Heterodontus is a small genus of sharks within the family Heterodontidae of the order Heterodontiformes.
First discovered in Australia in the 18th century in the form of the Port Jackson shark, it now includes 10 scientifically recognized species.
All Heterodontus species are relatively small, with the largest species reaching 1.65 m (5.5 feet) in length.
They are commonly known as bullhead sharks or horn sharks.
They first appeared in the fossil record in the Early Jurassic epoch, well before any of other Galeomorphii, a group that includes all modern sharks except the dogfish and its relatives.
They occur in warm-temperate and tropical continental waters of the western Indian Ocean, western and eastern Pacific, but are absent from the Atlantic and from oceanic insular waters.
These are sluggish, rare to uncommon night-active sharks, slowly swimming and crawling on rocky, kelp-covered and sandy bottom. Some species at least favor rocky crevices and caves, where they spend the day resting.
These sharks are oviparous, producing eggs in unique, large, spiral-flanged egg cases. At least two species lay eggs in specific ‘nesting’ sites. Eggs may take over five months to hatch, and young hatch at a large size, over 14 cm (5.5 inches).
They primarily feed on benthic invertebrates. Sea urchins are a favorite food, but crabs, shrimp and other crustaceans, abalone, top shells and other marine gastropods, oysters, polychaetes, sipunculid worms and more rarely small fish are also eaten.
Some Heterodontus sharks are encountered by divers, who have commonly harassed them.
Although regarded as harmless, these sharks can and do snap when provoked and occasionally pursue and bite their tormentors.
“The Heterodontiformes is a unique shark order comprising a single family and single extant genus,” said lead author Dr. William White from CSIRO Australian National Fish Collection and colleagues.
“They are also one of the earliest identified modern elasmobranch lineages with distinctly recognizable isolated teeth recorded from the Early Jurassic, approximately 175 million years ago.”
“These horn or bullhead sharks are characterized by their large, blunt heads with prominent crests above the orbits and a small mouth.”
“They also have a large spine in front of both dorsal fins and an anal fin.”
“Australia is home to two endemic Heterodontus species, i.e., Heterodontus galeatus and Heterodontus portusjacksoni, which occur in temperate waters,” they added.
“A third species, Heterodontus zebra, is found in the Western Central Pacific from tropical northern Australia extending further north to Japan.”
The newly-identified species, named Heterodontus marshallae, was previously considered to be conspecific with Heterodontus zebra from the Western Pacific.
Despite the coloration being similar between the two species, i.e., pale background with 22 dark brown bands and saddles, they differ consistently in two key aspects.
“Firstly, the snout of Heterodontus marshallae has a dark semicircular bar, usually bifurcated for most of its length vs. a pointed, triangular shaped dark marking in Heterodontus zebra,” the scientists said.
“Secondly, Heterodontus zebra has a dark bar originating below the posterior gill slits and extending onto anterior pectoral fin, which is absent in Heterodontus marshallae.”
Heterodontus marshallae is up to 60 cm (2 feet) in length, has a moderately robust body and a slightly humped nape.
The species occurs in northern Australia from west of Exmouth Peninsula east to at least off Bathurst Island in the Northern Territory.
“Heterodontus zebra is a primarily shallow water species occurring from close inshore to depths of at least 143 m (469 feet),” the authors said.
“In contrast, Heterodontus marshallae has only been recorded between 125 and 229 m (410-751 feet) off northwestern Australia.”
The discovery is described in a paper published in the journal Diversity.
Citation:
William T. White et al. 2023. Species in Disguise: A New Species of Hornshark from Northern Australia (Heterodontiformes: Heterodontidae). Diversity 15 (7): 849; doi: 10.3390/d15070849
This article by Natali Anderson was first published by Sci-News on 31 July 2023. Lead Image: Heterodontus marshallae. Image credit: Sci.News / White et al., doi: 10.3390/d15070849.
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