Shallow water tidal flat use and associated specialized foraging behavior of the great hammerhead shark ( Sphyrna mokarran )

Abstract

Evidence suggests the great hammerhead shark, Sphyrna mokarran,
is vulnerable to a variety of anthropogenic stressors, and is an
understudied species of shark due to its cryptic nature and wideranging
movements. While recognized as both a pelagic-coastal and
a highly mobile predator, minimal anecdotal evidence exist describing
shallow water habitat use by this species. This report describes six
cases in which a great hammerhead shark utilizes an inshore shallow
water flats environment (<1.5 m in depth), five of which involve prey
capture. These observations permitted identification of two novel
behaviors that may allow great hammerheads to inhabit these
shallow habitats: a (1) prey-capture technique termed ‘grasp-turning’
that involves burst swimming at tight turning angles while grasping
prey and (2) a post-predation recovery period whereby the shark
maintains head-first orientation into the current that may facilitate
respiration and prey consumption. These behavioral observations
provide insights into the natural history of this species.

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Marine and Freshwater Behaviour and Physiology
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Shallow water tidal flat use and associated
specialized foraging behavior of the great
hammerhead shark (Sphyrna mokarran)
Robert P. Roemer, Austin J. Gallagher & Neil Hammerschlag
To cite this article: Robert P. Roemer, Austin J. Gallagher & Neil Hammerschlag (2016):
Shallow water tidal flat use and associated specialized foraging behavior of the great
hammerhead shark (Sphyrna mokarran), Marine and Freshwater Behaviour and Physiology,
DOI: 10.1080/10236244.2016.1168089
To link to this article: http://dx.doi.org/10.1080/10236244.2016.1168089
Published online: 29 Apr 2016.
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MARINE AND FRESHWATER BEHAVIOUR AND PHYSIOLOGY, 2016
http://dx.doi.org/10.1080/10236244.2016.1168089
© 2016 Informa UK Limited, trading as Taylor & Francis Group
Shallow water tidal at use and associated specialized
foraging behavior of the great hammerhead shark (Sphyrna
mokarran)
Robert P. Roemera,c, Austin J. Gallaghera,c,d and Neil Hammerschlaga,b,c
aRosentiel School for Marine and Atmospheric Science, University of Miami, Miami, FL, USA; bLeonard and
Jayne Abess Center for Ecosystem Science and Policy, University of Miami, Coral Gables, FL, USA; cShark
Research and Conservation Program, University of Miami, Miami, FL, USA; dBeneath The Waves Incorporated,
Syracuse, NY, USA
ABSTRACT
Evidence suggests the great hammerhead shark, Sphyrna mokarran,
is vulnerable to a variety of anthropogenic stressors, and is an
understudied species of shark due to its cryptic nature and wide-
ranging movements. While recognized as both a pelagic-coastal and
a highly mobile predator, minimal anecdotal evidence exist describing
shallow water habitat use by this species. This report describes six
cases in which a great hammerhead shark utilizes an inshore shallow
water ats environment (<1.5 m in depth), ve of which involve prey
capture. These observations permitted identication of two novel
behaviors that may allow great hammerheads to inhabit these
shallow habitats: a (1) prey-capture technique termed ‘grasp-turning’
that involves burst swimming at tight turning angles while grasping
prey and (2) a post-predation recovery period whereby the shark
maintains head-rst orientation into the current that may facilitate
respiration and prey consumption. These behavioral observations
provide insights into the natural history of this species.
Understanding the habitat use and movements of large and highly mobile marine preda-
tors is inherently challenging due to their wide-ranging behaviors, the concealing nature
of the environment and their increasing rarity due to human exploitation (Nelson 1977).
However, these types of data can provide information for initiating eective conservation
and management of threatened species (Green et al. 2009; Sims 2010, Dulvy et al. 2014).
Biotelemetry approaches are commonly employed to investigate movements and behav-
iors of many dierent shark species, with most studies designed to determine high-use areas
and environmental preferences (Donaldson et al. 2008; Sims 2010; Hammerschlag et al.
2011a; Papastamatiou & Lowe 2012). While these studies can reveal new information on
the habitat utilization and movements of large shark species (e.g. Bonl et al. 2005; Skomal
et al. 2009), their success is predicated on several key conditions: locating individuals,
KEYWORDS
Shark; hammerhead; habitat
use; behavior; predation;
specializations
ARTICLE HISTORY
Received 2 September 2015
Accepted 29 February 2016
CONTACT Robert P. Roemer rproemer@rsmas.miami.edu
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2 R. P. ROEMER ET AL.
generating a suitable sample size, tag retention and functionality, community or stakeholder
acceptance, as well as ethical issues surrounding tagging impacts on animal health and
survival (Hammerschlag et al. 2014). While increased use and rapid advances in aquatic
biotelemetry are expanding our ability to document the behaviors of large sharks (Hussey
et al. 2015), other more traditional natural history approaches to studying shark behavior
(e.g. observation) can be overlooked and underutilized, possibly leading to incomplete
understanding of animal biology. Natural history approaches have long been viewed as
valuable for advancing the knowledge base of elasmobranch ecology and continue to pro-
vide important insights on species which are otherwise cryptic or rarely observed (e.g.
Strong et al. 1990; Klimley et al. 1992; Martin et al. 2005; Fallows et al. 2013). In particular,
natural history data gathered from local stakeholders or traditional ecological knowledge
may provide insights that could otherwise go undetected, since these individuals spend the
most time on the water and thus most likely to observe and interpret rare animal behaviors
(Huntington 2000; Drew 2005; Silvano and Valbo-Jørgensen 2008).
Large hammerhead sharks (family Sphyrnidae) are among the most specialized of extant
shark species, behaviorally (Gallagher et al. 2014a, 2014b), physiologically (Kajiura & Holland
2002; Mello 2009; Tricas et al. 2009; Gallagher et al. 2014a), and morphologically (Nakaya 1995;
Kajiura 2001; Kajiura et al. 2003; McComb et al. 2009). Although there was a petition to list
the great hammerhead on the US Endangered Species List, listing was not found to be war-
ranted (Miller et al. 2014). On the other hand, their vulnerability to anthropogenic stressors
such as sheries bycatch (Dudley & Simpfendorfer 2006; Zeeberg et al. 2006) and reported
wide-spread population declines, led to an ocial global listing of ‘Endangered’ by the IUCN
Red List (Denham et al. 2007). Despite their widespread distribution, relatively little is known
about their migratory patterns or habitat use across their life history. Presently, the vast major-
ity of what is known about hammerhead shark biology and ecology is based on studies on the
scalloped hammerhead shark (Sphyrna lewini). is is probably due to its gregarious behavior
at a limited number of predictable locations worldwide (Klimley & Nelson 1981; Klimley et al.
1988; Hearn et al. 2010; Hoyos-Padilla et al. 2014; Ketchum et al. 2014). Less is known about the
great hammerhead (Sphyrna mokarran), the largest of the hammerhead species which reaches
a maximum length of over six meters.
e great hammerhead is considered a nomadic and migratory coastal-pelagic/semi-
oceanic species (Compagno 1984; Queiroz et al. 2016). ere is consequently a paucity
of information on its habitat utilization and behavior. To our knowledge, there have only
been three published telemetry-tracking studies focusing on the movement of this species
(Hammerschlag et al. 2011b; Graham et al. 2016; Queiroz et al. 2016). Hammerschlag etal.
(2011b) documented a range extension for this species in the Atlantic Ocean based on a
female shark that migrated from the Florida Keys to north of the mid-Atlantic. In addition
to providing data corroborating this range extension, satellite tracking of 18 great hammer-
heads tagged in Florida by Graham et al. (2016) revealed all of their Core Habitat Use Area
(CHUA) fell within the combined waters of the Florida and US Exclusive Economic Zones
(EEZ). However, Queiroz et al. (2016) found that this species makes repeated movements
into the Atlantic Ocean associated with use of frontal zones where they are targeted by
commercial longline sheries. Much remains to be learned on the habitat use of this species,
especially in shallow inshore environments.
While this species is primarily found over continental shelves, island terraces, and deep coral
reefs (10–30 m, Compagno 1984; Compagno et al. 2005), it is known to occupy in-shore habitats
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MARINE AND FRESHWATER BEHAVIOUR AND PHYSIOLOGY 3
to feed on elasmobranchs such as stingrays. Strong et al. (1990) documented a natural predation
by S. mokarran on a southern stingray (Dasyatis americana) in 6 m depth of water, located 12km
east of North Bimini Island, Bahamas. Similarly, Chapman and Gruber (2002) documented
predation by a great hammerhead on a spotted eagle ray (Aetobatus narinari). is event took
place within a pass between two of the northernmost islets of small cays approximately 5km
south of South Bimini Island, Bahamas. Although the pass was fringed with re coral, creating
depths of less than 1 m, the predation event was conned to the pass, which dropped steeply
to a uniform depth of 3 m. Our own observations, combined with anecdotal evidence suggest
that great hammerheads may enter even shallower inshore habitats (<1.5 m depth) such as tidal
ats to feed, but no published data exist to substantiate these behaviors. It has been documented
that it may be benecial for elasmobranchs to forage in warm waters but return to cooler waters
to rest (Bernal et al. 2012). However, to our knowledge, there are no published reports of great
hammerhead predatory behavior within shallow tidal at ecosystems. Further investigations into
shallow water use by otherwise oshore species and potential mechanisms driving such behavior
could augment our understanding of how species tolerate environments with potentially lower
dissolved oxygen, higher temperatures, as well as persistent shing pressure that could render
sharks vulnerable to stranding, exhaustion, and capture.
In the present study, we used a combination of direct observation, discussions with
shers/stakeholders, and analysis of digital media generated by local shers/stakeholders
to describe new aspects of the natural history of the great hammerhead with a focus on
foraging within inshore shallow water at environments (i.e. seagrass ats and back reef
ats, <1.5 m depth). e information adds to our knowledge of the behavioral ecology of
this species and potentially provides avenues leading to future research.
Methods and results
Below, we describe six instances of inshore shallow water at (<1.5 m) habitat use by
S. mokarran, composed of our own personal observations, rst-hand reports gathered
from various stakeholders, and analysis of video obtained from local shing guides and
their clients. We used various forms of communicative instruments including social media
outlets, and in-person open-ended interviews with various stakeholder groups that spend
large amounts of time within tropical and sub-tropical inshore at ecosystems. Once a case
was found to be potentially pertinent to this study, we contacted the individuals who wit-
nessed the events, subsequently conducting more interviews to obtain supplementary data
on environmental parameters and details of the respective ats habitat. e data presented
below was based on opportunistic observations derived from dierent sources. e level of
detail and available information therefore varies by case. Additionally, more information
was attainable post hoc from our analyses of video compared with still photographic images.
Case 1
On 6 May 2012 at 14:30, study author Robert Roemer (RR) observed from a ~150 m dis-
tance while wading and shing for bonesh (Albula vulpes), a great hammerhead shark
inside a shallow water at on the east side of Rock Sound, Eleuthera Island, Bahamas (near
Poison Point, 24°49′0.01″N −76°11′59.99″W). e single individual was identied by its
distinctive sickle-shaped large dorsal n protruding entirely out of the water column. It
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4 R. P. ROEMER ET AL.
had an estimated total length (TL) of 2.5 m. e water depth the shark was occupying was
~0.7 m. is location contained shallow areas such that bonesh were observed with their
caudal ns and dorsal ns partially-to-fully exposed while likely feeding on benthic prey
items (‘tailing’, Crabtree et al. 1998). RR observed the shark move sinuously forward with
wide sweeps of its cephalofoil. is calm, non-erratic behavior was repeated within the at
habitat, for a total distance of roughly 15 m, and a period of 10min until the shark was
presumably alarmed and darted away, not to be seen again (a distance of ~80 m between
RR and the shark). At the time of observation, the at environment was large and relatively
uniform with water depth varying from 0.2 to 0.9 m during a low incoming tide. e ben-
thic habitat comprised various hard and so coral species, including rose coral, Manicina
aerolata and shallow-water starlet coral, Siderastrea radians. e following species of tele-
osts and elasmobranchs were also identied in the area both before and aer the shark was
observed: scrawled cowsh (Acanthostracion quadricornis), bonesh, yellow n mojarra
(Gerrescinereus), neonate and juvenile lemon sharks (Negaprion brevirostris), and several
species of stingrays (Dasatidae spp.).
Case 2
On 10 March 2014 at 10:22, a great hammerhead roughly 2.5 m TL was observed at a dis-
tance of less than ~8 m by Matthew Glaze, and was rst spotted at an approximate distance
of 270 m. Wave action was less than 0.3 m, on a sunny day with prevailing winds less than
5 knots and water clarity of 16 to 18 m. e lone hammerhead was occupying a shallow
back-reef in the Republic of Palau, on the south side of German and Lighthouse Channels
(7°17′15.7″N 134°27′46.9″E) during a low incoming tide. e approximate depth of the
occurrence and back-reef habitat was between 0.8 and 1.3 m with uniform benthic contour
for 450 m. e benthic substrate consists of various hard coral rubble created from constant
wave action and as a result is quite barren. It has minimal ora or fauna with the exception
of a fairly common presence of bubble algae, (Valonia spp.) and nominal reef teleosts. e
shark was rst sighted when a bait ball composed of sardines (genus Sardinella or possibly
Sardinops) was being actively predated on by a giant trevally, Caranx ignobilis (Figure 1(A)–
(C)). is feeding activity appeared to attract the great hammerhead from a distance of ~20
m. As it moved through the shallow reef at habitat, it performed erratic movements, and
employed rapid surges of speed in its attempts to feed on the sardine bait ball. e right
ank of the shark had numerous scarring/wounds. Also present in close proximity to the
bait ball were two nurse sharks (Ginglymostoma cirratum) and a spotted eagle ray (Aetobatus
narinari). Aer a total time of 15min, the hammerhead moved slowly into a deeper nat-
ural channel on the reef at (Figure 1(D)) and eventually into deeper waters o the reef
edge aer which it was not observed again. Other teleosts present on the reef at habitat
were houndsh (Tylosurus crocodilus), parrotsh (Scaridae spp.), wrasse (Labridae spp.),
butterysh (Chaetodontidae spp.), and angelsh (Pomacanthidae spp.). While the number
of anthropogenic disturbances is low on the reef at habitat, this area is subject to large,
direct discharges of raw sewage from Palau’s largest city, Koror. Information from this case
and associated photo documentation was provided to Matthew Glaze aer RR performed
a search on social media for professional photographers that have recorded hammerheads
in shallow water environments.
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MARINE AND FRESHWATER BEHAVIOUR AND PHYSIOLOGY 5
Case 3
On 30 January, 2015 at 16:15 during high-slack tide, Lorna Scribner (assistant lab manager
of the Bimini Biological Field Station) and John Rayeld (scientic volunteer) witnessed a
great hammerhead in South Bimini Island, Bahamas (25°41′51.4″N 79°17′28.4″W) within a
shallow water at environment. e backwater at was lined with mangroves and relatively
sheltered with an estimated average depth of 1.2 m and a substrate consisting mainly of
turtle grass (alassia testudinum). e male shark had a TL of approximately 2.5 m and
was rst observed from a distance of 10 m. e shark was seen swimming erratically with its
dorsal n protruding from the water column (Figure 2 (A)–(B)). It was probably in pursuit
of a spotted eagle ray, which breached out of the water column most likely to escape the
shark trailing close behind. e air temperature during observation was 21.6°C and the
sky was overcast. is evidence was acquired from John Rayeld aer several photographs
were posted and identied on his social media site. We subsequently followed up with the
observer to gather information relating to this case, including photographic documentation.
Case 4
On February 11th, 2011 at 10:45, a great hammerhead (estimated 2.0 m TL) was docu-
mented in South Andros Island, Bahamas, near Leaf Cay (23°44′31.1″N 77°51′10.8″W) on
the western end of the island. e observation was made by Andy Dober who was on a
guided y-shing ats expedition for bonesh. is shark was observed in a shallow water
Figure 1.(Colour online) A great hammerhead in the Republic of Palau: (A) lateral view of animal skimming
coral substrate; (B) dorsal fin of the same individual protruding almost entirely out of the water, fin may
appear dried due to extended heat exposure; (C) top-down view of animal turning on minimal radius; (D)
underwater view of animal showing ventral proximity to the back-reef habitat. Published with permission
of the copyright holder, Matthew Glaze.
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6 R. P. ROEMER ET AL.
sand at with an estimated depth of less than 0.9 m. e great hammerhead was initially
attracted by disturbances caused by a struggling bonesh which was hooked on y rod/reel
gear (Figure 3(A)). A lemon shark was also actively pursuing the hooked bonesh. Aer
17-s into the video evidence, the hammerhead had isolated the lemon shark, which was
still following the hooked bonesh. While in pursuit of the lemon shark, the hammerhead
exhibited rapid bursts of speed while performing several successive tight turns. At the 22-s
mark in the video, the great hammerhead executed a fast, straightforward motion in an
attempt to close with the lemon shark (Figure 3(B)–(D)). At this point, the hammerhead
made contact with the vessel, which appeared to cause it to abort its pursuit of the lemon
shark and slowly move o the at to deeper waters. Andy Dober provided video and envi-
ronmental information of this event aer RR interviewed several ats shing guides in the
Bahamas, then contacted AD.
Case 5
During the week of November, 25th 2014, y-shing guide William Benson witnessed and
recorded a great hammerhead while guiding a shing client. e hammerhead was situated
on a shallow at comprised mostly of so sediments o the island of Key West, Florida,
in close proximity to the Northwest Channel (24°35′29.8″N 81°56′17.8″W). e estimated
depth of the at was 0.9 m while the shark had an approximate TL of 2.1 m. e shark was
rst noticed because of the considerable amount of sediment it had disturbed during its
predation on a school of permit, Trachinotus falcatus, which were using the shallow at
to feed (Figure 4(A)–(D)). During the observed sequence, the shark exhibited rapid burst
swimming while also displaying several successive tight turns uctuating between one-half
and one-third of TL (Figure 5(E)–(H)). is was similar to the behavior observed in the
great hammerhead within Case 4. ese burst swimming events were rst observed 07-s
aer initial detection of the shark and continued throughout the entire observed episode
(24-s). Video evidence and environmental conditions were compiled and found to be per-
tinent following contact, a meeting, and an interview with William Benson.
Figure 2.(Colour online) (A and B). A single great hammerhead in South Bimini, Bahamas, exploiting a
mangrove lined seagrass flat environment approximately 1.2 m in depth while hunting elasmobranch
species. Published with permission of the copyright holder, John Rayfield.
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MARINE AND FRESHWATER BEHAVIOUR AND PHYSIOLOGY 7
Figure 3.(Colour online) Still frames pulled from video showing a hammerhead in pursuit of a lemon shark,
Negaprion brevirostris, in South Andros Island, Bahamas. (A) The individual great hammerhead orientates
towards the lemon shark. (B-D) The great hammerhead attacks the lemon shark. Arrows illustrate the
lemon shark during predation event. Published with permission of the copyright holder,Andy Dober.
Figure 4.(Colour online) Frames taken from video depicting a great hammerhead using a shallow water
flat in order to prey upon a school of permit, Trachinotus falcatus. (A) Initial observation of hammerhead.
(B) Sediment cloud in relation to individual hammerhead. Arrow details fin orientation at time of
initial observation in relation to size of sediment cloud. Published with permission of the copyright
holder,William Benson.
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8 R. P. ROEMER ET AL.
Case 6
On 22 March 2010 at 12:00, a great hammerhead shark of an estimated 3.65 m TL was
observed exercising erratic burst swimming behavior on a shallow at habitat with a mainly
sand substrate (Figure 6(A)) o the northwest corner of the Marquesas Islands
(24°35′28.1″N 82°09′00.1″W). William Benson was guiding a shing client (Gannon
Dudlar) and documented the entire sequence, which was provided to RR. e at habitat
was approximately 1.2 m deep and prevailing wind speeds averaged 15 knots. e tide was
outgoing, almost low-slack and the day sunny. e shark was swimming in a straight line at
the time the vessel approached. It exhibited a sudden and rapid burst of speed that propelled
it nearly 3.35 m (3min into the video observation). e shark then located and attacked a
nurse shark (~0.9 m in total length), situated on the benthic substrate (Figure 6(B)–(D)).
During the attack, the shark exhibited 22 circular turns at high speed within a tight radius
(Figure 7(A)–(B)). e great hammerhead continued to display tight rotations with limited
radii, (estimated to be one-half of TL) with the nurse shark still largely protruding from its
jaws (Figure 7(D)). Aer the nurse shark was rmly oriented within the jaws of the shark, the
great hammerhead established itself so that it was facing anteriorly into the current (Figure
8(A)–(B)). e hammerhead remained facing steadily into the current with the caudal n
of the nurse shark still projecting from its jaws. e shark propelled itself minimally so as
to remain stationary and did so for a span of 15min, undisturbed by the close proximity
of Benson and the vessel. Aer this period, the hammerhead moved slowly away from the
vessel. William Benson and Gannon Dudlar provided environmental conditions and video
evidence to RR and in interviews with WB and correspondence with GD.
Figure 5.(Colour online) Frames (E-H) detailing example of tight turning radius exhibited by the feeding
activity of the great hammerhead. Hammerhead dorsal fin labeled ‘i.’, and caudal fin labeled ‘ii.’ to help
deduce orientation. Published with permission of the copyright holder,William Benson.
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MARINE AND FRESHWATER BEHAVIOUR AND PHYSIOLOGY 9
Figure 6.(Colour online) (A) The initial observation of the estimated 3.65 m great hammerhead before a
predation event in Marquesas Islands. (B-D) The predation event on prey later identified as a nurse shark,
Ginglymostoma cirratum. Published with permission of the copyright holder, William Benson.
Figure 7.(Colour online) (A and B) Tight turn radius (roughly one-half of total body length) utilized by
a hammerhead in order to capture prey. (C) Distinguishable caudal fin of a nurse shark, Ginglymostoma
cirratum protruding from the jaws of the hammerhead. (D) ‘Grasp-Turning’ technique of hammerhead
with prey still protruding from its jaws. Dorsal fin labeled ‘i.’ caudal fin labeled ‘ii.’ and nurse shark labeled
‘iii’, respectively. Published with permission of the copyright holders, Gannon Dudlar and William Benson.
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10 R. P. ROEMER ET AL.
Discussion
Here we have documented and described six instances of inshore, shallow water (<1.5
m depth) habitat use by great hammerhead sharks. ese occurred on tidal ats within
various tropical and sub-tropical locations across the globe. ese six cases appear to be
instances of prey searching and hunting. Five of these cases involved erratic behaviors by a
single individual in pursuit of elasmobranch or teleost prey. An exception to this was Case
1, where the observed hammerhead was calm, using wide sweeps of its cephalofoil. We
suggest that this represents prey searching behavior before the shark had located a prey
item. e observations are noteworthy for the environment in which they occurred because
this species is characterized as a coastal-pelagic species, primarily occupying oshore areas
(Compagno et al. 2005).
Inshore shallow water habitats in tropical and sub-tropical marine ecosystems contain
high species richness and abundance and are oen regarded as important to the devel-
opment of numerous small and medium-sized teleost shes and elasmobranchs (Chong
et al. 1990; Blaber et al. 1995; Beck et al. 2001). Tidal at ecosystems such as tropical and
sub-tropical inshore shallow water ats are protected, provide shelter, and, are more dicult
for larger predators to access. ey have consequently been considered to be successful
nursery grounds (Reise 2012). ese ecological features can make inshore habitats valuable
hunting and foraging grounds for large predatory shes such as sharks (Hammerschlag
etal. 2010). Entrance into these habitats may, however, incur several costs to predators
due to tidal uctuations that can result in temporary periodicity of low dissolved oxygen
levels (Sakamaki et al. 2006) and other water-quality stressors within inshore tropical shal-
low waters (e.g. raw sewage euent in Case 2). As a result, predators using these habitats
may balance trade-os between potential environmental stressors and resource abundance,
thereby requiring specialized foraging or swimming techniques to exploit these environ-
ments. Two behaviors were identied during our video analyses that may be specializations
permitting great hammerheads to balance these trade-os and use shallow tidal ats in
pursuit of prey. ese are: (a) prey-handling behavior (Case 5, 6); and (b) a post-predation
energetic recovery behavior (Case 6).
Specialized prey handling behavior of a great hammerhead was rst described in detail by
Strong et al. (1990). During this observation, a great hammerhead (~3.0 m TL) utilized its
Figure 8.(Colour online) (A and B) Details the great hammerhead positioning itself, anteriorly into the
current defining a possible ‘recovery period’ of the shark post-predation. The nurse shark is labeled ‘i.’ for
easier perspective. Published with permission of the copyright holder,William Benson.
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MARINE AND FRESHWATER BEHAVIOUR AND PHYSIOLOGY 11
cephalofoil to pin a southern stingray, Dasyatis americana to the substrate (termed ‘pin and
pivot’). A similar behavior was observed by Chapman and Gruber (2002) of a single great
hammerhead (~3.6 m) capturing and consuming a spotted eagle ray. While both predation
events occurred in deeper water than reported in our study, they support the interpretation
that our sequences represent examples of specialized foraging behavior.
We documented ve cases where a great hammerhead grasped or attempted to grasp
teleost and/or elasmobranch prey in its jaws. In three of the documented cases, the ham-
merhead made multiple (up to 22) tight turns before the prey was consumed. We term this
behavior ‘grasp-turning.’ is may be a technique that allows hammerheads to use the force
exacted by the surrounding water to help keep prey within their mouths and maneuver
prey within their jaws to facilitate consumption (i.e. head down swallow). Performance of
‘grasp-turning’ behavior presumably provides the predatory shark with a tactical advantage
over the prey within the vertically restricted space of a tidal at.
e use of shallow water and performance of grasp-turning behaviors incur metabolic
and homeostatic costs for a large-bodied elasmobranch. Previous research has demonstrated
a rapid onset of anaerobic acidosis in exercise-stressed (sheries capture) great hammerhead
sharks (Gallagher et al. 2014c). While these behaviors may also be occurring in deeper
waters, metabolic costs and challenges to respiration from increased activity are likely higher
in shallow inshore waters that are oen warmer, more saline, and possess lower dissolved
oxygen concentrations (Belding 1929; Kitheka 1997; Meyer-Reil and Köster, 2000; Diaz
2001; Ridd & Stieglitz 2002; Hodoki & Murakami 2006). Our observations suggest, how-
ever, a potential mechanism for recovery from exercise stress in a sub-tropical inshore at
environment. e great hammerhead from the Marquesas (Case 6) re-positioned itself into
a strong, incoming current while propelling itself at a minimal rate to remain stationary for
15min. is likely maximizes oxygen uptake in the gills and promotes recovery from energy
expenditure and anaerobic acidosis during prey pursuit (Figure 8(A)–(B)). is action may
also help to further facilitate consumption: the high velocity water current keeping the nurse
shark situated within the jaws of the great hammerhead while consumption continues.
During exercise in teleosts, increased water ow over the gills is proportional to oxygen
uptake, even increasing in low-oxygen environments such as those of shallow tropical and
sub-tropical inshore waters (Randall 1982). Aer exercise, gill ventilation and water ow
interactions with the gills have eects on the blood respiratory and blood acid–base status
(Perry & Wood 1989). Teleosts remaining in low oxygen, elevated temperature environments
(like those of inshore tidal at ecosystems) are hindered from the respiratory gill ventila-
tor method and exhibit impaired recovery of tissue ATP and plasma glucose (Suski etal.
2006; Shultz et al. 2011). ey would therefore experience more acute need for recovery
from exercise. Benthic tropical elasmobranchs capable of stationary buccal pumping (i.e.
epaulette shark, H. ocellatum) can tolerate mild hypoxic environments and have a well-de-
veloped capacity for anaerobic metabolism (Wise et al. 1998). Bonnethead sharks (Sphyrna
tiburo) and blacknose sharks (C. acronotus) increase mouth gape, swim speed, and oxygen
consumption rates during hypoxic conditions (Carlson & Parsons 2001, 2003). e ham-
merhead behavior described in Case 6 may have served a similar function as those found
by Carlson and Parsons (2001, 2003). Although water quality parameters were not collected
in Case 6, it is likely great hammerheads occupying warm shallow waters will encounter
relatively lower dissolved oxygen by comparison with colder oshore waters. An animal’s
behavior is directly driven by its physiology (Ricklefs & Wikelski 2002), which is in turn
Downloaded by [McMaster University] at 11:33 29 April 2016

12 R. P. ROEMER ET AL.
driven by its environment. Based on the metabolic responses of great hammerheads during
exercise as well as water quality characteristics typical of shallow tropical and sub-tropical
tidal ecosystems, the observed great hammerhead behavior that we attributed to post-pre-
dation recovery is perhaps predictable.
Hammerhead sharks are a functionally, physiologically, and behaviorally specialized
group of species (Gallagher et al. 2014b). Here we provide a series of detailed opportunis-
tic observational accounts that provide insights into the habitat use and foraging behavior
of this species. We speculate that use of shallow tidal ats by great hammerhead sharks is
likely common and an important aspect of their overall feeding ecology. We also describe
several behaviors that we suggest are specializations that allow them to compensate for the
costs of foraging in shallow environments. is study also highlights the value of utilizing
natural history, social media, observational science, and local knowledge as a means for
advancing our understanding of a species that is dicult to study.
Acknowledgements
e authors would like to thank William Benson, Andy Dober, Gannon Dudlar, Matthew Glaze, John
Rayeld, and Lorna Scribner for their assistance in acquiring observational evidence. e authors
would also like to thank Rob and Susan Roemer for assistance with this publication.
Disclosure statement
No potential conict of interest was reported by the authors.
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