| Smalleye hammerhead | |
|---|---|
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Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Carcharhiniformes |
| Family: | Sphyrnidae |
| Genus: | Sphyrna |
| Species: | S. tudes |
| Binomial name | |
| Sphyrna tudes (Valenciennes, 1822) | |
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| Range of the smalleye hammerhead[3] | |
| Synonyms | |
Sphyrna bigelowiS. Springer, 1944 | |
Thesmalleye hammerhead (Sphyrna tudes), also called thegolden hammerhead orcurry shark, is a smallspecies ofhammerhead shark in thefamilySphyrnidae. This species was historically common in the shallow coastal waters of the westernAtlantic Ocean, fromVenezuela toUruguay. It favors muddy habitats with poor visibility, reflected by its relatively small eyes. Adult males and juveniles areschooling and generally found apart from the solitary adult females. Typically reaching 1.2–1.3 m (3.9–4.3 ft) in length, this shark has a unique, bright golden color on its head, sides, and fins, which was only scientifically documented in the 1980s. As in all hammerheads, its head is flattened and laterally expanded into a hammer-shaped structure called the cephalofoil, which in this species is wide and long with an arched front margin bearing central and lateral indentations.
The yellow-orangepigments of the smalleye hammerhead seem to have been acquired from thepenaeid shrimpXiphopenaeus kroyeri, the main food of juvenile sharks, and fromsea catfish and theireggs, the main food of adults. The golden color may serve to conceal it frompredators such as larger sharks. This species isviviparous, with the developingembryos sustained by aplacental connection formed from the depletedyolk sac. Females bear litters of five to 19 pups every year following agestation period of 10 months. Reproductive seasonality, litter size, and size atmaturity vary between geographical regions. Because of its abundance, the smalleye hammerhead is an economically importantbycatch ofartisanalgillnetfisheries throughout its range, and is used as food. In recent years,overfishing has caused marked declines in its numbers offTrinidad, northernBrazil, and probably elsewhere. Coupled with the smalleye hammerhead's low reproductive rate, this led theInternational Union for Conservation of Nature to list the smalleye hammerhead ascritically endangered in 2020.
Despite being one of the most easily recognizable sharks, the smalleye hammerhead has had a long history of taxonomic confusion that still remains to be fully resolved.[4] Its scientific name originated in 1822, with FrenchzoologistAchille Valenciennes' description ofZygaena tudes in thescientific journalMémoires du Muséum National d'Histoire Naturelle; thespecific epithettudes isLatin for "hammer". Valenciennes made reference to three specimens, one fromNice inFrance, one fromCayenne inFrench Guiana, and one from theCoromandel Coast ofIndia.[5] For over two centuries, though, taxonomists believed Valenciennes' account matched thegreat hammerhead, which thus became known asZygaena (laterSphyrna )tudes.[3] The smalleye hammerhead was known by a different name,Sphyrna bigelowi, coined byStewart Springer in a 1944 issue ofJournal of the Washington Academy of Sciences.[6]
In 1950, Enrico Tortonese examined the Nice and Cayenne specimens ofS. tudes (the Coromandel specimen having been lost in the interim), and concluded that they were not great hammerheads, but rather the same species asS. bigelowi.[7] Carter Gilbert concurred in his 1967 revision of the hammerhead sharks, noting that while the lost Coromandel specimen was probably a great hammerhead, none of the existing material belonged to that species. Thus,Sphyrna tudes became the accepted name for the smalleye hammerhead, taking precedence overS. bigelowi because it was published earlier, and the great hammerhead received the next available nameSphyrna mokarran. Gilbert designated the Nice specimen as thelectotype that would defineS. tudes, having priority over the Cayenne specimen (the paralectotype). This was meant to stabilize the name, but had the opposite effect.[3][8]
In 1981, Jean Cadenat and Jacques Blache revisited thetype specimens ofS. tudes and found that the lectotype from Nice is likely not a smalleye hammerhead, but rather afetalwhitefin hammerhead (S. couardi, likely asynonym of thescalloped hammerhead,S. lewini).[3][9] This would also explain the anomalous locality of the Nice specimen, as the smalleye hammerhead is not otherwise known outside of the Americas. By the rules ofbinomial nomenclature,Sphyra tudes should then become the valid name for the whitefin hammerhead, taking precedence overS. couardi, and the smalleye hammerhead would revert to beingSphyrna bigelowi. Taxonomists, though, have been reluctant to change the names again, preferring to keep the smalleye hammerhead asS. tudes.[3] For this solution to have official status would require a decision by theInternational Commission on Zoological Nomenclature (ICZN), to reject the Nice specimen as the lectotype and designate the Cayenne specimen in its place. The relevant petition to the ICZN has not yet been put forth.[1]
| Phylogenetic tree of hammerhead sharks:[10] |
Until the first detailed study of the smalleye hammerhead was carried out in 1985–86 by José Castro ofClemson University for theFood and Agriculture Organization of the United Nations, its distinctive golden coloration was unknown to science. The color fades after death, and thepigments leach into thepreservative, resulting in the "yellowish cast" of museum specimens being regarded as an artifact of preservation. The names "yellow hammerhead" or "golden hammerhead" are used by fishermen in Trinidad for this shark, and the latter was promoted for wider usage by Castro.[4][11] Anothercommon name for this species is the curry shark.[1]Phylogenetic analyses based onnuclear andmitochondrial DNA have found that the hammerheads with the smallest cephalofoils are the mostderived members of their lineage. The closest relative of the smalleye hammerhead appears to be thescoophead (S. media), and the two of them in turn form aclade with thesister species pair of thescalloped bonnethead (S. corona) and thebonnethead (S. tiburo).[10]
One of the smaller members of its family, the smalleye hammerhead can reach a length of 1.5 m (4.9 ft),[12] though 1.2–1.3 m (3.9–4.3 ft) is more typical,[13] and a weight of 9 kg (20 lb).[11] The body is streamlined and fairly slender. The mallet-shaped cephalofoil is wide and long, with a span measuring 28–32% of the body length; the leading margin forms a broad arch with indentations in the middle and on either side.[14] The cephalofoils of newborns are longer, more arched, and less indented in front than those of adults.[4] The eyes, placed at the ends of the cephalofoils, are proportionately smaller than in other hammerheads and equipped withnictitating membranes (protective third eyelids).[3][4] The nostrils are positioned just inside of the eyes, each with a well-developed groove running towards the center of the cephalofoil. The mouth is strongly curved, containing on either side 15–16 upper tooth rows and 15–17 lower tooth rows. The teeth have single narrow cusps with smooth or weakly serrated edges, that are angled in the upper jaw and upright in the lower jaw.[3][14]
The firstdorsal fin is tall and slightly falcate (sickle-shaped), originating behind thepectoral fin bases; its free rear tip lies over the origin of thepelvic fins. The second dorsal fin is smaller than the first, but still rather large, with a concave, trailing margin. The pelvic fins have nearly straight trailing margins. Theanal fin is taller and longer than the second dorsal fin. Thecaudal fin has a well-developed lower lobe and a notch near the tip of the upper lobe.[3] Thedermal denticles are oval with five horizontal ridges leading to marginal teeth.[15] The most distinctive trait of this species is its coloration; the back and dorsal fins are gray to yellowish gray, and the cephalofoil margins, flanks, underside, pectoral fins, pelvic fins, and anal fin are bright yellow to orange with a metallic or iridescent sheen. Newborn sharks are gray above, darkening on the first dorsal fin and upper caudal fin lobe, and whitish below. They gain a bright yellow cast on their undersides by a length of 45 cm (18 in), which turns to orange by a length of 50 cm (20 in). The golden color is brightest in sharks 55–70 cm (22–28 in) long, and tends to fade with the onset ofsexual maturity.[4]
The smalleye hammerhead is found along the eastern coast of South America from Uruguay to Venezuela, though it seldom occurs further west than theOrinoco Delta southeast ofTrinidad.[4] Reports of this species from offPanama,Mexico, and westernFlorida are unconfirmed; records from other parts of the world are most likely erroneous, resulting from its tangled taxonomic history.[1] This species inhabitsinshore murky waters 5–40 m (16–131 ft) deep, over muddy bottoms. Segregation occurs by sex and age; newborns and juveniles under 40 cm (16 in) long are found in the shallowest waters, moving deeper after a few months of life. Adult females are mostly found at depths of 9–18 m (30–59 ft), while larger juveniles and adult males are mostly found at depths of 27–36 m (89–118 ft).[4] This species is tolerant ofbrackish water, and can be found over asalinity range of 20–34 ppt.[16]
Four other species of hammerhead sharks overlap in range with the smalleye hammerhead - the small-sized scoophead and bonnethead, and the large-sized scalloped hammerhead and great hammerhead. Little competition occurs between these species because of their differing habitats and dietary preferences. The smalleye hammerhead is the dominant hammerhead in shallow, muddy areas, where highturbidity limits the utility of vision (hence its smaller eyes). Adult males and juveniles of both sexes form schools of uniform body size; these schools do not appear to relate to reproduction ormigration. Adult females are apparently solitary.[4][17]
Young smalleye hammerheads under 67 cm (26 in) long feed predominantly onpenaeid shrimp, mostlyXiphopenaeus kroyeri. Larger sharks feed mainly onbony fishes, especiallyariid sea catfish and theireggs. The shrimp and the surfacemucus layer and eggs of the catfish containcarotenoid pigments that appear to be the source of the sharks' golden color; whether the pigments in the catfish also ultimately come from the shrimp is uncertain. Another shark species in the region, theyellow smooth-hound (Mustelus higmani), also feeds on shrimp and has a yellowish color, albeit not nearly as bright.[4] This species has also been known to consumeswimming crabs,squid,grunts, and newborn scalloped hammerheads.[3] The smalleye hammerhead may fall prey to larger sharks such as thebull shark (Carcharhinus leucas), while smaller individuals may also be taken by bony fishes.[15] Its coloration may providecamouflage.[18] A knownparasite of this species is thehexabothriidmonogeneanErpocotyle schmitti;[19] it may also serve as a host to commoncopepodectoparasites such asEchthrogaleus coleoptratus,Pandarus satyrus, andP. cranchii.[15]
Like all hammerhead sharks, the smalleye hammerhead isviviparous: when the developingembryos exhaust their supply ofyolk, the depletedyolk sac develops into aplacental connection through which the mother delivers nourishment. Mature females have a single functionalovary and two functionaluteri.Ovulation occurs at the same time asgestation, allowing females to bear young every year.[4] The details of the smalleye hammerhead's life history vary across its range.[1] Off Trinidad, reproduction occurs on a well-defined annual cycle with mating in August and September, and birthing in late May and June of the following year. The females carry between five and 12 pups for 10 months, and make use of food-rich, shallow coastalbays as nursery areas. The newborns measure around 30 cm (0.98 ft) long, and males and females attain sexual maturity at 80 cm (31 in) and 98 cm (39 in) long, respectively.[4] By contrast, smalleye hammerheads off the northernBrazilian state ofMaranhão are substantially larger, with males maturing at over 92 cm (36 in) long and females at over 101 cm (40 in) long. As the litter size increases with female size, Maranhão sharks have been recorded carrying up to 19 pups. The seasonality of breeding also differs, with pregnant females found from June to October and January to April, and males in apparent reproductive condition from May to November and in March.[16]
Timid and harmless to humans,[12][13] the smalleye hammerhead iscaught incidentally by inshoreartisanal multispecies fisheries throughout its range, and marketed as food. It is the most or second-most important shark caught by such fisheries off Trinidad and Tobago, Guyana, and Brazil. Because of its head shape, individuals of all ages are readily caught ingillnets; small numbers are also caught on line gear and inbottom trawls.[1][4][17] The IUCN assessed this species ascritically endangered in 2020, as it is subjected to intense fishing pressure, often without management, by techniques ranging from beachseine netting totrawling, and is unable to replenish this harvest. Smalleye hammerheads are alsobycatch in commercial shrimp trawls, which are often run through the shark's shallow-water habitat. Anecdotal evidence suggests that smalleye hammerhead catches have declined significantly off Trinidad and northern Brazil, which are likely indicative of population trends in the rest of its range. The smalleye shark is federally protected in Brazil and Colombia.[1]
