Lab notebook: the great hammerhead (Sphyrna mokarran)

Lab 1

Specific objectives:

  • Document the external anatomy of the specimen
  • Identify the specimen using taxonomic features

The great hammerhead (Sphyrna mokarran) is from the family Sphyrnidae of the “ground sharks” or Carcharhiniformes [1] (Fig. 1).

Figure 1 External morphology of the great hammerhead shark, (Sphyrna mokarran)

The sheer size of S. mokarran hints at its predatory nature. Its serrated, pointed teeth are excellent for tearing apart flesh from prey like fish and rays, or perhaps crushing shells of crustaceans too [2], [3]. The ampullae of Lorenzini, located on the underside of its snout, are electroreceptors in the form of a network of mucus-filled pores helping in prey detection [4]. Its head’s unique “hammer” shape is called a cephalofoil, a unique evolution to enhance the shark’s vision. The position of the eyes on the ends of the cephalofoil allow the shark to always see above and below itself [5]. The cephalofoil also increases surface area for distribution of the ampullae of Lorenzini, thereby allowing the shark to locate prey more effectively [6]. The cephalofoil acts as a lifting surface, similar to the hydrofoil of a boat, and assists with sharp turns to attack prey [3], [7]. The two nares under the snout act as nostrils where water can pass through, allowing the shark to “smell” and assist with prey detection. The shape of the head means the nares are located further apart which may enhance olfaction.

The two dorsal, pelvic, and anal fins act as vertical stabilisers (in males, the pelvic fin is modified into a reproductive organ known as a clasper). The two pectoral fins help to steer while swimming and, like a plane, provide lift in the water. The caudal fin provides thrust, and the caudal fin of S. mokarran has a smaller lower lobe than the upper lobe, indicating that it may spend much time swimming close to the seabed and that speed is not as essential for this species. S. mokarran likely has carangiform or thunniform propulsion because of the large, muscular tail.

The colouration of S. mokarran is known as countershading, whereby the top surface is darker than the lighter underside. This means that the shark is camouflaged from prey above it (looking down to the deep, dark ocean) and from prey below it (looking up at the light, sunlight, surface waters). This will help the shark ambush its prey effectively and, for younger/smaller individuals, avoid detection from carnivores looking for a hammerhead-shaped snack. Its skin is comprised of dermal denticles: rough, teeth-like scales that would help reduce swimming-induced drag and protect them from predators and parasites. The lateral line extends the length of the shark sides and helps orient the shark to movement and sound. As a sensory organ, it works in conjunction with the ampullae of Lorenzini to assist in prey detection.

Lab 2

Specific objective:

  • Describe the functions of its organs as it relates to the ecology/biology of the specimen

Sharks lack an operculum and instead have gill slits that act as openings to the gills [8], allowing seawater to flow over them and oxygen to be extracted for respiration. From viewing photographs of S. mokarran, it does not appear to have a spiracle, like other sharks do (except requiem sharks). The spiracle is a small hole behind the eye that opens to the buccal cavity, assisting sharks to take in water over their gills whilst stationary, suitable for sharks that spend much time on the seafloor [9]. The lack of a spiracle coincides with the pelagic nature of S. mokarran and infers that they must be ram ventilators who must continuously swim forward to encourage water flow over the gill filaments through the mouth or gill slits [9].

Elasmobranchs are unique in that the morphological structure of their jaw means that it is suspended by a musculoskeletal sling [10]. S. mokarran are even more unique in that their head is dorsoventrally compressed and laterally expanded to form the cephalofoil. Because the evolution of the cephalofoil is such a drastic, morphological difference among Carcharhiniforms, trade-offs have been made to the pharyngeal apparatus (as well as other systems that occupy the head) [11]. Additionally, the teeth are triangular and serrated (Fig. 2).

One observation [7] of S. mokarran predation on a stingray saw the shark use its cephalofoil to pin the ray to the seafloor on two occasions, each time taking a bite from the wings of the ray, rendering it incapacitated. The hammerhead was then able to consume the immobile ray easily.

Figure 2 Teeth in dried jaws of great hammerhead shark, Sphyrna mokarran. Doug Perrine. (2013). received from on 29 September 2021.

There is limited research on S. mokarran digestion specifically, so it is helpful to infer similar characteristics from other species with a similar diet. The digestive tract begins at the mouth, where a strong jaw and sharp, replaceable teeth can rip flesh or crush prey into swallowable sizes. This is where the first type of digestion takes place – mechanical.

Food travels through the oesophagus from the mouth, where striated muscles and secreted mucous assist food into the stomach [12]. The stomach of most sharks is J-shaped [13], although there are some exceptions. The bonnethead shark (Sphyrna tiburo) exhibits a straight (I-shaped) stomach [14]. Because S. tiburo is from the same genus as S. mokarran, it may seem wise to assume that S. mokarran has an I-shaped stomach too, but S. tiburo is omnivorous and digests seagrass, whereas S. mokarran is strictly carnivorous. Like other vertebrates, the cells on the stomach walls of an elasmobranch secrete mucous to protect it from the acidic gastric juices that biochemically digest food stored in the stomach [12]. Some sharks are known to undergo gastric evacuation, whereby undigestible contents of the stomach are regurgitated out of the mouth [11], [12], although it is unknown is S. mokarran can do this. Furthermore, some elasmobranch species can regulate gastric acid secretion, likely in times of fasting due to low prey availability [15].

Nutrients are transported to the intestine from the stomach, which is relatively shorter than most vertebrate intestines. Leigh et al. (2017) recommend separating elasmobranch intestines into three sections: proximal, spiral, and distal. The proximal region gives way to the spiral region or spiral valve. The anatomy of the spiral valve varies among species but can have between 2–50 turns and is thought to increase surface area for absorption of nutrients and/or slow the rate at which food travels through the intestine, therefore, increasing time available for digestion [14], [16]. Furthermore, the spiral valve ensures that oversized items (e.g., bones) cannot pass through their lower intestine and allows them to be sufficiently broken down first or regurgitated.

After the spiral valve is the distal intestine, characterised by thicker and more muscular walls to accommodate the accumulation of faeces [12]. As pressure increases on the rectal walls, nerve impulses are sent to the brain for muscles to relax, and faeces are passed through the cloaca, which serves as the anus as well as the genitals and urinary duct [12]. Another species from the same genus as S. mokarran, the scalloped hammerhead (Sphyrna lewini), demonstrates an increased gastric evacuation rate with increased meal size [17], so this may be the case for S. mokarran as well. The amount of speculation I’m having to undergo for S. mokarran based off closely related species is evidence that S. mokarran needs more research and investigation to better understand it.

The small, relative size of the elasmobranch digestive tract may make room for the large liver. Baldridge (1970) found that the liver accounted for 3.83% and 9.5% of total body weight in two S. mokarran individuals (imagine a 100kg man having a 3.8 kg or 9.5 kg liver!!). Elasmobranchs do not have swim bladders like bony fish and instead rely on the liver, which is saturated in oil, to maintain buoyancy [19].  The liver contains lightweight oils, increasing its buoyancy and, along with its fins, gives it the lift it needs to prevent sinking.

Overall, not much is known about the reproductive biology of S. mokarran. Male specimens of S. mokarran have two claspers inside each pelvic fin [20] that deposit sperm into the female’s cloaca [21]. Based on evidence from other shark species, it is apparent that females can store sperm in their shell gland for up to 16 months [21]. S. mokarran are viviparous (birthing live young), and the ova has a yolk sac that, once depleted, turns into a structure similar to a placenta [1], [21]. They usually litter between 6–42 pups after a gestation of ~11 months [1], although one female was known to have littered a record 55 pups [22].

Lab 3

Specific objective:

  • Provide information about the age and growth of the specimen

The maximum reported age for S. mokarran is 30 years [1], although one specimen was estimated to be around 40–50 years old [22].

S. mokarran are the largest hammerhead species, and a male can reach a maximum total length (TL) of up to 610 cm, although typically, they will average 370 cm TL [1]. Due to their lack of otoliths, cartilaginous fish are aged by counting banding patterns on their sagittal vertebrae (Fig. 3). The von Bertalanffy growth function (VBGF) (Fig. 4) shows that, at birth, S. mokarran is around 50-70 cm.

Figure 3 Taken from [23]: “Sagittal vertebral section from a 4-year-old great hammerhead, Sphyrna mokarran, illustrating the banding pattern and annuli used to assign age. Scale bar = 2 mm.”
Figure 4 Taken from [23]: “The best fit von Bertalanffy growth model for male and female great hammerhead sharks, Sphyrna mokarran, collected in the northwestern Atlantic Ocean and the eastern Gulf of Mexico.”

The VBGF (Fig. 4) shows that males grow faster than females but reach a smaller asymptotic size than females. This growth is likely due to different energy requirements for the sexes for somatic growth and reproductive development [23].

S. mokarran reaches sexual maturity between 210–300 cm total length, with males tending to reach maturity at a smaller size than females [1].

Lab 4

Specific objective:

  • Provide information about the reproductive dynamics and life history of your chosen specimen

The embryonic sex ratio of S. mokarran is close to 1:1 [24]. It is gonochoric, with no interesting or outstanding sexual dynamics to note [1].

Rigby et al. (2019) describe S. mokarran as aplacental viviparous, whereas Froese and Pauly (2021) describe the species as viviparous with a yolk-sac placenta. Either way, they birth live young that have hatched from an egg in-utero.

There is not much research into the spawning behaviour of S. mokarran, which is perhaps reflective of their naturally elusive lifestyle. Stevens and Lyle (1989) note mating scars on females, which indicates, like many shark species, the courtship process can be seemingly violent with the male holding onto the females with his teeth during copulation.

S. lewini were observed in a large group off the Galapagos Islands and were thought to be amidst a courtship ritual where the largest females dominated the centre of the group, and the males attempted to access them to mate [26]. This could provide insight into the mating rituals of S. mokarran, although S. mokarran populations are substantially smaller than S. lewini, and they have yet to be observed in such large numbers.

One account of mating S. mokarran in the Bahamas reported two individuals ascending in 21m of water as they spiraled around one another and copulated at the surface [27].

S. mokarran birth between 6–42 pups every two years [25]. Their parental mode is not well researched. Many elasmobranchs offer no maternal care once the pup is born, so it can be assumed that this is the same for S. mokarran. However, a study on the scalloped hammerhead (S. lewini) and the Carolina hammerhead (S. gilberti) illustrated that neonatal hammerheads are likely to rely on maternal provisioning in the first few weeks after birth [28]. Therefore, an increased maternal investment may be a part of the life history strategy of S. mokarran. Again, further research is crucial to understand their reproductive and life histories further.

There is regional variation in the size and age range of S. mokarran sexual maturity. As before mentioned, this species reaches sexual maturity between 210–300 cm total length, with males maturing from 225–269 cm and females maturing from 210–300 cm [1], [25]. Age-at-maturity for females is estimated to be 5.5–8.3 years in Atlantic and Pacific populations [25], [29].

S. mokarran eggs hatch in-utero and embryonic individuals spend 11 months in their mother’s uterus, and newborns are around 50–70 cm total length and are then known as pups [25], [29]. At birth, they resemble S. mokarran adults in external morphology (Fig. 5). They grow rapidly until ten years of age, where their growth rate reduces [23], likely because they have reached sexual maturity and fitness rather than size becomes more critical for courtship and survival.

Figure 5 Neonatal great hammerhead, Sphyrna mokarran, pups. By Apex Predators Program, NOAA/NEFSC –, Public Domain,

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[10]    P. J. Motta, “Prey Capture Behavior and Feeding Mechanics of Elasmobranchs,” in Biology of Sharks and Their Relatives, J. C. Carrier, J. A. Musick, and M. R. Heithaus, Eds. Boca Raton: CRC Press, 2004, pp. 165–202.

[11]    J. M. Brunnschweiler, P. L. R. Andrews, E. J. Southall, M. Pickering, and D. W. Sims, “Rapid voluntary stomach eversion in a free-living shark,” J. Mar. Biol. Assoc. United Kingdom, vol. 85, no. 5, pp. 1141–1144, 2005, doi: 10.1017/S0025315405012208.

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[15]    R. D. Day, I. R. Tibbetts, and S. M. Secor, “Physiological responses to short-term fasting among herbivorous, omnivorous, and carnivorous fishes,” J. Comp. Physiol. B Biochem. Syst. Environ. Physiol., vol. 184, no. 4, pp. 497–512, 2014, doi: 10.1007/s00360-014-0813-4.

[16]    C. Bucking, “Feeding and Digestion in Elasmobranchs: Tying Diet and Physiology Together,” in Fish Physiology, vol. 34, Academic Press, 2015, pp. 347–394.

[17]    A. Bush and K. Holland, “Food limitation in a nursery area estimates of daily ration in juvenile scalloped hammerheads,” J. Exp. Biol. Ecol., vol. 278, pp. 157–178, 2002.

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[20]    M. Aidan, “Why Do Sharks Have Two Penises?,” Biology of Sharks and Rays, 2021. (accessed Sep. 27, 2021).

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