Parasitic crustaceans are important factors in the ecology and life history of reef fishes. With the biomass and energy they removed from their hosts being considerable and influencing behavior, physiology, and survivorship. Gnathiid isopod crustaceans represent the most prominent group of fish parasites in tropical and subtropical reef systems, where they are both abundant and diverse and are the primary food item of cleaner fishes.
Gnathiids have unusual life histories in which only the three juvenile stages feed on blood and body fluids of fishes, while in the adult stage, they no longer feed. Given that feeding requires only a brief association with the host, while each interfeeding interval last days, Gnathiids spend most of their life freeliving in the benthos. Given the many ways in which Gnathiids impact their hosts, there are compelling reasons to include them in ecological studies of coral reefs.
However, their unique life history and the small number of researchers who study them creates a barrier to the development, implementation, and dissemination of reliable reproducible sampling methods to collect them for research. Light traps have long been used to collect small marine organisms at night. They take advantage of and are based on the fact that many nocturnally active organisms, including arthropods, are attracted to light.
Traditionally, they have been used to collect planktonic organisms in the water column. However, the basic principles can be applied to collecting free swimming organisms that are active near the benthos. Here we present a light trapping method adapted for collecting free living stages of Gnathiid isopods near the ocean bottom in remote coral reef environments such as the Philippines.
In collecting remote areas, these traps offer some advantages over other methods that have been developed for collecting these organisms. They're highly portable and durable and they require only three parts, which are easily obtainable and inexpensive. They're also negatively buoyant.
Because they're dependent on light for attraction, they're only effective at night for collecting nocturnally active species. They also attract more than the target species, requiring sorting of the samples under a dissecting microscope to obtain the target organisms. Light traps.Construction.
Light traps are best constructed from commercial PVC tubes originally designed for plumbing purposes. Use 20 to 25 centimeter diameter PVC cut to 30 to 40 centimeters in length. To both ends of the tube, add PVC caps with transparent acrylic funnels inserted in the center of the opening and glue in place with transparent epoxy glue.
Ensure that one end of the tube has a screw on otherwise removable lid and that both ends are water tight when the trap is closed. Light source. Before deployment, an underwater torch, such as a Fantasea nano spotter 6023, should be turned on and placed in the tube facing the transparent funnel such that the light from the underwater torch illuminates the outside in front of one side of the tube.
The light attracts a variety of small nocturnal organisms, including Gnathiids, and drives them to swim into the tube with the clear funnel. Once they've entered the tube, they're unable to escape due to the geometry of the light trap and the continued presence of a light source.Placement. Fill light traps with the light turned on and with sea water and secure both ends.
Place traps on the sea floor in the sand or rubble next to coral heads or other complex structures known to attract fish. Focus the light cone inwards towards the areas where fish aggregate. In shallow water, the traps can be placed by breath hold diving.
Deeper deployment requires scuba.Retrieval. Organisms will remain in the trap once the batteries of the lights have expired and the light is no longer illuminated. This provides flexibility in the time in which the traps are retrieved.
Immediately prior to retrieving the trap, seal the openings of both the funnels on either end of the tube with a piece of clay or rubber stopper sealing, keeping all the seawater and the contained organisms inside.Transport. Once the traps have been retrieved from the bottom, they can be carried by divers to a boat or swam ashore. However, it's best to maintain traps as close as possible to ambient seawater temperature once they've been removed from the ocean.
Transport them to the laboratory for processing as soon as possible since no gas or water exchange will take place once removed from the ocean. Laboratory processing. Storing and filtering samples.
Once a light traps are removed from the ocean and brought back to the lab, empty their contents into buckets with fresh sea water. Add aeration to keep organisms alive until filtering. Filter the contents of the bucket by pouring through a funnel lined with 50 to 100 UM plankton mesh.
Then empty the contents to a hundred milliliter container of fresh seawater. Use a pipette to draw from this small container to place aliquots of the sample into a Petri dish for microscopy. Repeat until the entire sample has been processed.
Gnathiid isopod identification and rearing. Because light trap samples will attract multiple species of small invertebrates, samples must be carefully screened to identify and remove Gnathiids isopods. 10 to 20 times magnification is best for this task.
Identifying Gnathiids at the family level does not require living specimens. However, adult Gnathiids, which are seldom caught in light traps, are needed for morphological species ID and for breeding. Thus, in cases where Gnathiids need to be kept alive of rearing, gently remove them with a pipette and place them in small plastic containers of fresh seawater.
Representative results. The sampling in the central Philippines, the outline trap design was used when traps were set overnight. One to 1, 343 Gnathiids per trap were collected.
These included both fed and unfed juvenile stages. These results clearly demonstrate the effectiveness of light traps for collecting Gnathiid isopods under the study conditions. This method for Gnathiid collection is effective and sufficiently flexible for a variety of field sites and scientific questions.
For example, in a study by Artim et al in 2020 in the Caribbean, where they used light traps to quantify coral cover and fish biomass on Gnathiid abundance.Conclusion. While other techniques have been developed and used for collecting Gnathiids, this technique is particularly efficient. While the light traps described here attracts a wide range of small mobile invertebrates, the composition of this bycatch is highly variable.
However, when set in habitat that includes rubble and fish, Gnathiids are caught consistently. Although this demonstration study focused on sites in the Philippines, traps of a similar design have also been used successfully for studies at sites in the Caribbean region and the Great Barrier Reef.
