ABSTRACT Several factors that influence the evolution of the unusual head morphology of hammerhead sharks (family Sphyrnidae) are proposed but few are empirically tested. In this study we tested the ‘enhanced binocular field’ hypothesis (that proposes enhanced frontal binocularity) by comparison of the visual fields of three hammerhead species: the bonnethead shark, Sphyrna tiburo, the scalloped hammerhead shark, Sphyrna lewini, and the winghead shark, Eusphyra blochii, with that of two carcharhinid species: the lemon shark, Negaprion brevirostris, and the blacknose shark, Carcharhinus acronotus. Additionally, eye rotation and head yaw were quantified to determine if species compensate for large blind areas anterior to the head. The winghead shark possessed the largest anterior binocular overlap (48 deg.) and was nearly four times larger than that of the lemon (10 deg.) and blacknose (11 deg.) sharks. The binocular overlap in the scalloped hammerhead sharks (34 deg.) was greater than the bonnethead sharks (13 deg.) and carcharhinid species; however, the bonnethead shark did not differ from the carcharhinids. These results indicate that binocular overlap has increased with lateral head expansion in hammerhead sharks. The hammerhead species did not demonstrate greater eye rotation in the anterior or posterior direction. However, both the scalloped hammerhead and bonnethead sharks exhibited greater head yaw during swimming (16.9 deg. and 15.6 deg., respectively) than the lemon (15.1 deg.) and blacknose (15.0 deg.) sharks, indicating a behavioral compensation for the anterior blind area. This study illustrates the larger binocular overlap in hammerhead species relative to their carcharhinid sister taxa and is consistent with the ‘enhanced binocular field’ hypothesis.
Publié par Denham, J., Stevens, J.D., Simpfendorfer, C., Heupel, M.R., Cliff, G., Morgan, A., Graham, R., Ducrocq, M., Dulvy, N.K., Seisay, M., Asber, M., Valenti, S.V., Litvinov, F., Martins, P., Lemine Ould Sidi, M., Tous, P. & Bucal, D.
Sphyrna mokarran. The IUCN Red List of Threatened Species
ABSTRACT A large, widely distributed, tropical hammerhead shark largely restricted to continental shelves. Sphyrna
mokarran is highly valued for its fins (in target and incidental fisheries), suffers very high bycatch
mortality and only reproduces once every two years, making it vulnerable to over-exploitation and
population depletion. Generally regarded as solitary, and is therefore unlikely to be abundant wherever
it occurs. Previously observed from Mauritania to Angola, reportedly abundant from November to
January in Senegal, and in October in Mauritania, stocks have since collapsed and it is recognized as one
of the four most threatened species by member states of the Sub Regional Fishing Commission.
Although there is very little species specific data available, the absence of recent records give cause to
suspect a decline of at least 80% in the past 25 years. Fishing proceeds unmanaged and unmonitored,
resulting in an assessment of Critically Endangered in the Eastern Atlantic. Although not targeted in the
Northwest Atlantic and Gulf of Mexico it is taken as by-catch in several fisheries and suffers greater than
90% vessel mortality. Two time series data sets (pelagic logbook, large pelagic survey) have shown a
decline in the catch of Sphyrna spp. since 1986. Difficulties in species identification and accurate
recording make an assessment of this species very difficult, however low survival at capture makes it Endangered in the Northwest Atlantic and Gulf of Mexico, based on a suspected decline of at least >50%
over the past 10 years. The decline is poorly documented and has not been curtailed. In the Southwest
Indian Ocean this species is assessed as Endangered based on a continued decline in catch rate of 79%
reported for the period 1978 to 2003. It is uncertain whether these declines reflect highly localized stock
depletion or whether they reflect a general decline in the Southwest Indian Ocean, but large numbers of
longline vessels have been reported to be operating illegally in coastal waters of the western Indian
Ocean where they are targeting primarily hammerhead sharks and giant guitarfish Rhynchobatus
djiddensis. Sphyrna mokarran is found along the northern coast of Australia. A large increase in the
illegal, unregulated and unreported (IUU) fishing in northern Australia in the last few years points to
great concern that this species is being increasingly targeted for its valuable large fins. Recent Risk
Assessments of northern Australian elasmobranchs indicate that it may be 'high-risk' however, due to a
lack of data to form the basis of an accurate assessment, the species is considered Data Deficient in
Australia at the present time. Further investigation of its status there is required. Given its vulnerability
to depletion, low survival at capture and high value for the fin trade this species is considered to meet
the criteria for Endangered globally based on the available evidence for declines of >50%. There is an
urgent need for data collection in other parts of its range, but considering the high value of its fins and
high fishing pressure in other parts of its range, similar declines are likely to have occurred elsewhere.
highly vulnerable to fishing pressure, whether directed or incidental. It is therefore assessed as
ABSTRACT From 1994–2005 the Commercial Shark Fishery Observer Program (CSFOP) placed fishery observ-ers aboard US bottom longline vessels engaged in directed fishing for sharks in the region from New Jersey to Louisiana, USA. Observers routinely recorded species specific at-vessel mortality as related to enduring the stress of longline capture. Data for 5 species of sharks (sandbar Carcharhinus plumbeus, blacktip Carcharhinus limbatus, dusky Carcharhinus obscurus, tiger Galeocerdo cuvier, scalloped hammerhead Sphyrna lewini, and great hammer-head Sphyrna mokarran) were analyzed in this study. Multiple stepwise linear regressions indicate that age group, soak time and bottom water temperature can be used as predictors of at-vessel mortality and that size restrictions, size selective gear, restricting the soak time and time/area closures may be beneficial to fisheries targeting large coastal sharks.
ABSTRACT Many hypotheses have been advanced to explain the adaptive significance of the sphyrnid cephalofoil, including potential advantages of spacing the olfactory organs at the distal tips of the broad surface. We employed comparative morphology to test whether the sphyrnid cephalofoil provides better stereo-olfaction, increases olfactory acuity, and samples a greater volume of the medium compared to the situation in carcharhiniform sharks. The broadly spaced nares provide sphyrnid species with a significantly greater separation between the olfactory rosettes, which could lead to an enhanced ability to resolve odor gradients. In addition, most sphyrnid species possess prenarial grooves that greatly increase the volume of water sampled by the nares and thus increase the probability of odorant encounter. However, despite a much greater head width, and a significantly greater number of olfactory lamellae, scalloped hammerhead sharks do not possess a greater amount of olfactory epithelial surface area than the carcharhiniform sandbar sharks. Therefore, sphyrnid sharks might not possess any greater olfactory acuity than carcharhinids. Despite this, there are clear olfactory advantages to the cephalofoil head morphology that could have led to its evolution, persistence, and diversification. persistence, and diversification.
ABSTRACT The accidental capture of large animals such as sharks, manta rays, sea turtles, and dolphins in pelagic trawler fisheries remains controversial because it threatens biological diversity in many biogeographical regions, including the subtropical eastern North Atlantic. Bycatch rates observed during more than 1400 trawl sets off Mauritania, Northwest Africa, are shown to have been considerable during the past 4 years, with high animal abundance in Summer when the Northwest African shelf is occupied by subtropical water. We demonstrate the urgency for bycatch reduction and evaluate the use of species-selective gear, a conservation method immediately available and immediately effective in waters fished through international access agreements. A modification tested in commercial trawls during the observer program guides pelagic megafauna deflected by a filter to an escape tunnel along the bottom of the trawl. This “excluder” reduces bycatch mortality of the most vulnerable megafauna species by at least 40–100%.
ABSTRACT The peculiar head morphology of hammerhead sharks has spawned a variety of untested functional hypotheses. One of the most intuitively
appealing ideas is that the anterior foil acts, as in canard-winged aircraft, to increase maneuverability. We tested this hypothesis
by determining whether juveniles of two hammerhead species (Sphyrna tiburo and S. lewini) turn more sharply, more often, and with
greater velocity than a juvenile carcharhinid shark (Carcharhinus plumbeus). Although the hammerheads were more maneuverable,
further investigation revealed that they do not roll their body during turns, suggesting that the cephalofoil does not act as a steering
wing. We also show that hammerhead sharks demonstrate greater lateral flexure in a turn than carcharhinids, and that this flexibility
may be due to cross sectional shape rather than number of vertebrae.
ABSTRACT Selection to maximize electroreceptive search area might have driven evolution of the cephalofoil head morphology
of hammerhead sharks (family Sphyrnidae). The enhanced electrosensory hypothesis predicts that the wider head
of sphyrnid sharks necessitates a greater number of electrosensory pores to maintain a comparable pore density.
Although gross head morphology clearly differs between sphyrnid sharks and their closest relatives the carcharhinids,
a quantitative examination is lacking. Head morphology and the distribution of electrosensory pores were compared
between a carcharhinid, Carcharhinus plumbeus, and two sphyrnid sharks, Sphyrna lewini and S. tiburo. Both
sphyrnids had greater head widths than the carcharhinid, although head surface area and volume did not differ
between the three species. The raked head morphology of neonatal S. lewini pups, presumably an adaptation
to facilitate parturition, becomes orthogonal to the body axis immediately post-parturition whereas this change
is much less dramatic for the other two species. The general pattern of electrosensory pore distribution on the
head is conserved across species despite the differences in gross head morphology. Sphyrna lewini has a mean of
3067 158:9 SD pores, S. tiburo has a mean of 2028 96.6 SD pores and C. plumbeus has a mean of 2317
126.3 SD pores and the number of pores remains constant with age. Sphyrnids have a greater number of pores
on the ventral surface of the head whereas C. plumbeus has an even distribution on dorsal and ventral surfaces.
The greater number of pores distributed on a similar surface area provides S. lewini pups with a higher density of
electrosensory pores per unit area compared to C. plumbeus pups. The greater number of ampullae, the higher pore
density and the larger sampling area of the head combine to provide hammerhead sharks with a morphologically
enhanced electroreceptive capability compared to comparably sized carcharhinids.
At least eight species of sharks of the families Carcharhinidae and Sphyrnidae use Cleveland Bay in northern Australia as a communal nursery area.Carcharhinus dussumieri, C. fitzroyensis, C. limbatus andC. tilstoni use the bay as a seasonal primary nursery, with juveniles occurring in it for only a few months each year immediately after birth. Alternatively,Carcharhinus sorrah, Rhizoprionodon acutus andR. taylori use the bay as a year-round primary and secondary nursery, with juveniles remaining in it up to the size at maturity. AdultR. taylori also persist in the bay, a behavioural pattern possibly explained by their small maximum size. While present immediately after birth the type of utilisation pattern displayed bySphyrna lewini could not be clarified in this study. Although diets of these species in the bay are similar, there is probably little direct competition for food due to the highly productive habitats in the bay supporting an abundance of food resources. The highest numbers of juveniles occur when prey species are the most abundant, and when temporal separation of some seasonally-occurring species of sharks in effect.
ABSTRACT The hammerhead sharks offer one of the most striking examples of evolutionary divergence in form and function. The greatly expanded head or cephalofoil may be as great as half their body length in the winghead shark Eusphyra blochii or only slightly enlarged as in the bonnethead shark Sphyrna tiburo. The function of the laterally expanded head is unclear, but hypotheses include greater maneuverability and lift, better olfactory resolution, enhanced binocular and lateral vision, and superior electro- or mechanoreception. This project investigates the function and evolution of the hammerhead shark cephalofoil utilizing a variety of species with divergent head shapes (E. blochii, the great hammerhead S. mokarran, smooth hammerhead S. zygaena, the scalloped hammerhead S. lewini, scalloped bonnethead S. corona, scoophead S. media, and S. tiburo). This study will: establish an evolutionary history of the hammerhead sharks using modern molecular DNA analysis; examine the functional and anatomical differences in the sensory, feeding and respiratory structures among the hammerhead sharks; compare the sensory capabilities of the expanded cephalofoil to the closely related blacknose shark, Carcharhinus acronotus with its more typical pointed head; investigate the role of the expanded head in locomotion and maneuverability; and finally, reveal the evolutionary sequence of structural and functional changes that have resulted in this striking head form. These goals will be accomplished by integrating the expertise of evolutionary biologists, sensory physiologists, functional morphologists, and fluid engineers along with utilizing the specialized shark research facilities of Mote Marine Laboratory in Florida. This collaborative study will identify the selective pressures that have resulted in the evolution of this bizarre head form, and increase our knowledge of the structure, function, and evolution of sharks, an overfished and depleted group of marine predators. This project promotes training, teaching, and learning at all educational levels. Undergraduate and graduate students will be cross-trained in evolutionary biology, sensory physiology, anatomy and biomechanics. This research will be integrated into teaching at the graduate and undergraduate levels at three universities. Because of the broad public interest in sharks the PIs will provide public outreach and education for school children to adults through newspaper and magazine editorials, television documentaries, and museum displays.
ABSTRACT The hammerhead sharks Eusphyra blochii, Sphyrna mokarran and S. lewini form part of the incidental
catch of a commercial gill-net fishery off northern Australia. Of the specimens sampled between June
1980 and December 1986, 46% of S. mokarran, 41% of E. blochii and 31% of S. lewini were females.
Few adult female S. lewini were caught and it is suggested that these occur offshore of the study area.
In northern Australia, the usual size at maturity of male E. blochii, S. lewini and S. mokarran is 108,
150 and 225 cm total length (TL), ,and of females is 120, 200 and 210 cm TL, respectively. S. mokarran
and E. blochii gave birth in January and February/March, respectively, after a gestation period of
10-11 months. S. Iewini appears to have a more extended seasonal cycle: the young are born between
October and January after 9-10 months gestation. Size at birth is about 45-50 cm TL in E. blochii
and S. lewini, and 65 cm TL in S. mokarran. Mean litter size is 12 in E. blochii, 15 in S. mokarran
and 17 in S. lewini. Individual E. blochii females breed every year, whereas S. mokarran females
probably breed every other year. Fish are an important component of the diet of all three species as,
to a lesser extent, are cephalopods for S. lewini and crustaceans for S. rnokarran and E. blochii