The smalltail shark (Carcharhinus porosus) is a species of requiem shark, and part of the family Carcharhinidae. It is found in the western Atlantic Ocean, from the northern Gulf of Mexico to southern Brazil. It inhabits shallow waters close to shore, particularly over muddy bottoms around estuaries. It tends to swim low in the water column and forms large aggregations segregated by sex. A slim species generally not exceeding in length, the smalltail shark has a rather long, pointed snout, a broad, triangular first dorsal fin, and a second dorsal fin that originates over the midpoint of the anal fin base. It is plain gray in color, without prominent markings on its fins.
The diet of the smalltail shark consists mainly of bony fishes such as croakers, while crustaceans, cephalopods, and smaller sharks and rays may also be consumed. It is viviparous, meaning the developing embryos are sustained by a placental connection. Females bear litters of two to 9 young on a biennial cycle, following a roughly 12-month gestation period. The smalltail shark is often caught as bycatch and may be used for meat, fins, liver oil, cartilage, and fishmeal. It seems to have declined significantly since the 1980s. Therefore, the International Union for Conservation of Nature has listed it as critically endangered.
Taxonomy and phylogeny
Italian naturalist Camillo Ranzani published the first scientific description of the smalltail shark in an 1839 volume of Novi Commentarii Academiae Scientiarum Instituti Bononiensis. He named the new shark Carcharias porosus from the Greek porus ("pore"), referring to the prominent pores behind its eyes. The type specimen, a -long male from Brazil, has since been lost. This species was moved to the genus Carcharhinus by later authors. Its Trinidadian name is puppy shark.
The evolutionary relationships of the smalltail shark are uncertain. Based on morphology, Jack Garrick in 1982 and Leonard Compagno in 1988 tentatively placed it in a group defined by the whitecheek shark (C. dussumieri) and the blackspot shark (C. sealei). This grouping was equivocally supported by Gavin Naylor's 1992 allozyme-based phylogenetic analysis. Alternately, a 2011 phylogenetic study by Ximena Vélez-Zuazoa and Ingi Agnarsson, based on nuclear and mitochondrial genes, found close relationships between the smalltail shark, the daggernose shark (Isogomphodon oxyrhynchus), the blacknose shark (C. acronotus), and the finetooth shark (C. isodon). The Pacific smalltail shark (C. cerdale) was once mistakenly synonymized with C. porosus, until 2011 when José Castro resurrected it as a distinct taxon. An undescribed species closely similar to C. porosus is known from Southeast Asia.
Description
The smalltail shark is a slender-bodied species with a fairly long, pointed snout. The leading margin of each nostril is enlarged into a narrow, pointed lobe. The large, circular eyes are equipped with nictitating membranes, and behind them is a series of prominent pores. The mouth bears short furrows at the corners and contains 13–15 tooth rows on either side of both jaws (usually 14 upper and 13 lower). The upper teeth are tall and triangular with strong serrations, becoming increasing oblique towards the sides. The lower teeth are comparatively narrower and more upright, with finer serrations. The five pairs of gill slits are short.
The small pectoral fins are falcate (sickle-shaped) with relatively pointed tips. The first dorsal fin is broad, forming nearly an equilateral triangle in adults, with a blunt apex; it originates over the pectoral fin rear tips. The second dorsal fin is small and originates over the midpoint of the anal fin base. No ridge exists between the dorsal fins. The pelvic fins are small with pointed to narrowly rounded tips, and the anal fin has a deep notch in its trailing margin. The asymmetrical caudal fin has a strong lower lobe and a longer upper lobe with a ventral notch near the tip. The dermal denticles mostly do not overlap; each has three to five horizontal ridges leading to posterior teeth, with the central one the longest. This shark is plain gray to slate above and whitish below, with a faint lighter stripe on the flanks. The pectoral, dorsal, and caudal fins may darken toward the tips. The smalltail shark reaches a maximum known length of , though is typical. Females grow larger than males.