November 10, 1996

Tracy Hamilton

At a presentation that I gave on Friday night in the Crary Lab on the subject of my work, I was happy to share some of the Weddell seal recordings that I had just finished making. Afterwards, Tracy Hamilton, a researcher and diver approached me with the suggestion that it might be interesting to try and record the seals vocalizing using an underwater video camera. I was intregued by the idea--particularly since it might afford me the opportunity to get a sense of how loud the seals' voices are. If I knew the distance between the hydrophone and a vocalizing seal, and could actually see this distance to measure it, then a sound pressure level could be obtained. Surveys had already been conducted wherein the salinity and density of the water was measured, and this would also be important for measuring sound speed and pressure levels. It seemed like a wonderful idea. Moreover, as a certified diver, Tracy said that she and Rob Robbins, the ASA Dive Coordinator, would be willing to travel out to Little Razorback Island and place the camera near a dive hole for me. They had seen a number of seals coming up for air at the dive holes that had been bored a few weeks earlier.

I secured permission for use of the camera equipment and Tracy and Rob made their arrangements. In order to get a sense of the possibilities, Tracy suggested that I accompany her, while she set up the video camera just offshore from MacTown as part of an experiment she was developing to watch sea urchin and starfish interaction. I was eager to tag along and help out.

Tracy is down here working with Donal Manahan, Adam Marsh and Patrick Leong on sea urchin ecology. The title of their project is "Metabolic Physiology during Embryonic and Larval Development of Antarctic Echinoderms." The research, out of the Biological Sciences Department of the University of Southern California (USC), is an attempt to understand how marine invertebrates, such as sea urchins, develop in the frigid waters of Antarctica. What they have discovered is that the larvae are found to be active in the water for many months before the appearance, or blooming, of the phytoplankton upon which they feed. Until recently it was thought that the larvae somehow made due by resorting to alternative food sources including bacteria and perhaps "dissolved organic material." Based on the data they have collected, the USC team now suggests that Antarctic echinoderms have developed specialized strategies to cope with starvation conditions. They appear to be able to exist for up to a year without feeding. The direction of their current enquiry focuses on the physiology and metabolism of this process--which is particular to antarctic larvae.

Adam Marsh

Patrick Leong

Tracy said to meet at the Dive Locker at 11:30am and we would load up a Spryte and head down to one of the fish huts just offshore. I arrived and found Tracy running through her diving equipment in the tank room and Rob preparing cameras and sample bags; Tracy was planning to collect about a dozen sea urchins for her experiment.

Dive Locker	Tank Room
Rob gets gear together for the dive.	Suiting up.

The day was clear and warm, with the temperature climbing to the upper 20s. The fish hut was one of several located near MacTown. We pulled up along the side of the hut and offloaded the camera and diving gear. Rob fired up the generator to power the video equipment and they proceeded to get dressed for the dive. Diving in water which is about 29 degrees is tricky and drysuits are used--with numerous underlayers. I was interested in observing the ritual of their getting dressed, the pacing and attention to detail. As I had never been around a dive site, Tracy and Rob explained to me the various emergency procedures and talked through what they were planning and how long they would be under. I helped fit the seals for their mittens and with adjustments to tanks and weight belts. Rob had already assembled the camera and lowered it through the hole in the ice; they were then going to position the camera and collect sea urchins. The next step would be a return to the fish hut, where Tracy would prepare the sea urchins and they would have a look at the video monitor to see about the framing and whether the light was focused properly.

With everything ready, Rob sat for a minute at the dive hole--thinking through the dive and then jumped in and vanished. Tracy looked to me for a signal for when the hole was clear and she hopped in, pausing for me to hand her the sample bag. The water boiled and roiled with air from their tanks, bringing shards of plate ice to the surface. I could see the strobe flash of beacon lights from the down line and an aquamarine halo where the video was set up. Turning to the video monitor, I watched the camera being adjusted and the sandy bottom come to life. Several fish swam into the light and rested within the chain ring that Tracy had left at the bottom for scale and reference.

After about 30 minutes, the ice hole began to churn and the ice which had formed broke up. I scooped with a dip net to clear the hole and waited. Rob surfaced first, and I took his tank and weight belt and together we gave Tracy a hoist out of the water. She had gathered a dozen sea urchins (pictured below) and set to injecting them with a small dose of potassium chloride--to contract muscles and cause them to expel their gametes. The purpose of this was to simulate spawning. What Tracy was looking for was a sense of how this aspect of reproduction might play out and at what depths in McMurdo Sound. The urchins and their ooze of eggs and sperm would be left at the site where she placed them, and starfish would move in rather quickly for feeding. "We spend a lot of time in the lab looking at these creatures at the biochemical level; it is important to have a sense of their natural history and how they behave."

Rob contemplates the dive.	Tracy drops into the dive hole...
...and disappears into the depths.	Sea Urchins

The bottom was sandy and she had not seen too many starfish, so she was not very optimistic about the outcome--but this would be a good test situation. If the gear held up, she would repeat the experiment out at Little Razorback, where the rocky bottom is dense with life. After enough time had passed and it was safe for their second descent, they got dressed and suited up. This was a short dive--8 minutes. I watched the monitor as Tracy placed 7 sea urchins within a chain circle. The fish approached and started immediately grazing on the discharge of eggs. Back in the hut, Tracy turned on the video recorder: a time lapse machine that would take a picture every 1/2 second for the next 24 hours.