Tales from Oceanic Islands:
The Biogeography of Insular Marine Gastropods
from off Brazil
by José H. Leal
A newly created
volcanic island is devoid of life, until it is colonized by
organisms from other, geologically older areas. The classical
theory of island biogeography discusses how physical factors,
such as the surface area of an island, its age, and its distance
from the mainland interact to determine how quickly such an
island will be colonized and how many species are likely to
live there. Other factors besides physical ones are important
too. Each particular species has a certain mixture of capabilities
and limitations that define its ability to disperse and colonize
an island. The study of the processes that make it possible
for these shallow water benthic (bottom-living) creatures to
cross stretches of deep water and colonize a geologically young
oceanic island is a paramount aspect of marine biogeography.
Most true oceanic islands have
originated from volcanic activity in the deep ocean floor. As
a result of plate tectonics and sea-floor spreading through
geological time, weaker regions of the oceanic crust have been
punctured sporadically by hot magma from below. Big piles of
solidified lava and debris produced in these areas during periods
of intensified volcanic activity create true oceanic islands
and submerged seamounts. Such islands occur off the coast of
Brazil: in the western extremity of the Romanche Trench off
northeastern Brazil are the Fernando de Noronha Archipelago
and Atol das Rocas; more to the south, in the eastern end of
the Vitoria-Trindade Seamount Chain (also known as Martin Vaz
Fracture Zone), are the islands of Trindade and Martin Vaz Archipelago.
Created since the birth of the Atlantic some 100 million years
ago, these islands have formed chains perpendicular to the South
American coast. In the case of this particular ocean, the islands
and seamounts situated closer to the continent are usually older
than those found in the open ocean, a consequence of the local
direction of sea-floor spreading.
My dissertation project at the
Rosenstiel School of Marine and Atmospheric Science/University
of Miami consists of a biogeographical study of the prosobranchs
(hard shelled, benthic, or bottom-dwelling, gastropods) from
these islands in the southwestern Atlantic Ocean. The main goals
of my research are to survey the prosobranchs found in these
islands and to attempt to determine why they are there, i.e.,
how the faunal composition of each island can be determined
by the interaction between the physical factors of the island
and the capabilities and limitations of the each species present
More than 320 species of mollusks
live in these islands, over a third the number of species Professor
Rios examined in 1985 for the entire Brazilian coast. The mollusks
included in my study come from samples I collected, by SCUBA
in the shallows and dredging in the depths, as well as from
E.C. Rios' collection in the Museu Oceanografico do Rio Grande,
Brazil, and from other museum collections worldwide.
The most fundamental (and time-consuming)
part of my project is the accurate taxonomic identification
of these species, a task I am performing with much help from
specialists in the U.S. and abroad. For close examination and
illustration of very small shells and protoconchs, I have used
extensively the scanning electron microscope (SEM) at the Electron
Microscopy Laboratory at the Rosenstiel School of Marine and
Atmospheric Science. Figures 1-15 exemplify my SEM micrographs
of some of these adult micromollusks, juvenile shells and protoconchs.
One of the reasons I chose this
study of the Brazilian oceanic islands as my dissertation project
is that it contributes to a Brazilian federal survey of these
islands aimed at establishing marine national parks and natural
reserves in these pristine localities. One of them, Atol das
Rocas, has already been designated an ecological reserve. Proposals
have been forwarded to the Brazilian government suggesting similar
procedures for Fernando de Noronha and Trindade Island. Trindade
Island has, in fact, recently been spared the threat of construction
of a military base.
A basic objective in my project
is to investigate the source of the prosobranchs that have colonized
the islands, and at the same time to test some propositions
in insular biogeography. Another aspect to my study is a computer-assisted
comparison of prosobranch faunas among the islands, and between
each island and the adjacent Brazilian coastal areas. Special
attention is paid to the presence of endemic species on any
island or group of islands. Endemism is a delicate subject to
approach, considering that a quarter to a fifth of the species
involved in the project are unnamed, and that, despite Rios'
pioneer efforts, a large part of the Brazilian coastal molluscan
fauna is still unknown.
The type of reproductive strategy
used by different species may affect faunal composition from
island to island. In the development from embryo to adult, some
prosobranch species go through a pelagic, or planktonic swimming
larval stage. Others undergo direct development, hatching as
miniature adults from a brood pouch or egg capsules, and settling
down immediately to the life of an adult. The presence of a
planktonic larval stage allows a species greater opportunities
for dispersal by ocean currents, so that it may colonize distant
areas, provided adequate environmental conditions are present,
and the arrival of larvae is frequent enough to enable the new
population to survive. Larval transport by large oceanic currents
occurs in the same way as when seeds and spores of plants are
carried by the wind, facilitating the dispersal of these plants
from one area to another.
During this free-living, swimming
period, a larva can be an active feeder, or planktotrophic,
eating microscopic algae and organic particles; or it can be
lecithotrophic, nourished only by its own yolk supply. The ability
to feed while in the larval stage enables planktotrophic types
to last longer than lecithotrophs, thus giving them a larger
potential for dispersal, just as lecithotrophs are, in turn,
better "dispersers" than direct developers. Isolated,
geologically young oceanic islands have a larger percentage
of planktotrophic species, or species that evolved from them,
than coastal areas or oceanic islands near the coast.
Determining reproductive strategies
is an important step in the investigation of a species' relative
predominance in the different islands. This is accomplished
(1) by direct data from the literature, when life histories
are known, (2) through comparisons of the larval shell or protoconch
morphology when life history is known only for another, closely
related species, and (3) when necessary, by complete inference,
using protoconch morphology. Planktotrophic larvae usually build
protoconchs with elaborate sculptures and more than two or three
whorls. Also, planktotrophic protoconchs are frequently divided
into a small, well-defined embryonic shell (protoconch 1) and
a larger larval shell (protoconch U). A sinuous double curve
in the external lip of the planktotrophic larval shell, called
the sinusigerous lip, allows space for the winglike velar lobes
of the larva. This sinusigerous lip is visible in the illustration
of the larval shell of the widely distributed planktotroph,
Cypraea cinerea Gmelin, 1791, in Figures 7-8. Lecithotrophic
types have a poorly defined embryonic shell; the larval shell
has 1.5 to 2 whorls, and no sinusigerous lip. In direct developers
the entire protoconch is the embryonic shell.
Dr. P. Bouchet of the Museum
National d'Histoire Naturelle, Paris, and I have studied the
general distribution of prosobranchs and their types of reproductive
strategies in ten seamounts and islands along the Vitoria-Trindade
Seamount Chain, and have a paper in preparation. We have found
the ratio of planktotrophs to lecithotrophs in both Trindade
and Martin Vaz to be just about the same as in the closest continental
area, 1,100 km to the west, even though the total numbers of
species decreases from the continent towards the open ocean.
This fact apparently contradicts observations made in other
areas of the world where planktotrophs are relatively more frequent
than lecithotrophs in oceanic islands. But the apparent contradiction
can be explained by the presence of the chain itself: the submerged
but relatively shallow seamounts are usually only 100 km apart,
so shorter-lived lecithotrophic larvae probably colonize the
seamounts in a stepping-stones fashion, moving in stages from
continent to seamount to seamount.
However, in the case of direct
developers, there has been a marked reduction in numbers in
the easternmost seamounts in the chain (towards the ocean).
Groups in which the eggs and juveniles are unusually large,
as in the family Volutidae, are completely absent from the chain.
In examining material from Atol
das Rocas and Fernando de Noronha, off northeastern Brazil,
I am finding that their prosobranch faunas are apparently much
richer and more diverse than those of Trindade and Martin Vaz.
The warmer equatorial waters and the larger number of habitat
types available in these localities, including small coral reefs,
probably explain this difference. Endemic species are infrequent,
perhaps owing to the relatively smaller distance from the coast,
but are present nonetheless; several endemic microprosobranchs
have already been examined (e.g., Parviturbo sp., Figure 2;
"undetermined barleeid," Figure 14).
Another curious aspect of oceanic
islands is that they can provide relatively stable environments
for species or genera that otherwise went extinct in other areas
because of some shift in their environment. Possible examples
of such differential extinction in my research are Rissoina
indiscreta Leal & Moore, 1989, from Atol das Rocas and northeastern
Brazil, a species apparently closely related to the Indo-West
Pacific R. turricula Pease, 1861 and to a large complex of fossil
(Miocene and Pliocene) species in tropical seas. The tun shell
genus Malea is known at present only from the Indo-West and
Eastern Pacific, and in the Atlantic Ocean only from Atol das
Rocas and Fernando de Noronha, but has been reported by Olsson
& Petit from the late Miocene and Upper Pliocene of Florida,
Venezuela and Colombia. Another significant example is Neritopsis
atlantica Sarasua, 1973, family Neritopsidae. The family has
extensive Paleogene fossil representation, but only two living
species are known, one in the Indo-West Pacific and one in the
Atlantic Ocean. Netitopsis atlantica was described from Cuba,
and we have found several recently dead shells of the species
in Trindade Island, representing a very odd type of distribution
for this relatively large (1.5 cm width) species.
In the end result, my project
has also had more general consequences. In allowing me to work
with a large number of species and the processes affecting their
distribution, I have been led to a better understanding of the
origins and relationships of the molluscan fauna in the southwestern
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_____ & D.R. Moore. 1989.
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