Tracking Ocean Sunfish, Mola mola, with Pop-Up Satellite
Archival Tags in California Waters
As summer draws to a close and the tourist hordes thin, a bizarre visitor starts appearing more frequently in the Monterey Bay National Marine Sanctuary. The ocean sunfish, Mola mola, is sighted here year-round, however September through November provide the most likely chances for encountering these uniquely shaped open ocean travelers. With a strikingly abridged appearance, molas are the world’s heaviest bony fish. We know they can grow to a weight of more than 2,250 kilograms (5,000 pounds), on a diet primarily of jellyfish. But exactly where these leviathans travel, feed, mature, and reproduce remains a mystery.
|Local researchers are part of a global effort to track ocean sunfish. photo 2003 Mike Johnson
In 2000, with help from National Geographic, the Monterey Bay Aquarium, and others, a team began conducting mola research using genetics and satellite tagging technology. Genetic analyses have been conducted at the University of South Florida under the direction of Steve Karl with assistance from J. Todd Streelman and Anna Bass. I am joined on the domestic satellite tagging team by John O’Sullivan, Heidi Dewar, Chuck Farwell, Brett Hobson, and Eddie Kisfaludy. (For individual affiliations, see www.oceansunfish.org.)
The genetic results thus far have been fascinating. Between the Atlantic and Pacific oceans, Mola mola populations appear clearly divided, while intra-oceanic differences between northern and southern hemispheres appear nominal. More extensive analyses of South Pacific and North Atlantic samples will further clarify these preliminary findings. Globally, we have located several significantly genetically divergent individuals from Australia and South Africa, and these likely represent two new ocean sunfish species.
The California Mola mola population analyses have yielded very interesting results. We looked at a special group of genes known as microsatellites, which are commonly highly variable in fishes and can provide a genetic fingerprint of individuals. Surprisingly, there are a larger number of individuals sharing similar fingerprints than expected. Although results are preliminary and there are several things that might cause this, it is consistent with early indications that the population size has recently been reduced – possibly due to fishing pressure. We must collect more data before we can be sure of the direct cause of the modest loss of genetic variation, but these findings advance our knowledge significantly.
Unfortunately, Mola mola make up more than 25 percent of the California drift net bycatch – the single largest species component, according to Rand Rasmussen of the Southwest Fisheries Science Center. While these low energy, passive fish appear to survive their time in the nets, we have little way of gauging the long-term survival of individuals that have been caught and released. With more extensive genetic analyses, we can start to decipher if these extensive incidental captures may be adversely affecting our California population.
Tagging efforts (using pop-up satellite archival tags, PSATs) are revealing individual daily movements and diving behaviors. The tags record temperature, depth, and light intensity, from which location is deduced. At a preprogrammed time, the PSAT releases from the animal to the surface, where it transmits data to orbiting Argos satellites, which in turn relay the data to a ground station and to computers. The beauty of this technique is that it collects large amounts of data without bothering the fish again. In addition, since the tag is attached for several months to years, the fish has time to recover from the tagging event and presumably display normal behavior patterns.
In the fall of 2000 we tagged four individuals off San Diego and acquired data from one fish’s movements between August 2000 and March 2001. This mola traveled south to the middle of the Baja peninsula, then returned north to Catalina Island, where the tag released on March 17 as planned (Figure 1, opposite).
The fish’s dive behavior depended on location. In August it spent half its time in the upper five meters (80 percent above 40 meters). It rarely forayed into deeper water, below 200 meters. Moving south, its time in deep water increased. By October, it spent half its time between 10 and 40 meters and nearly a quarter of its time between 100 and 300 meters. In December it spent a quarter of its time between 40 and 60 meters and approximately 40 percent of its time below 200 meters. By March, returning north, the fish split its time between 10 and 20 meters and 100 to 200 meters.
While more than 60 percent of its time was spent in water above 10‹ C, the mola also frequented temperatures below 7.5‹ C more than 16 percent of the time. Together, these temperature/depth data suggest that the fish may have been making repeated forays to the deep scattering layer, returning to the upper waters to thermally recharge. Its decreased time in the upper five meters between October and March may be relate to surface wave action that typically increases during winter months. Additional tags and data will allow us to test these two hypotheses further.
|Figure 1. Movement of satellite-tagged Mola mola 18567 off the coast of southern California and Baja between August 2000 and March 2001. Lines indicate longitudinal limits of movement.
In August 2003 our team deployed another six tags off San Diego – five as part of the Tagging of Pacific Pelagics-Census of Marine Life Project (www.coml.org/descrip/topp.htm). These tags will release in the spring of 2004. Our ultimate aim is to combine tagging data with regional oceanographic data, fisheries observers’ sightings, and aerial censuses to deduce how Mola mola are using the California current. We hope our long-term tracking work will shed insight into diurnal behaviors as well as seasonal movement patterns. With this knowledge, fishermen and managers may be able to reduce the amount of mola bycatch.
This California work is part of a global effort to track ocean sunfish and record species prevalence and distribution in all tropical and temperate oceans. To date, we have tagged twenty-two individuals in California, Japan, South Africa, and Australia, including five sharp-tailed mola (Masturus lanceolatus) in Taiwan. Next September we will tag Mola mola in Bali, Indonesia. Taken together, the genetic and tagging efforts are beginning to reveal the intimate details of the mysterious mola and its role in the open ocean ecosystem. For continuing updates and published papers, see www.oceansunfish.org.
Sea Studios Foundation
New Ways to Get Energy from Light:
Novel Microbial Processes in Monterey Bay
The oceans, including the waters of the Monterey Bay National Marine Sanctuary, abound with microbial life. Marine waters are teaming with microorganisms – about 1 million microbes in each milliliter of seawater. The vast majority of these tiny organisms are not contaminants or pathogens, but rather are central components of oceanic biota. Though small in size, the diverse biochemical and physiological activities of these microorganisms help to sustain the balance of energy, elements, and nutrients in the oceans. As Pasteur said so well, “The very great is achieved by the very small.” In a very real sense these tiny microbial chemists help to maintain the oceanic ecosystem.
Despite the general importance of picoplankton (microbial plankton with cell diameters less than 2.0 µ), their biological properties are still not well understood. One reason is that it has been difficult to study these microbes in the laboratory using standard microbiological methods. Even when cultures are available, it is difficult to recreate the ecologically relevant biological interactions in a test tube. So, it has become necessary to redirect research efforts towards studying these microbes on their own turf. This involves the use of new strategies, techniques, and technologies, ranging from single cell analyses to stable isotope analyses, genomics, and physical biochemistry. The application of new techniques to study Monterey Bay picoplankton is leading to a deeper understanding of the biology and ecology of these tiny organisms.
Our National Science Foundation Microbial Observatory project, conducted at the Monterey Bay Aquarium Research Institute, is exploring the application of new technologies, developed in conjunction with the human genome project, to study natural microbial communities. It’s now possible, using advanced biotechnologies, to recover and analyze large portions of microbial genomes directly from seawater, sidestepping the problem of poor recovery by cultivation. The DNA sequence of these microbial genomes can provide important clues about the nature and ecological function of naturally occurring picoplankton. For example, an entirely new type of light-driven energy generation was recently discovered using such genomic approaches to ecological questions, as described below.
|Scanning electron micrograph of marine picoplankton, showing their very small size and different shapes. However, microbial diversity is not revealed by simple morphology, but rather by the complex and diverse physiology and biochemistry found within different microbial species.
There are several strategies for decoding the DNA of naturally occurring microbes. One approach is to capture and analyze very large DNA fragments (100,000 base pairs in size or greater), which can contain more than one hundred genes. Each gene encodes a functional protein, the building blocks that make up individual microbial cells. The decoding of these genes can lead to important clues about the properties and activities of microbes in nature. For instance, we decoded one large DNA fragment from an uncultivated marine bacterium (dubbed ‘SAR86’) that is abundant in Monterey Bay waters. Unexpectedly, we found a new type of photoprotein (a pigment containing protein that interacts with light) that we showed can be used by the microbes to generate cellular energy from sunlight. These photoproteins (known generally as rhodopsins) had never before been found in bacteria, so this was an unexpected finding: our newly applied genomic approach to environmental microbiology had revealed a new type of light-driven energy generation (phototrophy) in a very abundant planktonic marine bacterium. In fact, we can now show that these microbes and similar photoproteins are widespread in the sea, from polar regions to the tropical open ocean. A general take-home lesson here (also known from many other local studies), is that what we learn in Monterey Bay can often lead to useful insights about the oceans worldwide.
The initial discovery of the rhodopsin photoprotein was originally determined in laboratory studies. These showed us that indeed, the recombinant rhodopsin could generate energy from light. But do these laboratory experiments really tell us about natural oceanic processes? Once we had determined the biochemical properties of the novel rhodopsins, the search for this photoprotein in waters of Monterey Bay began. In natural picoplankton communities here, we found that the photoprotein was indeed present and functionally expressed in marine picoplankton of the bay. Further surveys in other locations, ranging from Antarctica to Hawaii, revealed that variants of the photoprotein are widely distributed in the world’s oceans and that they come in different colors. In deep waters, these microbial rhodopsins appear ‘tuned’ to blue light, the light that is most abundant at greater depth. In shallower waters, the rhodopsins are ‘tuned’ to absorb green light, which is more available at the surface but not at depth. So, the rhodopsin-containing bacteria have adapted to different conditions through-out the photic zone, shallow and deep. They are an abundant component of the picoplankton and certainly contribute to picoplankton productivity throughout the world’s oceans.
Many questions remain to be answered. Do the rhodopsin containing bacteria ‘fix’ CO2, like plants? How much carbon and energy do these microbes contribute to the food web? Are these microbes entirely reliant on light for growth? Further work in the sanctuary is bound to help resolve some of these questions.
Edward F. Delong
Monterey Bay Aquarium Research Institute