How Deep Is Deep?
Museum-Hunting for the Deepest Atlantic Mollusks
by José H. Leal,
Questions often arise concerning
the distinction between "shallow-water" and "deep-water"
shells. Where should we draw the line between deep and shallow
water? Although we use these expressions very often in shell
collecting and other natural history contexts, there is not
a well-established, all-purpose boundary separating "shallow"
from "deep." The terms are usually used in comparative
situations. For instance, for someone who collects shells on
the beach or at low tide, deep-sea species may be those that
are not easily collected on the shoreline or on exposed sand
flats. For them, deep-water species may be rarer species that
require more sophisticated collecting methods, including snorkeling,
SCUBA, or dredging.
Although there is not a rigid
rule validating this, scientists may use "shallow-water"
to indicate species living on the continental shelf bottom.
The shelf is the area of ocean bottom that is just a continuation
of the continental mass, usually extending to depths between
150 and 300 meters (or 500 and 1,000 feet.) Marine biologists
like to use the term "sublittoral species" for organisms
that live on the continental shelf; true "deep-water species"
would appear only below that depth limit. "Deep-water"
bottoms can be further subdivided into the archibenthal or bathyal
zone (between 300 and 3,000 meters, or 1,000 and 10,000 feet),
the abyssal zone (between 3,000 and 6,000 meters, or 10,000
and 20,000 feet), and the hadal or ultra-abyssal zone (below
6,000 meters, or 20,000 feet). The latter category includes
only areas of submarine trenches, where the ultimate deep-water
species are found! The deepest point of all oceans, about 11,000
meters (36,000 feet), is located in the Marianas Trench, western
Upon my return to South Florida
in 1994 following a two-year postdoctoral appointment at the
Smithsonian Institution's Division of Mollusks, I went to work
at the University of Miami's Rosenstiel School of Marine and
Atmospheric Science (RSMAS). In my search for "deep-water"
(bathyal and abyssal) mollusks on the shelves of the University's
Marine Invertebrate Museum, I ran into several big five-gallon
jars, all filled to the rim with pieces of wood, seagrass blades,
seeds of land plants and, curiously enough, coarse chunks of
rusty metal that looked like the solidified remains of molten
lava. All of that was immersed in alcohol and, after almost
30 years of maceration, the concoction had turned into a solution
the color of dark tea.
As I started to empty the contents
of the big jars onto sorting trays, the key to the mystery appeared
in the form of little tags with capital letters and numbers.
The University of Miami station "numbers" are always
preceded by one or two initials. The letters stand for the names
of their now legendary research vessels; the numbers represent
stations. Their well-kept station records disclosed all the
information I needed: location, including latitude and longitude,
dates, depths, personnel working on board, kind of gear used
(mostly big deep-sea trawls), all indicating exactly in what
part of the world's oceans the samples were obtained. The pickled
plant material came from the floors of the Cayman and Puerto
Rico trenches, in the Caribbean region! As I carefully sorted
through the alcohol-preserved plant material under the microscope,
I finally found what I was so eagerly looking for: small, white,
yellowish, or brown limpets, with a maximum size of about a
quarter-of-inch in length. And what a surprise! According to
the station records of R/V Gilliss, the material from station
GS-109 in the Puerto Rico Trench, a trawl haul obtained at 8595
meters (28,000 feet) and yielding many limpet specimens, came
from near the deepest spot in the Atlantic Ocean! Although shallower
than the Marianas Trench, the Puerto Rico Trench is as impressively
deep as Mount Everest is high.
The Cayman and Puerto Rico trenches
basically represent tectonic plate boundaries, or areas of the
ocean floor where the gigantic individual plates that cover
the Earth's surface are in contact, colliding or sliding past
each other. As a result, earthquakes and volcanic activity abound
at and near trench areas. Submarine volcanic activity produces
the rusty, metallic nodules I found inside the jars. These are
usually blends of iron, manganese, and other minerals.
Trenches are frequently close
to tropical islands, and there is steady accumulation of plant
debris falling from these nearby land, mangrove, and seagrass
environments onto the trench floors. It is by a fortuitous combination
of circumstances that herbivorous hadal limpets like the ones
I had found are furnished with a permanent supply of plant food.
Mollusks living in the less-populated, shallower abyssal zone
(between 3500 and 6000 meters, or 11,500 and 20,000 feet), must
depend upon the "rain" of occasional fish carcasses
and other animal debris falling from the upper layers of the
water column (in the case of scavenger species), or upon random
encounters with prey (in the case of predators). So it seems
somehow ironic that mollusks living in the very deepest parts
of the oceans must rely on food coming directly from land or
from very shallow water areas! That situation represents a kind
of "short circuit" in the classic ecological model
for deep-sea food webs. Most descriptions of deep-sea food-webs
involve a large number of feeding levels based on the large
physical distance separating primary producers (plants, which
are absent in the dark deep-sea) from predators and scavengers,
and the absence of herbivore groups such as keyhole and true
After a couple of years of studies,
and with the help of my colleague, M.G. Harasewych, curator
of mollusks at the Smithsonian Institution in Washington, D.C.,
and, like myself, also deeply (!) involved with this type of
mollusk, I prepared a manuscript summarizing the results of
our research. The work we did was funded in part by a 1995 COA
Research Grant Award I received. All limpets found at the RSMAS
Museum can be grouped into four species belonging to two different
families in the Order Cocculiniformia. The Cocculinidae are
Fedikovella caymanensis Moskalev, 1976 and a new species also
belonging to a new genus; the Pseudococculinidae are Caymanabyssia
spina Moskalev, 1976, and Amphiplica new species.
Fedikovella caymanensis Moskalev, 1976
Scale line = 1mm
New Genus, new species
Inset shows internal apical septum.
Scale line = 1mm
Caymanabyssia spina Moskalev, 1976
Scale line = 1mm
Amphiplica new species
Scale line = 1mm
The two named species were first
found by Lev Moskalev, from the Russian Academy of Sciences,
after extensive field work carried out in the early 1970s by
Russian research vessels in the Caribbean Sea. All four species
are fully adapted to their ultra-deep-water lifestyles. They
lack eyes, for there is no light available for vision; instead,
they rely on smell and touch, having batteries of cilia, or
miniature tentacles, near or around their mouths. They are equipped
with sturdy radulae capable of scraping the hard wood or other
plant material. Their shells are covered by very thick periostracums
that help protect the calcium carbonate underneath from the
chemical attack of acids present in the surrounding water. The
organic protein content in their shells is also higher than
in other gastropods, a resource that further enhances protection
against shell dissolution by acids. Cocculiniform limpets are
hermaphroditic, each animal having both female and male reproductive
systems. Finding a mate can be difficult in deep-water conditions,
and hermaphroditism ensures that when one mating event occurs,
two animals are fertilized through mutual cross fertilization,
thus increasing the chances for perpetuation of the species.
The ultra-deep-sea limpets certainly
compelled me to reevaluate my notions about deep-water mollusks
and to stimulate me to research one of the last frontiers of
the world's ocean.
Director, The Bailey-Matthews
Shell Museum, P.O. Box 1580, Sanibel Island, FL 33957
See also Dr. Leal's
article, "Deep-Sea Mollusks
-- An Introduction".