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Braz. J. Biol. vol.69 no.3 São Carlos Aug. 2009
Caracteres não métricos nas duas espécies de Sotalia (Gray, 1866)
Fettuccia, DC.I,*; da Silva, VMF.I; Simões-Lopes, PC.II
ILaboratório de Mamíferos Aquáticos - INPA, Av. André Araújo, 2936, Aleixo, CEP 69060-001, Manaus, AM, Brazil
IILaboratório de Mamíferos Aquáticos - LAMAQ, Departamento de Ecologia e Zoologia, CCB, Universidade Federal de Santa Catarina - UFSC, CP 5102, CEP 88040-970, Florianópolis, SC, Brazil
Analyses of non-metric characters of the skull and cervical vertebrae were performed among samples of dolphins of the genus Sotalia from the north, northeast and south Brazilian coast (S. guianensis) and also samples from the Amazon River Basin (S. fluviatilis) as part of an osteological descriptive study. The results demonstrated that there was a higher percentage of occurrence of fenestrae in the occipital region (66%) and cervical ribs in the cervical vertebrae (87%) in the riverine species. The vomer in wide shape was more frequent in the riverine species (57%), followed by the intermediate (32%) and narrow shape (11%), that was found to be more frequent in the marine species (66 to 76%). In relation to the lacerate anterior foramen, it was observed that an open/elongated shape is more common in the riverine species (88%). Most samples in the marine species present this foramen divided by a spike shaped projection (72 to 98%). The ventrally visible location of the hypoglossal foramen was more often observed externally displaced in S. guianensis (88 to 98%), while in S. fluviatilis, most samples (87%) presented this foramen internally displaced to the jugular notch, and not visible in ventral view. The fluvial species seems to present neoteny (or maintenance of juvenile characters in adults) in relation to the position of the pterygoids and in development of lacerate anterior foramen.
Keywords: Morphology, Sotalia, osteology, skull, cervical vertebrae.
Este é um trabalho osteológico descritivo entre exemplares do gênero Sotalia da costa Norte, Nordeste e Sul do Brasil e exemplares fluviais da bacia amazônica, analisando caracteres não métricos no crânio e nas vértebras cervicais. A frequência de ocorrência de fenestras na região occipital (66%) e de costelas cervicais (87 %) foi maior na espécie fluvial (S. fluviatilis). Na espécie fluvial, a forma do vômer largo foi mais frequente (57%), seguida da forma intermediária (32%) e estreita (11%). A forma do vômer estreito foi mais frequente na espécie marinha (S. guianensis) (66 a 76%). Em relação ao forâmen lacerado anterior, foi observado que a forma aberta/alongada é mais comum na espécie fluvial (88%). Na espécie marinha, a maioria dos exemplares apresenta este forâmen dividido por uma projeção em forma de espinho (72 a 98%). A localização do forâmen hipoglossal visível ventralmente foi mais observada em S. guianensis (88 a 98%), enquanto que em S. fluviatilis, a maioria dos exemplares (87%) apresentou este forâmen deslocado internamente à reentrância jugular, não podendo ser observado em vista ventral. A espécie fluvial parece apresentar neotenia (ou manutenção de caracteres juvenis no adulto) em relação ao posicionamento dos pterigoides e ao desenvolvimento do forâmen lacerado interior.
Palavras-chave: morfologia, Sotalia, osteologia, crânio, vértebra cervical.
The genus Sotalia (Gray, 1866) was until recently considered monospecific, however recent molecular genetics (Cunha et al., 2005; Caballero et al., 2007), and geometric morphometric (Monteiro-Filho et al., 2002) studies revealed the existence of two species: the marine species Sotalia guianensis (P.J. van Bénéden, 1864) (gray dolphin) and the riverine species Sotalia fluviatilis (Gervais, 1853) (tucuxi).
The marine species is broadly distributed along the tropical and subtropical Atlantic coast of South and Central America, having been recorded from Florianópolis, Santa Catarina, Brazil (27º 35' S and 48º 34' W) (Simões-Lopes, 1988; Borobia, 1989) to Honduras (15º 58' N and 85º 42' W) (da Silva and Best, 1996). The riverine species is endemic to the Amazon Basin, occurring from Belém (at the mouth of the Amazon River), in Brazil, to the rivers of Peru, Colombia and Ecuador (Borobia et al., 1991, da Silva and Best, 1996).
Osteological descriptions for the genus Sotalia have been published by Miranda-Ribeiro (1936); Casinos et al. (1981); Borobia (1989); Menezes and Simões-Lopes (1996); da Silva and Best (1996); Alves Júnior and Monteiro-Neto (1999); Avila et al. (2002); Fettuccia and Simões-Lopes (2004), Simões-Lopes (2006). With the exception of Simões-Lopes (2006), these studies were mainly based on traditional studies of morphometrics. There are no previous comparative studies using non-metric characteristics such as, for example, the comparison of forms or presence and absence of determined characters for this genus. Therefore, this study aims to compare the two species of the genus Sotalia using non-metric characters.
2. Material and Methods
A total of 149 specimens were analysed from five distinct Brazilian states: Mammal Collection of National Institute for Amazônia Research (INPA), Manaus, Amazonas (AM); Emilio Goeldi Museum (MPEG), Belém, Pará (PA); Aquatic Mammals Laboratory (LMA) of the Department of Ecology and Zoology, Federal University of Santa Catarina (UFSC), Florianópolis, Santa Catarina (SC); as well as the Osteological Archive of the Association of Research and Preservation of Aquatic Ecosystems (AQUASIS), Caucaia, Ceará (CE) (Tables 1-4). The INPA collection contains riverine specimens (S. fluviatilis) from the Amazonas State (AM), marine specimens from the coast of the Amapá (AP) and estuarine specimens (S. guianensis) from Pará State (PA). The other collections only possess marine specimens from their respective localities. Forty-four specimens of S. fluviatilis and 105 specimens of S. guianensis were analyzed (AP = 24; PA = 02; CE = 41; SC = 38). The marine specimens were divided into 3 large areas: NOR (including specimens from AP and PA), for the specimens from the coast of Amapá and Pará States, CE for the specimens off the Ceará coast and SC for specimens from the Santa Catarina coast.
The skulls were compared using non-metric characters, in accordance with Perrin et al. (1982) and two newly proposed characters (Table 5; Figures 1-4). The cervical vertebrae were analyzed according to the presence or absence of cervical ribs.
In order to observe ontogenetic variation, the characters were evaluated by age classes (calf, juvenile, immature and adult) (adapted from Dawbin et al., 1970). Variations between the sexes were not analysed in this study due to the small sample size.
The terminology used for skeletal bones followed Flower (1885), Kraglievich (1937), Lessertieur and Saban (1967), Rommel (1990) and Simões-Lopes (2006).
Of the 22 non-metric characters analysed, only six were informative for the distinction between the two species.
A high frequency of fenestrae in the occipital region was observed, near the foramen magnum in S. fluviatilis (66%; n = 44). In S. guianensis the percentage of occurrence of fenestrae varied between 31% (specimens from NOR, n = 26), 10 % (CE, n = 41) and 13% (SC, n = 39).
The number of fenestrae near the condyle was also larger in the riverine species, varying from one to four per individual. In both species, these structures were found in all age classes.
In the marine species, the specimens from CE and NOR exhibited one to three fenestrae, while those from SC exhibited only one to two. Fenestrae can occur only on one side (more common in the marine species) or on both sides of the occipital condyle (more common in the riverine species). In newborn and young specimens, other openings in the occipital region (located above the fenestrae) were observed, corresponding to the fontanelles, formed by the union of the exoccipital, parietal and supraoccipital bones (Figure 5).
In the region of occipital protuberance, in general, the occurrence of fenestrae was less frequent: 4.5% in S. fluviatilis (n = 44), 10% for S. guianensis of CE (n = 41), 5% for specimens from SC (n = 38) and no occurrence for marine specimens from NOR (n = 26). In some specimens of the two species, small fenestrae of irregular contour in the basioccipital were observed. In the marine species, such fenestrae occurred in 2% (n = 41) and 8% (n = 39) of the specimens from CE and SC, respectively. In the riverine species, these fenestrae occurred in 9% (n = 43) of the specimens. These openings were not observed in the marine specimens from NOR.
There was a prevalence of one form of vomer for the marine species, and another form for the riverine species (Figure 6). The wide form (wider in the posterior region) was more frequent in the riverine species, occurring in 57% (n = 37) of specimens, followed by the intermediate form (32%) and the narrow form (11%) (Figure 7). In the marine species, the narrow form was most frequent, occurring in 76% (n = 21) of the specimens from NOR and in 66% (n = 38) of the specimens from CE. The intermediate form of the vomer occurred in 9.5% (n = 21) of the specimens from NOR and in 31.5% (n = 38) of the specimens from CE. This characteristic was added later to the analysis, thus the specimens from SC were not included in the analyses. This characteristic seems to be related with individual variation and not with development.
The contact, anterior and posterior, between pterygoids exhibited no variation, being separate (gap >1 mm) in 100% of specimens of both species (except for INPA specimen 130 from AP, whose pterygoides had medial contact). In riverine species it was observed that generally there was a greater distance between pterygoids in the posterior portion, forming an inverted "v". Young individuals of marine species seems to present this same greater distance in the posterior portion. This could indicate neoteny in the riverine species, but in order to be able to interpret this variation a more thorough morphometric study is needed.
3.4. Anterior lacerate foramen
The predominant form of anterior lacerate foramen was distinct in the two species (Figures 8 and 9). In S. fluviatilis, the majority of specimens (88%, n = 43) exhibited open/elongated anterior lacerate foramen. In S. guianensis, the majority of the specimens analysed exhibited foramen divided by a projection in the form of a spine: 72% (NOR, n = 25), 77.5% (CE, n = 40) and 98%, (SC, n = 38). This spine projection, which rarely develops in adults of the riverine species, seems to be associated with ontogenetic development in the marine species, leading to the assumption that it is related to neoteny. A more detailed study with a larger number of young and juveniles of both species could provide an answer to this question. In the three marine samples analysed, it was observed that some individual adults exhibited a formation of fusion points between the projection and one side of the lacerate foramen.
3.5. Hypoglossal foramen
The location of hypoglossal foramen next to the jugular notch was considerably higher in S. guianensis: 88% (NOR, n = 25), 95% (CE, n = 41) and 98% (SC, n = 39). In this species, this foramen is generally visible in the ventral view, externally displaced to the jugular notch. Conversely, in the riverine species, the majority of specimens (87%, n = 45) exhibited this foramen internally displaced to the jugular notch, not easily observed in the ventral view (Figures 10 and 11). It is worth noting, however, that variation exists in the proximity of this foramen to the jugular notch in both species, where some individuals exhibit a displaced foramen, being either separated or together with the notch. This characteristic doesn't seem to be related with development.
3.6. Cervical vertebrae
In the cervical vertebrae, projections on the seventh cervical vertebrae (Ce7) associated with the transversal canal (= vertebrarterial canals), are called pleuroapophiseal plates or cervical ribs (Figures 12). The occurrence of these structures was higher in S. fluviatilis (87%; n = 31) than in S. guianensis from CE (9%, n = 23) and from SC (19%, n = 32). Furthermore, the size of the ribs is proportionally smaller in the riverine species than in the marine species. In S. guianensis, the cervical ribs were generally observed only on one side (right or left), while in S. fluviatilis they were observed on both sides. The UFSC specimen 1117 exhibited this structure on the on the right side of the third cervical vertebra (Ce3). The cervical ribs were observed in all age classes.
The presence of fenestrae commonly observed in the occipital region, near the condyles of Sotalia fluviatilis was also recorded in juveniles and adults of Pontoporia blainvillei (Gervais and d'Orbigny, 1844) (Pinedo,1991). However, in Pontoporia, the number of fenestrae was greater than that observed for the genus Sotalia.
In the genus Sotalia, the occurrence of fenestrae next to protuberances of the occipital region was low compared with the region next to the condyles. In the basioccipital, the presence of fenestrae was less frequent in immature specimens than in mature specimens. In Pontoporia blainvillei and in Stenella the occurrence of this structure was rare and occasional, respectively (Pinedo, 1991; Perrin et al., 1982).
Considering that the fenestrae observed next to the condyles in S. fluviatilis do not have the same origin as the fontanelles present in newborn individuals, given their different location, the origin and function of these fenestrae are not clear.
The cranial floor is not generally included in morphologic studies and revealed interesting results. Simões-Lopes (2006), analysing specimens of S. guianensis in the southern region, observed that the laminar posterior process of the vomer is narrower than the lamelar pterygoid processes.
In the same study, the author found the converse in S. fluviatilis - where the vomer is broader and the lamelar pterygoid processes narrower. In the present work we verified that this characteristic described by Simões-Lopes (2006) was present in the majority of the marine specimens analysed. However, in the samples from CE and AP, we observed that some specimens also possessed a broad vomer, similar to the riverine species, herein called the "wide form". Apparently, this characteristic can be used to distinguish the two species, but it is important to note that a small percentage of marine specimens possess a broad vomer just as some riverine specimens possess a narrow vomer. Thus, the use of this characteristic should be considered in combination to others when separating the species.
In relation to the posterior alignment of the vomer, the arrangement anterior to the lamelars pterygoid processes was observed in 100% of the cases for the marine species. In the riverine species (n = 39), 18% were observed with the posterior edge of the vomer aligned with the pterygoids, while in another 18% the posterior edge of the vomer extended beyond the pterygoides. Dawbin et al. (1970) attribute the alignment of the vomer in relation to the posterior pterygoid processes to age. For these authors, in sub-adults Peponocephala electra (Gray, 1846) the vomer extends to the level of the sutures between the pterygoides and basioccipital and in adults, the vomer extends beyond the adjacent pterygoides. This pattern was not observed in the current study, since all adults of S. guianensis were observed with the posterior suture of the vomer anterior to the pterygoides. Moreover, the variation observed in the riverine species does not appear to be related to ontogenetic development, but rather to individual variation.
The pterygoides were found to be medially separated by a projection in the tip of the palatines in all specimens, corroborating the data found in the literature (van Bénéden, 1875; Flower, 1885; Miranda-Ribeiro, 1936, da Silva and Best, 1994; 1996; Avila et al., 2002; Simões-Lopes, 2006). The variation in the form of the posterior projection of the left pterygoide was very subtle between the two species.
4.4. Anterior lacerate foramen
The anterior lacerate foramen is formed by two foramens (the optic foramen and the orbitorotundum foramen). These two structures are divided by a "wall" (Yamagiwa, et al., 1999) here called a spike-shaped projection. In S. guianensis, the majority of the adult specimens (between 72 and 98%) exhibited this projection between the optic and the orbitorotundum foramen. On the other hand, the narrow form was only observed in adults, suggesting that with time, this spiny projection fused with one side of the lacerated foramen (as observed in some marine specimens). In S. fluviatilis, in contrast, the absence of this projection was more common (88%), and could indicate neoteny.
4.5. Hypoglossal foramen
The location of the hypoglossal foramen varied between the two species. In S. guianensis the hypoglossal foramen generally meets between the crest of the basioccipital and the paraoccipital process, more precisely in the jugular notch as it occurs in other species of marine Delphinidae, for example, Grampus griseus (Cuvier, 1812) (Yamagiwa et al., 1999) and Tursiops truncatus (Montagu, 1821) (Rommel, 1990). In contrast, the hypoglossal foramen in S. fluviatilis, in the majority of cases, was found to be internally displaced.
4.6. Cervical vertebrae
The occurrence of cervical ribs was higher (87%) in the riverine species. The presence of pleuroapophiseal plates was initially suggested for some groups of mammals, especially monotremes and marsupials (Lessertisseur and Saban, 1967). Unilateral or bilateral processes in the cervical vertebrae are common in some groups of mammals (including Homo), and the costal rudiments are associated with the vertebral or vertebrarterial foramens (Lessertisseur and Saban, 1967). These authors also mention their presence in marine mammals as Orcinus orca (Linnaeus, 1758), Tursiops truncatus and Balaenoptera sp. Such processes have been considered serially homologous to the cervical ribs, also appearing in the literature as costal plates or pleuroapophiseal plates (Flower and Lyddeker, 1891). The presence of cervical ribs is an intriguing characteristic in comparative anatomy, as these structures are typically observed in reptiles (Paula Couto, 1979; Ferigolo, 1987). In S. guianensis, these structures have already been reported in about 22.5% (n = 31) of the specimens of the southern region (Fettuccia and Simões-Lopes, 2004). In this work, considering a larger sample, the observed frequency of cervical ribs in the specimens from SC was 19% (n = 33), and of 9% (n = 23) from CE. Cervical ribs in mammals are examples of atavism (reappearance of an extinct character, common to ancestral lineages that rarely occur in current populations). Other cases of atavism are described in the literature as the occurrence of vestigial posterior members in whales and extra-numeric teeth in bats and sea lions (Bejder and Hall, 2002; Rui and Drehmer, 2004 and Drehmer et al., 2004). According to Hall (1984), there are four basic criteria for the recognition of an atavism: 1) persistence of the characteristic in adult life; 2) absence of this characteristic in the parents or recent ancestors; 3) occurrence in one or a few individuals within a population; and 4) similarity or identity with the same character exhibited by all the members of the ancestral population. Moreover, if the incidence of the character is still relatively high in a population, it is considered a polymorphism (Hall, 1984). Thus, considering the high occurrence of cervical ribs in specimens of the riverine species, these structures do not appear to be atavistic, suggesting the need for a more detailed study with a more representative sample number for a more consistent conclusion.
Acknowledgements - The authors would like to thank Angel Enzo Crovetto, César Drehmer, Fernando Rosas, Lena Geise and two anonymous referees for their helpful suggestions on improving the manuscript. We would also like to thank Nina Best for the English review. Special thanks to the Museu Paraense Emilio Goeldi (MPEG) and Associação de Pesquisa e Preservação de Ecossistemas Aquáticos (AQUASIS). This work was supported by grants from CNPq and IEB.
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Received December 7, 2007
Accepted August 27, 2008
Distributed August 31, 2009 (With 12 figures)