Searching for the Rivulus

Dr. George O’Meara, professor emeritus of Physiology and Ecology at the University of Florida’s Medical Entomology Laboratory (FMEL) at the Oslo Riverfront Conservation Area (ORCA) in Vero Beach, Florida, was the guest speaker at the 14 February 2015 FMEL-Pelican Island Audubon conservation stewardship class. His talk on Life in the Pits and the Treetops focused on critters that live in the water that pools in epiphyte plants ā€” the treetop part ā€” and those that live down in the burrows dug out by the Great Atlantic Land Crab (Cardisoma guanhumi) ā€” the pits part. To find out what creatures live down in the crab holes besides the crab requires this custom-made pump.
Dr. O’Meara started his presentation in the FMEL Boathouse classroom before taking everyone out onto the mosquito impoundment dike to search for crab holes. Earlier speakers included Janice Broda talking about epiphytes, plants that grow on other plants but are not parasitic, and my talk on local woodpeckers.
Dr. O’Meara collects water from the impoundment to pour down the crab hole to make it easier to pump out. Though the crabs burrow down to the water table, more water needs to be added making the hole easier to pump out if the hole is flooded. My many walks along this isolated trail has occasionally been in the company of some rather large alligators meaning scooping water out of the impoundment is a bit more adventuresome than it looks.
Inserting the hose down a candidate crab hole. This is a temporary inconvenience for the crab, who is not affected by the operation.
Pouring in the water.
Pumping the water out of the crab hole and through a sieve into a tray. Assisting Dr. O’Meara is Dr. Richard Baker, professor emeritus of Genetics and Cytogenetics at FMEL and current president of Pelican Island Audubon Society.
The usual end result pumps out large numbers of Crabhole Mosquito (Deinocerites cancer) larvae that mature down in the water of the hole. Today, unfortunately, was a bit of a bust with only two mosquito larvae being seen in the water. The holy grail of this operation would be to pump out a Mangrove Rivulus (Rivulus marmoratus), a small self-fertilizing hermaphroditic fish that also lives down in the water of the crab burrows and feeds on the mosquito larvae and other small life in the hole. Sadly, no Rivulus were seen this day either. One would think all these small creatures are trapped down in the crab hole but high tides or flooding allows them to migrate to other crab holes or other areas of their liking and, of course, when the mosquitos mature, they can fly in and out of the holes as they please. So it is not an isolated existence for the Land Crabs down in their burrows, they have a lot of company with them.

Visit the Oslo Riverfront Conservation Area blog by Janice Broda.

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