An overview on the role of Hexanchiformes in marine ecosystems: biology, ecology and conservation status of a primitive order of modern sharks

Abstract

The large size, high trophic level and wide distribution of Hexanchiformes (cow and frilled sharks) should position this order as important apex predators in coastal and deep‐water ecosystems. This review synthesizes available information on Hexanchiformes, including information not yet published, with the purpose of evaluating their conservation status and assessing their ecological roles in the dynamics of marine ecosystems. Comprising six species, this group has a wide global distribution, with members occurring from shallow coastal areas to depths of c. 2500 m. The limited information available on their reproductive biology suggests that they could be vulnerable to overexploitation (e.g. small litter sizes for most species and suspected long gestation periods). Most of the fishing pressure exerted on Hexanchiformes is in the form of commercial by‐catch or recreational fishing. Comprehensive stock and impact assessments are unavailable for most species in most regions due to limited information on life history and catch and abundance time series. When hexanchiform species have been commercially harvested, however, they have been unable to sustain targeted fisheries for long periods. The potentially high vulnerability to intense fishing pressure warrants a conservative exploitation of this order until thorough quantitative assessments are conducted. At least some species have been shown to be significant apex predators in the systems they inhabit. Should Hexanchiformes be removed from coastal and deep‐water systems, the lack of sympatric shark species that share the same resources suggests no other species would be capable of fulfilling their apex predator role in the short term. This has potential ecosystem consequences such as meso‐predator release or trophic cascades. This review proposes some hypotheses on the ecology of Hexanchiformes and their role in ecosystem dynamics, highlighting the areas where critical information is required to stimulate research directions.

Redescription of Hexanchus nakamurai Teng 1962, (Chondrichthyes: Hexanchiformes: Hexanchidae), with designation of a neotype

“…vitulus) did not reveal any distinctive differences between those from the western North Atlantic and the western North Pacific (Ebert, 1990). Taniuchi & Tachikawa (1991) followed in synonymising the two species with H. nakamurai being the senior synonym and most subsequent authors considering the two species to be synonymous (Last & Stevens, 1994, 2009Compagno et al, 2005;Barnett et al, 2012;Ebert & Stehmann, 2013;. Herman et al (1994) later resurrected H. vitulus as a valid species separable from H. nakamurai based on the following odontological differences.…”

“…Size at birth is about 40 to 43 cm TL. Barnett et al (2012) provides an overview on the biology of this little known species. Compagno (1984) commented that comparison of the original descriptions of H. griseus nakamurai and H. vitulus strongly suggested that these two species were synonymous, but he did not have any specimens with which to A B C compare these two nominal species directly.…”

New record and morphometry of the bluntnose sixgill shark Hexanchus griseus (Chondrichthyes: Hexanchidae) in Baja California Sur, Mexico

“…Such conditions coincide with the known habitat of H. griseus in its area of distribution, i.e., hypoxic environments with low temperature and high hydrostatic pressure (Compagno, 1984;Barnett et al, 2012). Although this species mainly inhabits the continental slope, juveniles have been reported to make incursions into neritic environments (over the continental shelf) during the night, when they undergo vertical migration possibly related to feeding (Compagno, 1984;Andrews et al, 2007Andrews et al, , 2009Andrews et al, , 2010Comfort, 2012).…”

“…In Mexico, information relative to H. griseus is insufficient for realize a proper assessment of the status of its population; furthermore, the limited information on its reproductive biology suggests that this taxon may be vulnerable to fishing (Barnett et al, 2012). The punctual catches recording of H. griseus taking advantage of them to produce scientific knowledge about basic aspects of its biology, will allow better understanding of the ecological niche occupied by this species in the national territory.…”

“…The shark specimen was transported on ice to the Laborato- (Ebert, 2002(Ebert, , 2003, in both coastal and deep-water ecosystems (Barnett et al, 2012). This study provides the first record of H. griseus that includes detailed morphometric data of a complete individual.…”