King Midas Slitsnail

Offshore New Providence, Bahamas, Day 1,1999

Dr. Harasewych talking slitsnails

With this dispatch from sea, we begin an extraordinary expedition to the Bahamian archipelago, to search for slitsnails, wonderfully strange and primitive deep-sea gastropods living at crushing depths in inky black waters. Dr. Jerry Harasewych (right) from the Smithsonian Institution is leading the team to the seafloor in his effort to understand the natural history of the second largest living group of animals on Earth, the mollusks.Research Vessel Edwin Link docked at Harbor Branch Oceanographic Institute

By midnight Saturday, every member of the crew was on board the Research Vessel Edwin Link, and four hours later the ship left the dock at Nassau for open water. Out at sea, 10-12 foot waves breaking over the bow forced us to sheltered water--vestiges of hurricane Irene. The ship's crew and the research team held their first dive meeting Sunday morning at 7:30 am, and the sub was launched at 8:00 am with two scientists and two crew.

 

Submersible launch

One can tell from the energy and excitement that launching the submersible is the real beginning of the expedition. Diving through pitch black, the sub held at a depth of 2,500 feet off the northwest corner of New Providence. With lights and cameras turned on, the pilot slowly cruises the steep slopes while the team searches for samples. The landscape looks lunar; steep slopes are layered with fine chalky-looking sediments. Mostly, there is a look of desolation, a starkly beautiful underseascape that comes wonderfully alive when the powerful lights of the sub shine over it. In fact, the few individual corals and sponges that we see are dusted with sediment. A light and continuous rain of organic sediment filters down. It is "snowing" shells of planktonic organisms from near the surface. Without light and photosynthetic organisms, the possibility of life at this depth seems View from the submersibleimprobable. But we see a number of animals, never abundant, yet there. Tiny fishes, odd and wonderful--like little birds, occasional squid and octopuses, sea stars, brittle stars, sea cucumbers (that swim away when disturbed), a giant isopod, and then the deepest dwelling slitsnail known, the King Midas slitsnail (Perotrochus midas). Midas stands out among the beige sediment, its shell a reflection of iridescent pearl and gold. The animal has a large ivory-colored foot and head, and strangely, two small eyes, or light receptor organs.

The search for slitsnails is by no means random. Jerry knows that Midas occurs only within a narrow temperature range, between 8.0--10.0 degrees centigrade. There is a thermal gradient running from the warm surface waters to the cold, deep Midas feeding on a spongewaters surrounding the Bahamas. The search for Midas is carefully planned. Using depth charts and knowledge of deep sea temperatures in the region, and locating steep slopes, all increase the likelihood of finding this beautiful animal. In fact, Midas is found on such steep slopes that if one is knocked loose by the mechanical arm, it may tumble 50 feet or more and is hardly ever recovered.Juvenile Midas

On our first day Jerry dove twice accompanied by José. Five specimens of Midas were collected on the first dive, three on the second, a success for the team, or as Jerry says, "it's like walking a straight line across a tiny fraction of Africa looking for elephants. What are the chances that one will cross your path?" Actually, what this team is doing is much harder than that. The lights on the sub only illuminate a small area of the Adult Midas slitsnail on the seafloorlandscape, so peering out at the seafloor is like looking through a keyhole. Adding even more difficulty to the situation is the optical effect of the sub's Plexiglas bubble. Looking through the bubble, everything appears much smaller than actual size, so a four inch diameter shell looks to be about the size of a dime.

The team will be diving for Midas over two days, four dives altogether, and then the ship will head off to Chub Cay in search of another gastropod, Andanson's slitsnail (Entemnotrochus adansonianus). Future reports will explain more about the sub and other equipment, and the kinds of research the team is carrying out.

 

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Smithsonian, National Museum of Natural History