David Thistle

Current Research Interests

• Do sediment-dwelling species have large ranges in the deep sea?
• Environmental effects of CO₂ sequestration in the deep sea
• Emergence - benthic animals that leave the seabed at night

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Do sediment-dwelling species have large ranges in the deep sea?

The sediment-covered deep-sea floor is the largest habitat on Earth. Despite the extreme conditions (low temperatures, high pressures, and little food input), the number of species present at a given deep-sea location for some taxa can rival that of the most species-rich groups in the most species-rich habitats on Earth.

The nature of this richness is controversial. Some studies have reported that a few species had ranges of 100's to 1,000's of kilometers, but many species were found at only a single location. In other work, many more species were reported to have large ranges. Worse, the existing data may not be completely informative because the decisions about the assignment of individuals to species were made on the basis of morphology, which can fail to distinguish species that are biologically separate.

The question of species' range sizes in deep-sea sediments needs to be resolved because it matters profoundly to conceptual models of the ecology of deep-sea sediments. If most species have small ranges, students of the deep sea will want to understand, for example, how species' ranges are bounded in an environment that appears to have few barriers. Alternatively, if most species have large ranges, investigators will want to study issues such as how individual species maintain genetic cohesion over 100's to 1,000's of kilometers.

The objective of this project is to evaluate the possibility that some sediment-dwelling deep-sea species have large ranges. In 2008, we (Dr. Trisha Spears and I) will sample one station at 2,700 m and one at 3,700 m depth at each of four latitudes (47.6° N, 42.6° N, 36.5° N, and 32.6° N) on the continental slope off the west coast of the United States; the most widely separated stations will be ˜1,900 km apart. The ubiquitous and speciose harpacticoid copepods will be the target group. Each adult harpacticoid from each sample will be assigned to a species on the basis of morphological characters. Each adult will then be cut in two; the posterior portion will be used for DNA-sequence analysis, and the anterior will be retained as a voucher. The DNA-sequence analysis will be used to test our hypotheses that particular individuals belong to particular species. When the morphological and DNA-sequence data agree, we will know the distribution of the individuals of a species among stations and thus that the species' range is at least as large as the distance between the stations where it was found. In this way, we hope to begin to unravel the puzzle of the size of species ranges in the deep sea.

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Emergence

General background

For many years, benthic animals were considered to live their adult lives in the seabed. In the 1970's, evidence began to accumulate that this view was incorrect for some species. Since that time, workers have found that species of many taxa (e.g., polychaetes, amphipods, and harpacticoid copepods) have a life style that is primarily benthic but includes time in the water column at night.

Part 1: The Light-Shift Experiment

Background

Although much has been learned, emergence is still very incompletely described. We do not know the temporal pattern of species' emergence in detail. Some information about the cues that cause animals to emerge are known for some taxa, in particular, the importance of light, but the importance of endogenous rhythms in the light response and the possibility that cues in the sediment might also be important have not be explored. To improve our understanding, Dr. Kay Vopel, who was a post-doctoral fellow with me, Michael Teasdale, who was a master's degree student, Ires Hoedl, an undergraduate visiting from the University of Vienna, and I did an experiment designed to (1) provide the most detailed temporal description of species-level emergence ever done nonacoustically, and (2) to determine if the cue for each species' emergence was light level or an endogenous rhythm. We focused on harpacticoid copepods.

Design

To obtain organisms for study before each run, we collected cores from a site on the north Florida shelf at 6 m depth. The cores were mounted in a flume (Figure 1) and exposed to field temperature and salinity. We did two types of experimental runs. (Figure 2) In one type, we decreased the light in parallel with the decrease in the field. In the second type, we delayed the decrease by six hours. To make sure that the experimental conditions we created in the flume were appropriate, we made microelectrode measurements of oxygen, REDOX, and pH in the field. (Figure 3)

During each run, the water in the flume was moving but at a velocity far less than the erosion velocity of the sediment. If a harpacticoid emerged, it was carried by the flow into a sieve in the flume outflow. We changed the sieve every 20 minutes, giving us an unparalleled opportunity to learn of the details of emergence timing and to determine whether the animals time their emerge with a biological clock or whether they use a cue from the environment.

Figure 1. A view of the flume showing the electronics, the motorized micromanipulator carrying the microelectrodes, and the light source.

Figure 2. The left panel shows the experimental light regime designed to mimic the decline of light in the field. The right panel shows the decline of light shifted six hours later than in the field.

Figure 3. Dr. Kay Vopel measuring profiles of oxygen and pH in the sediment of the study site.

Kay Vopel, Michael Teasdale, and Iris Hoedl in the lab at the FSU Marine Laboratory during the light-shift experiment.