View source for Bonnethead

{{Short description|Species of shark}}
{{Use dmy dates|date=November 2020}}
{{speciesbox
| name = Bonnethead shark
| status = EN
| status_system = IUCN3.1
| status_ref = <ref name=iucn>{{cite iucn |author1=Pollom, R. |author2=Carlson, J. |author3=Charvet, P. |author4=Avalos, C. |author5=Bizzarro, J. |author6=Blanco-Parra, M.P.|author7=Briones Bell-Iloch, A.|author8=Burgos-Vázquez, M.I.|author9=Cardenosa, D. |author10=Cevallos, A. |author11=Derrick, D. |author12=Espinoza, E. |author13=Espinoza, M. |author14=Mejía-Falla, P.A.|author15=Morales-Saldaña, J.M.|author16=Navia, A.F. |author17=Pacoureau, N. |author18=Pérez Jiménez, J.C. |author19=Sosa-Nishizaki, O. |year=2021|amends=2020 |title=''Sphyrna tiburo'' |page=e.T39387A205765567|doi=10.2305/IUCN.UK.2021-3.RLTS.T39387A205765567.en|access-date=20 May 2023}}</ref>
| status2 = CITES_A2 
| status2_system = CITES 
| status2_ref = <ref name="CITESAppendices">{{Cite web|title=Appendices {{!}} CITES|url=https://cites.org/eng/app/appendices.php|access-date=14 January 2022|website=cites.org}}</ref>
| image = Sphyrna tiburo SI.jpg
| image2 = Sphyrna tiburo SI3.jpg
| genus = Sphyrna
| species = tiburo
| authority = ([[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]])
| range_map = Sphyrna tiburo distribution map.svg
| range_map_caption = Range of the bonnethead shark
}}

The '''bonnethead''' ('''''Sphyrna tiburo'''''), also called a '''bonnet shark''' or '''shovelhead''',<ref>{{cite web |title=Definition of BONNETHEAD |url=https://www.merriam-webster.com/dictionary/bonnethead |website=www.merriam-webster.com |publisher=Merriam-Webster, Incorporated |access-date=8 February 2021 |language=en}}</ref> is a small member of the [[hammerhead shark]] genus ''[[Sphyrna]]'', and part of the family Sphyrnidae. It is an abundant species in the [[littoral zone]] of the [[North Atlantic]] and the [[Gulf of Mexico]], is the only shark species known to display [[sexual dimorphism]] in the [[Morphology (biology)|morphology]] of the head, and is the only shark species known to be [[Omnivore|omnivorous]].

==Description==
The shark is characterized by a broad, smooth, spade-like head:  it has the smallest cephalofoil (hammerhead) of all ''Sphyrna'' species. The body is grey-brown above and lighter on the underside. Typically, bonnethead sharks are about {{convert|80|-|90|cm|ft|abbr=on}} long, with a maximum size of about {{convert|150|cm|ft|abbr=on}}.<ref>{{FishBase species|genus=Sphyrna|species=tiburo|year=2021|month=11}}</ref>
The generic name ''Sphyrna'' probably derives from a misspelling of '' sphyra '', the Greek word for "hammer"; the specific name ''tiburo'' derives from the Spanish word {{Lang|es|tiburón}}, meaning "shark".<ref>{{cite web |first=Christopher |last=Scharpf|title=Family SPHYRNIDAE Bonaparte 1840 (Hammerhead Sharks) |website=The ETYFish Project|url=https://etyfish.org/sphyrnidae/ |access-date= 1 September 2024}}</ref><!-- Note, FishBase has the epithet meaning wrong, despite citing ETYFish-->

<gallery mode="packed">
Sphyrna tiburo head.jpg|Head, underside
Sphyrna tiburo head2.jpg|Head, upper side
Sphyrna tiburo upper teeth.jpg|Upper teeth
Sphyrna tiburo upper teeth posterior.jpg|Upper teeth, posterior
Sphyrna tiburo lower teeth.jpg|Lower teeth
Sphyrna tiburo lower teeth posterior.jpg|Lower teeth, posterior

</gallery>

==Morphology==

===Sexual dimorphism===
Bonnethead sharks are the only sharks known to exhibit sexual dimorphism in the morphology of the head. Adult females have a broadly rounded head, whereas males possess a distinct bulge along the anterior margin of the cephalofoil. This bulge is formed by the elongation of the [[Anatomical terms of location#Directional terms|rostral]] cartilages of the males at the onset of sexual maturity and corresponds temporally with the elongation of the [[clasper]] cartilages.<ref>{{cite journal |pmid=16110088 |year=2005 |last1=Kajiura |first1=S. M. |title=The sexually dimorphic cephalofoil of bonnethead sharks, Sphyrna tiburo |journal=The Biological Bulletin |volume=209 |issue=1 |pages=1–5 |last2=Tyminski |first2=J. P. |last3=Forni |first3=J. B. |last4=Summers |first4=A. P. |doi=10.2307/3593136 |jstor=3593136 |s2cid=357244 }}</ref>

===Pectoral fins and swimming===
[[File:Sphyrna tiburo SI2.jpg|thumb|245px|Dorsal view showing the pectoral fins]] 
The [[pectoral fins]] on most fish control pitching (up-and-down motion of the body), yawing (the side-to-side motion), and rolling. Most hammerhead sharks do not yaw or roll and achieve pitch using their cephalofoils. The smaller cephalofoil of a bonnethead shark is not as successful, so they have to rely on the combination of cephalofoils and their large pectoral fins for most of their motility. Compared to other hammerheads, bonnethead sharks have larger and more developed pectoral fins and are the only species of hammerhead to actively use pectoral fins for swimming.{{citation needed|date=August 2016}}

==Evolution==
Using data from [[Mitochondrial DNA|mtDNA]] analysis, a scientist has found that the evolution of hammerhead sharks probably began with a taxon that had a highly pronounced cephalofoil (most likely that similar to the [[winghead shark]], ''Eusphyra blochii''), and was later modified through selective pressures. Thus, judging by their smaller cephalofoil, bonnethead sharks are the more recent developments of a 25-million-year evolutionary process.<ref>{{Cite web|title=Hammerhead shark study shows cascade of evolution affected size, head shape|url=https://www.sciencedaily.com/releases/2010/05/100518113132.htm|access-date=2020-11-24|website=ScienceDaily|language=en}}</ref>

==Distribution and habitat==
This species occurs on both sides of the American coast, in regions where the water is usually warmer than {{convert|70|F|C}}. In the [[Atlantic Ocean|Atlantic]], it ranges from [[New England]], where it is rare, to the [[Gulf of Mexico]] and [[Brazil]], and in the [[Pacific Ocean|Pacific]] it ranges from southern [[California]] to northern [[Peru]]. During the summer, it is common in the inshore waters of [[the Carolinas]] and [[Georgia (U.S. state)|Georgia]]; in spring, summer, and fall, it is found off [[Florida]] and in the Gulf of Mexico. In the winter, the bonnethead shark is found closer to the [[equator]], where the water is warmer.{{citation needed|date=August 2016}} While it is still abundant in the North Atlantic Ocean and Gulf of Mexico, it has become significantly less common in the [[Caribbean Sea]], and has been nearly [[Local extinction|extirpated]] from most of its [[Atlantic Ocean|South Atlantic]] and Pacific range.<ref name="iucn" />

It frequents shallow estuaries and bays over seagrass, mud, and sandy bottoms.<ref name=iucn/>

==Ecology==

===Behavior===
The bonnethead shark is an active [[tropical]] shark that swims in small groups of five to 15 individuals, although schools of hundreds or even thousands have been reported. They move constantly following changes in water temperature and to maintain respiration. The bonnethead shark sinks if it does not keep moving, since hammerhead sharks are among the most negatively [[buoyant]] of [[marine (ocean)|marine]] [[vertebrate]]s.  <!-- The bonnethead shark uses a unique type of [[cerebrospinal fluid]] to let others know it is nearby. Like other sharks, it is capable of [[electroreception]] to detect its prey.  This system allows the bonnethead shark to alert other sharks of their presence.{{citation needed|date=August 2016}} -->

===Diet===
The shark feeds primarily on [[crustacean]]s, consisting mostly of [[Callinectes sapidus|blue crab]]s, but also [[shrimp]], [[mollusk]]s, and small fish. Its feeding behavior involves swimming across the seafloor, moving its head in arc patterns like a metal detector, looking for minute [[Electromagnetism|electromagnetic]] disturbances produced by crabs and other creatures hiding in the sediment. Upon discovery, it sharply turns around and bites into the sediment where the disturbance was detected. If a crab is caught, the bonnethead shark uses its teeth to grind its [[carapace]] and then uses suction to swallow.{{citation needed|date=August 2016}} To accommodate the many types of animals on which it feeds, the bonnethead shark has small, sharp teeth in the front of the mouth (for grabbing soft prey) and flat, broad [[Molar (tooth)|molar]]s in the back (for crushing hard-shelled prey).

Bonnetheads also ingest large amounts of [[seagrass]], which has been found to make up around 62.1% of gut content mass. The species appear to be omnivorous, the only known case of plant feeding in sharks.<ref>{{cite web|url=http://news.nationalgeographic.com/2017/06/bonnethead-sharks-eat-seagrass/|archive-url=https://web.archive.org/web/20170630025407/http://news.nationalgeographic.com/2017/06/bonnethead-sharks-eat-seagrass/|url-status=dead|archive-date=30 June 2017|title=This Shark Eats Grass, and No One Knows Why|date=29 June 2017|author=Hannah Lang|website = National Geographic}}</ref> The shark may perform this activity to protect its stomach against the spiny carapaces of the [[Callinectes sapidus|blue crab]] which it feeds on.<ref name=aquarium>{{cite book |last=Michael |first=Scott |title=Aquarium Sharks & Rays |publisher=T.F.H Publications, Inc. |date=2001 | location=Neptune City, NJ}}</ref> A 2018 study with a carbon [[Isotopic labeling#Stable isotope labeling|isotope-labelled]] seagrass diet found that they could digest seagrass with at least moderate efficiency, with 50±2% digestibility of seagrass organic matter, and had [[cellulose]]-component-degrading enzyme activity in their hindgut.<ref name="LeighPapastamatiou2018">{{cite journal|last1=Leigh|first1=Samantha C.|last2=Papastamatiou|first2=Yannis P.|last3=German|first3=Donovan P.|title=Seagrass digestion by a notorious 'carnivore'|journal=Proceedings of the Royal Society B: Biological Sciences|volume=285|issue=1886|year=2018|page=20181583|issn=0962-8452|doi=10.1098/rspb.2018.1583|pmid=30185641 |pmc=6158537}}</ref><ref>{{cite web |url=https://www.theguardian.com/environment/2018/sep/05/bonnethead-omnivorous-shark-species-identified |title=First known omnivorous shark species identified |newspaper=The Guardian |date=5 September 2018  |author= Ian Sample |access-date=5 September 2018 }}</ref>

===Reproduction===
The bonnethead shark is [[viviparous]]. Females reach sexual maturity around {{convert|32|in|cm}}, while males reach maturity around {{convert|24|in|cm}}. Four to twelve pups are born in late summer and early fall, measuring {{convert|12|to|13|in|mm|abbr=on}}.

Bonnetheads have one of the shortest gestation periods among sharks, lasting only 4.5–5.0 months.<ref name=iucn/>

A bonnethead female produced a pup by [[parthenogenesis]]. The birth took place at the [[Henry Doorly Zoo]] in Nebraska; DNA analysis showed a perfect match between mother and pup.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/6681793.stm |website=BBC News |title=Captive shark had 'virgin birth' |date=23 May 2007}}</ref>

==Conservation==
The bonnethead was formerly classified as a [[least-concern species]] by the [[IUCN]]. It is heavily targeted by commercial and recreational fisheries and constitutes up to 50% of all small shark landings in the Eastern US, but is still reasonably abundant there as well as in the Atlantic Coasts of [[the Bahamas]] and [[Mexico]]. However, significant declines have been reported in the [[Caribbean Sea]] and Atlantic [[Central America]] and massive declines along with widespread disappearance on the Atlantic Coast of [[South America]] as well as most of the shark's [[Pacific Ocean|Pacific]] range, leading it to be uplifted to "[[Endangered species|Endangered]]" in 2020.<ref name=iucn/>

Since October 2021, ''S. tiburo'' has been classified as Largely Depleted by the IUCN.<ref name="iucn green status 31 October 2021">{{cite web |author=Pérez-Jiménez, J.C. |date=2021 |title=''Sphyrna tiburo (Green Status assessment)'' |url=https://www.iucnredlist.org/species/39387/205765567 |website= The IUCN Red List |access-date=20 February 2022}}</ref>

==References==
{{Reflist}}

==External links==
{{Commons category|Sphyrna tiburo}}
{{Portal|Sharks}}
* {{ITIS |id=160502 |taxon=Sphyrna tiburo |access-date=23 January 2006}}
* [https://shark-references.com/species/view/Sphyrna-tiburo Species Description of Sphyrna tiburo at www.shark-references.com]
* {{sealifephotos|158517}}

{{Selachimorpha|C.}}
{{Taxonbar|from=Q338891}}

[[Category:Sphyrna]]
[[Category: Viviparous fish]]
[[Category: Fish of the Americas]]
[[Category: Fish of the Dominican Republic]]
[[Category: Fish of the Caribbean]]
[[Category: Fish of the Western Atlantic]]
[[Category: Western American coastal fauna]]
[[Category: Fish described in 1758]]
[[Category: Taxa named by Carl Linnaeus]]