preserve and store them in a freezer.
To measure and categorize larvae into different stages of development, I observe them under a microscope that can record landmark points on the larval body in three dimensions. After determining the proportion of individuals that failed to develop to the normal 2 or 4-arm pluteus stage (pictured below), I use the landmarks to calculate the lengths of different skeletal features to determine how much the larvae had grown. At the end of each trial, I will have observed hundreds to thousands of dead larvae and once all of them have been counted and measured, I can begin to analyze the data and learn whether the phthalates are having a significant effect on their development.
This project is supported by Dr. Robert Podolsky and the Fort Johnson REU Program, NSF DBI-1757899.