Dicephalous v. diprosopus sharks: record of a two‐headed embryo of Galeus atlanticus and review of the literature

Abstract

As far as is known, this paper gives the first description of a two-headed shark embryo belonging to an oviparous species, Galeus atlanticus (Carcharhiniformes: Scyliorhinidae). The specimen was detected among 797 embryos intended for cardiovascular studies, which represents a defect incidence of 0·13%. Each head had a mouth, two eyes, a brain, a notochord and five gill openings on each side. The two heads fused behind the gills. On the single body, there were four anticipated dorsal fins, two anterior, right and left and two posterior, right and left. Ventrally, the specimen possessed two pairs of pectoral fins, a pair of pelvic fins and one anal fin. Two adjacent notochords, two neural tubes and two dorsal aortas ran along the body, which bent 180° at its posterior portion. There were two hearts, two oesophaguses, two stomachs, two livers, but a single intestine with a spiral valve. Previous reports of conjoined twins in sharks are scarce and only refer to oviparous and ovoviviparous species. Seven dicephalous sharks reported so far were similar to the specimen described here, namely, with two totally separated heads on one body. Instead, only one case of diprosopus shark has been reported; it had a single body and a single head with partial duplication of the face. Two further cases described in the literature as dicephalous or simply as abnormal sharks should be better regarded as diprosopus, while another three cases, also considered dicephalous, showed a mixture of characteristics of diprosopia and dicephalia.

Abnormalities in two shark species, the blue shark, Prionace glauca, and the school shark, Galeorhinus galeus (Elasmobranchii: Carcharhiniformes), from the Canary Islands, eastern tropical Atlantic

“…Many of the morphological abnormalities have mainly been recorded in the embryonic phase. In sharks, the bicephalism and diprosopia anomalies have been documented in placental viviparous (Goto et al 1981, Muñoz-Osorio et al 2013, Dos Santos and Gadig 2014 and aplacental viviparous (Lozano- Cabo 1945, Delpiani et al 2011, Sans-Coma et al 2016, Capapé and Ali 2017. Although these congenital morphological abnormalities are poorly understood, they can be due to several causes, either natural or anthropogenic (Heupel et al 1999, Moore 2015.…”

“…Bicephalism and diprosopia abnormalities have mainly been documented in sharks (Table 1), but they are commonly confused (Sans-Coma et al 2016). While the former anomaly is related to two-headed conjoined twins, with two totally separated heads on one body or trunk (Bondeson 2001), the latter anomaly refers to a single trunk and a single head where some craniofacial parts are duplicated (Hähnel et al 2003, Biasibetti et al 2011.…”

The first record of diprosopus tetrophthalmus in the South Atlantic Ocean: The case of Prionace glauca (Elasmobranchii: Carcharhiniformes: Carcharhinidae) in Brazil

“…Ehemann et al (2016) reported an additional case of bicephalia for the species. Hevia-Hormazábalet al (2011) reported the first case of diprosopus tetrophthalmus in sharks, in a specimen of P. glauca caught off Chile, although according to the review by Sans-Coma et al (2017) this would be only the third case. Okazaki et al (1987) defined diprosopus as consisting of a single neck and body and duplication of craniofacial structures; the term tetrophthalmos is applied to an embryo with four eyes, in which the median eyes may be partially fused or separate, but share a central orbit.…”

“…This study presents the first confirmed case of diprosopus tetrophthalmus found in the Atlantic Ocean (the other case was found in the Pacific Ocean off Chile). Sans-Coma et al (2017) suggested that two cases reported as bicephalous (Bosinceano 1934, Mancini et al 2006) would be better considered as diprosopus. Muñoz-Osorio et al (2013) described an anomalous specimen with a duplication of facial elements, which suggests a diprosopia; and the presence of two vertebral columns, which suggests bicephalia.…”

“…Sans-Coma et al (2017) reviewed 14 published cases of bicephalus and diprosopus in elasmobranchs. For the blue shark, Prionace glauca (Linnaeus, 1758), cases reported as bicephalous were described by Goto et al (1981), Mancini et al (2006), and Galván-Magaña et al (2011).…”