The IUCN/SSC Shark Specialist Group
Shark News 14: March 2002
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Molecular Genetics of some Brazilian Sharks
Manuel Furtado-Neto, Universidade Federal do Ceará, Brazil and
Steve Carr, Memorial University of Newfoundland, Canada
Sharks are excellent organisms for the study of molecular evolution in
vertebrates because there is an abundant stratigraphic record based on
fossil teeth which permits accurate calibration of rates of DNA and
protein sequence evolution. Literature on molecular genetics of sharks
from South America is rare. A study on the evolution of Squatinidae
sharks based on DNA sequences is presented here.
The family Squatinidae, (angel sharks), comprises a single genus
that includes fifteen extant species. Three species of the genus Squatina,
commonly known as 'cações-anjos' in Brazil, are endemic to the
continental shelf of southeastern South America, between latitudes
24°00'S and 42°00'S: Squatina argentina, S. guggenheim and S.
occulta. Fisheries for angel sharks are of great economic importance in
Rio Grande do Sul State, Brazil.
Vooren and Silva (1991) described the species Squatina occulta
which differs from S. argentina by the shape and relative size of the
pectoral fin, from S. guggenheim by the lack of a dorsal row of spines,
and from both species by the tooth formula and colour of the dorsal
body surface. Before their description of S. occulta and re-description
of S. guggenheim, only one species of Squatina was thought to occur
along the southern coast of South America. S. occulta and S. guggenheim
were misidentified as S. argentina in some studies.
In our study, PCR (polymerase chain reaction) was used to amplify
401-base pair sequences of the mitochondrial DNA cytochrome b
gene from each species. DNA sequences of this gene from the three
species of Squatina from southern Brazil were analysed. Phylogenetic
analyses were performed with the Phylogenetic Analysis Using
Parsimony (PAUP) program. The maximum parsimony tree was obtained
with the heuristic search algorithm (Swofford 1993).
The phylogenetic analyses performed indicate that the Squatina
species from southern Brazil constitute a monophyletic group, with the
newly described S. occulta being more closely related to S. guggenheim
than to S. argentina in all analyses, although S. occulta and S.
guggenheim differ in morphological aspects (e.g. presence or absence
of dorsal spines, tooth formula), in growth parameters (maximum total
length and total weight, total length and total weight at birth), and in
reproductive parameters (fecundity, total length at maturity, diameter
and mass of the mature follicle) (Vooren and Silva 1991).
The molecular phylogeny obtained here suggests that the similarities
between S. occulta and S. argentina are shared ancestral characteristics. S.
occulta and S. argentina have many similarities in growth and reproductive
parameters. Furthermore, the spatial distributions of these two species
overlap partially in the continental shelf of southern Brazil in depths of 60-
200 m, whereas S. guggenheim is rarely found in depths greater than 80 m
(Vooren and Silva 1991). The close relationship between S. occulta and S.
guggenheim observed in this molecular study is supported by the number
of functional ovaries. The present phylogeny suggests that a single functional
ovary is a synapomorphy between S. occulta and S. guggenheim. S.
argentina is the only one of the three angel sharks species from southern
Brazil that maintains the "ancient character" (Vooren and Silva 1991) of
paired functional ovaries in the female, while S. occulta and S. guggenheim
have a single functional ovary on the left-hand side of the body cavity. In
other species of Squatina, such as S. japonica, S. dumeril, and most
specimens observed of S. californica, only the left ovary is functional,
whereas S. oculata and S. squatina have two functional ovaries (Natanson
and Caillet 1986). Ovaries are paired structures in most elasmobranchs,
but it was observed that they can be asymmetrical in adult sharks of the
orders Carcharhiniformes, Pristiophoriformes and Squatiniformes.
Phylogenetic analyses of mitochondrial DNA sequences has shown
that the angel sharks (Squatiniformes) and saw sharks
(Pristiophoriformes) are sister groups (Kitamura et al. 1996).
Vooren and Silva (1991) suggested that speciation in angel sharks may
occur with minor changes in body form, and major changes in growth and
reproductive parameters. The results of the present study suggested that S.
occulta and S. guggenheim, the two species that share the reproductive
characteristic of a single functional ovary, are the most closely related pair
of species among the three species from southern Brazil. S. argentina, which
has two functional ovaries and has the highest fecundity (7-11 embryos), is
the only species that lives in depths below 200 m (up to 500m). S. occulta is
found in depths between 60-200m and has an intermediate fecundity
between the three species (4-10 embryos). S. guggenheim occurs from 0-
60m and has the lowest fecundity (3-8 embryos) (Vooren and Silva 1991).
Figure 1. Hypothesis of evolution of three species of angel sharks (S. argentina, S. guggenheim and S. occulta) from southern Brazil as predicted by mitochondrial.
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The present phylogenetic analysis suggests that evolution of the genus
Squatina in southeastern South America waters may have occurred from
deeper to shallower waters (Figure 1). The results indicate that S. argentina
was the first species of Squatina to occupy the continental shelf in depths
of 200m or more. Fossil records suggest that the genus Squatina has existed
since the Upper Jurassic (Capetta 1987). S. occulta and S. guggenheim
have evolved more recently, and speciation probably occurred as an
adaptation to life in shallower waters on different types of substrata. The
different colour patterns observed in the three species have also been cited
as evidence of adaptation to different types of ocean bottom (Vooren and
Silva 1991). If this hypothesis is true, S. guggenheim, the species that lives
between 0-80m, is the most recent species among the three Squatina from
southern Brazil.
References
Cappetta, H. 1987. Chondrichthyes. II. Mesozoic and Cenozoic Elasmobranchii.
Vol. 3B: handbook of paleoichthyology. Gustav Fischer, Stuttgart, Germany.
Kitamura, T., A. Takemura, S. Watabe, T. Taniuchi and M. Shimizu. 1996.
Molecular phylogeny of the sharks and rays of superorder Squalea based on
mitochondrial cytochrome b gene. Fisheries Science 62(3): 340-343.
Natanson, L. J., and G. M. Cailliet. 1986. Reproduction and development of the
Pacific angel shark, Squatina californica, off Santa Barbara, California.
Copeia 1986 (4):987-994.
Swofford, D. L. 1993. PAUP: Phylogenetic Analysis Using Parsimony, version 3.1.
Manual. Illinois Natural History Survey, Chapaign, IL.
Vooren, C. M., and K. G. Silva. 1991. On the taxonomy of the angel sharks from
southern Brazil, with the description of Squatina occulta sp.n. Rev. Brasil.
Biol. 51(3):589-602.
Manuel Furtado-Neto
Universidade Federal do Ceará
Fortaleza, Ceará, Brazil
Fax: 001-709-7374710
Email: mfurtado99@yahoo.com
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