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SEAS: Student Experiments At Sea

Field notes

The deep-sea mussel data were collected during a research cruise to the East Pacific Rise study site in May 2005. These field notes were recorded at sea aboard the Research Vessel Atlantis during that cruise. They detail important information about the at-sea portion of the classroom-to-sea mussel lab.

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Collecting mussels from the ocean floor

Using a submersible to collect deep-sea mussels

Because of the abundance of mussels in the area, we were able to get samples of mussels with almost every dive. The number of mussels in each sample varied because of the uneven distribution of animals, but also because of the challenges of working in this environment. Remember, deep-sea mussels live at the bottom of the ocean, 2500 meters deep, in pitch darkness, and we need a deep-sea vehicle like Alvin to find and collect them. The submarine pilot uses Alvin's robotic manipulator arm to grab the mussels, using just enough force to pull them up without crushing them. Often, the mussels are clumped, held together by byssal threads, and it's easy to collect a whole clump. Other times, we get only a few. With each grab, the mussels were placed inside the biobox (a heavy-duty plastic box used to store samples) and brought to the surface.

As soon as Alvin was on deck, the mussels were removed from the biobox, placed in cold seawater and then stored in a walk-in refrigerator (Temperature = 2.8°C/37°F) until dissection. Dissections were typically on the same day as collection. In most cases, the biologists in Dr. Shank's lab also sampled tissue from these mussels for genetic analysis.

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A note on size distribution

To avoid any bias in our data based on size or age of mussel, we attempted to collect mussels from a range of sizes (e.g., small to large). Small mussels (<60 mm), however, are more difficult to find at vents, since they prefer the underside of basalt rocks. Collecting the smaller mussels means breaking up the basalt rock, which is difficult to do. As such, small mussels are not well represented in this dataset.

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Six samples of mussels!

By the end of the cruise, mussel samples were obtained from six different locations along the East Pacific Rise vent study site (see table and map below for details).

There follows a 5-column table comparing samples.

Sample no Collection date Dive date Sample location Number of mussels
1 05/03/05 4104 marker 89, near Ty and Io vents 25
2 05/06/05 4109 marker 1, East Wall 8
3 05/06/05 4109 marker 0405C, East Wall; 3m from marker 1 8
4 05/07/05 4010 marker 82B, near Ty and Io vents 21
5 05/08/05 4011 Choo Choo 26
6 05/10/05 4013 East Wall, marker 0404B 25
Map of vent sites at the East Pacific Rise study site

Locations of vents where deep-sea mussels were collected. The red area on the map is the tallest part of this part of the East Pacific Rise ridge. Green and blue areas are deeper and colder.

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Dissection details: what did we see?

Deep-sea mussel
Byssal threads

For each mussel, we measured shell length and then opened the mussel to examine the body cavity. Following the same volume displacement proceduress used by students in the classroom, we dissected mussels and measured gill tissue volume and total body tissue volume (see a slideshow of the mussel dissection). We then calculated the ratio of gill volume to total body volume. We use ratios so that we can make a fair comparison between individuals of different sizes.

By the way, on nearly all of the deep-sea mussels collected, we noticed a large number of byssal threads covering the shells (see picture). These threads, made of incredibly strong collagen, serve as a means for the mussels to attach themselves to the substrate.

Reddish, flattened worm with many bristles
Polychaete worm

One of the first things we noticed inside many of the mussels was an abundance of eggs. So these mussels were apparently healthy and reproductive. We also saw small polychaete worms in many of the mussels (see picture). This particular species of worm lives inside mussels and very little is known about it. As the mussel irrigates its own gills and filters food by moving sea water in and out of its shell, the worm likely lives off of the particles floating around inside. It may also live off of the mucus formed by the mussel, but we are not entirely sure about this. Polychaete means "many bristles," which is obvious when you look closely at this worm.

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Quick statistics: six samples or one?

By the time we finished the dissections and entered the data, everyone was curious to find out if there was a consistent proportion of gill tissue to total body tissue across samples. We ran simple descriptive statistics (i.e., mean, standard error, min, max, median) and looked to see if there were differences between the samples. Dr. Breea Govenar graciously helped us run other statistics (One-way Analysis of Fit, and post-hoc comparison of the means using Tukey-Kramer) to determine if the sample means were significantly different or could be pooled and treated as one. Although the statistics did suggest a "significant difference" between the collections, the actual difference in means was small and was likely not biologically significant considering the precision of the measurement techniques. This points out the importance of careful interpretation of observations and of your data, rather than strict reliance on statistical interpretation. It also means that the dataset of deep-sea vent mussels may be treated as one collection for purposes of this lab.

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A special note of "Thanks"

The scientists on this cruise have been incredibly generous in helping us obtain these mussels for dissection, in particular Dr. Tim Shank (left) and his lab of graduate students and postdocs. With almost every dive, Dr. Shank made sure that a subset of mussels was collected for the Classroom To Sea Laboratory. He also shared space in his bustling lab each evening, and answered countless questions as we did the dissections. Thanks, Dr. Tim!

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