A dorsal fold in Gymnura micrura (Bloch and Scheneider, 1801) (Chondrichthyes: Gymnuridae)

ENVIRONMENTAL SCIENCES

A dorsal fold in Gymnura micrura (Bloch and Scheneider, 1801) (Chondrichthyes: Gymnuridae)

Jorge Luiz Silva Nunes^I,*; Nivaldo Magalhães Piorski^II

^ICentro de Ciências Agrárias e Ambientais; Universidade Federal do Maranhão; BR 222; km 4, s/n; 65500-000; Chapadinha - MA - Brasil

^IIDepartamento de Oceanografia e Limnologi; Universidade Federal do Maranhão; Av. dos Portugueses s/n; Campus Universitário do Bacanga; 65080-040; São Luís - MA - Brasil

ABSTRACT

This paper reports a dorsal fold which is a membranous structure located on the tail of two juvenile butterfly rays, Gymnura micrura (Bloch & Scheneider, 1801), caught through artisanal fishery in the shallow waters of Maranhão State (Brazil).

Key Words: Elasmobranchii, rays, teratogeny, South America, Brazil

RESUMO

Neste manuscrito registra-se uma nadadeira dorsal em dois espécimes juvenis de Gymnura micrura (Bloch and Scheneider, 1801) capturadas pela pesca artesanal em águas rasas do estado do Maranhão (Brasil).

INTRODUCTION

The family Gymnuridae belongs to the order Rajiformes and possess two genera and fourteen species, most of them distributed in the coastal zone (Figueiredo, 1977; Compagno, 1984). In Brazil, the family is represented by two species of the genus Gymnura: G. altavela (Linnaeus, 1758) and G. micrura (Bloch and Scheneider, 1801). Both species are widely distributed throughout the coastal areas of the Atlantic Ocean with some records from the Indian Ocean. However, in this part of the world, the records of G. micrura probably refer to other species (Eschmeyer, 2004). In South America, the species seems to have a disjunct distribution, with G. micrura occurring in the north and G. altavela in the south (Figueiredo, 1977; Compagno, 1984, Humann and Deloach, 2002; Léopold, 2004; Froese and Pauly, 2007), although Menni and Stehmann (2000) and Eschmeyer (2004) have cited G. micrura from southern Brazil.

These two Gymnura species are coastal and benthic animals of shallow waters inhabiting the sandy and muddy bottoms in estuaries and mouths of rivers. G. micrura presents a variable color pattern, with a gray, light to olive green, pink or dark brown dorsal region, with or without dispersed spots, and a white tending to yellow ventral surface. Morphologically, the species is characterized for presenting a disc width larger than the total length, reduced disc length and pelvic fins with straight margins, which in profile resembles a quadrilateral. The back, as well as the tail, is deprived of fins (Bigelow and Schroeder, 1953; Compagno, 1984).

This paper reports the occurrence of a dorsal fold (congenital disorder) located on the tail of two specimens of G. micrura caught in the Maranhão State (Brazil).

MATERIAL AND METHODS

Between October 1997 and February 1999, a total of 18 G. micrura rays were caught along the coast of Maranhão. During this period, the specimens were obtained in cruises of the LABOHIDRO I boat and bought from the local fisheries. The catches were performed in cruises between the depths of 8 to 25 meters using nets of 500 to 1000 fathoms, 4 fathoms long and a mesh of 8 centimeters (Stride, 1992; Nunes et al, 2005). Morphometric data were obtained using a steel pachymeter, according to Last (2004). The tails were X-rayed using the Funk 1070 odontological X-ray equipment (10mA, 60kV). The specimens were deposited in the Fish Collection of the Oceanography and Limnology Department of the Universidade Federal do Maranhão (Maranhão Federal University), under the numbers CPDOL 97610 and CPDOL 99611.

RESULTS AND DISCUSSION

From the eighteen G. micrura studied specimens, only two of them presented a dorsal fold. The anatomic formation observed corresponded to a small membranous expansion located in the middle point of the tail (Fig.1-2). This structure has nothing to do with the keel reported by Bigelow and Schroeder (1953), which is located on the upper surface of tail. The analysis of the X-ray images suggested an adipose structure deprived of sustentation rays. This condition was similar to the additional structure observed by Ben Brahim and Capapé (1997), which reported a case of a supplementary dorsal fin in the ray Torpedo torpedo (Linnaeus, 1758), caught in the Tunisian Mediterranean Sea.

Old illustrations and descriptions of elasmobranchii show great morphological variations and treat as "monstrosities" the specimens that present teratogenic anomalies. (Bureau, 1890 apud Ben Brahim and Capapé, 1997). In many ray species, congenital disorders were responsible for the creation of new taxa, which were subsequently rejected after refined studies concerning their embriology (Müller and Henle, 1841 apud Rosa et al., 1996). Reporting a teratogenic structure in the fins of Potamotrygon motoro Müller and Henle, 1841, Rosa et al. (1996) observed that this anomaly did not compromise the biological activities, given that the nutritional conditions were considered normal. For the taxonomical identification of the studied specimens, morphometrics (Table 1) and morphological features presented by Bigelow and Schroeder (1953), Figuereido (1977) and Compagno (1984) indicated that all the specimens belonged to the species G. micrura, including the two individuals with the dorsal membranous structure.

The fold observed in G. micrura from Maranhão was probably related to some teratogenic effect or pedomorphosis process, deserving further studies in order to explain these questions.

Received: April 18, 2006;

Revised: May 24, 2007;

Accepted: July 08, 2008.

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Publication Dates

• Publication in this collection
  06 May 2009
• Date of issue
  Apr 2009

History

• Received
  18 Apr 2006
• Reviewed
  24 May 2007
• Accepted
  08 July 2008