Variables e instrumentos

The crown morphology of the denticles at the fin tip (area C), the anterior margin (area D), the

posterior margin (area E), and the free rear tip (area H) were found to differ markedly. This is likely

to be associated with the varying hydrodynamic forces acting on the fin during swimming.

The fin tips (area C) of both dorsal and pectoral fins are characterised by highly overlapping

denticles, with more extensive ridging occurring in some species. For pectoral fins, both bi-cresting

of the primary ridge and the prominence of denticles with a large primary ridge is a common feature

in this region. Bi-crested ridges are hypothesized to be a hydrodynamic adaptation that produces

mixing vortexes in areas of adverse pressure gradients, thus reducing drag (Raschi & Tabit 1992,

Wagner 1996). Furthermore, turbulent flow is strongest at the distal tips of wings, called a ‘wing-

tip’ or ‘fin-tip’ vortex. This phenomenon creates increased drag at the fin tip, which increases with

swimming the denticles in this area show modifications for drag reduction such as imbrication,

increased ridging, and bi-crested primary ridges (e.g.Carcharhinus plumbeus).

The leading edge (area D) of both dorsal and pectoral fins is characterised by thicker,

abutting (closely associated) denticles and denticle crowns with greatly reduced ridges and cusps.

The boundary layer, created by the flow of water across the fin during swimming, is thinnest at the

leading edge of the fin. Consequently, there is less drag in this area and modifications that reduce

frictional drag, such as ridges and cusps, are reduced (Raschi & Tabit 1992). Crown smoothness is

also attributed to protective-type denticle morphology (Raschi & Tabit 1992). As the leading edge

of the fin would be subject to higher mechanical wear, the need for protection at this area would

also influence crown morphology.

The trailing edge of the fin (area E) of both dorsal and pectoral fins is characterised by

highly imbricated (overlapping) denticles and the prevalence of denticle crowns with extensive

ridging, obvious cusps, and microrelief. This area is subject to increased drag resulting from

boundary layer separation effects (Moore 1953). To combat drag in this area the denticle crowns

have more developed ridges and cusps that increase surface area and, consequently, increase surface

turbulence close to the skin. This results in highly efficient mixing in the boundary layer (close to

the skin), reducing boundary layer thickness and preventing separation of the layer at the posterior

edge of the fin (Burdack 1973, Bushnell & Moore 1991, Raschi & Elsom 1986). Cusps are

structures that are susceptible to damage (e.g. breaking off from the crown) and the high amount of

cusping, ridging, and crown overlap in this area suggests that crown morphology in the trailing edge

of the fin is influenced more by hydrodynamic function than a requirement for protection.

The structure of the free rear tip (area H) differs considerably between dorsal and pectoral

fins, which can be observed in the crown morphology of denticles at these areas. Denticles at the

free rear tip of the dorsal fin are characterised by crowns with extensive ridging and prominent

and cusps. The function of the free rear tip of the dorsal fin may act as a rudder during swimming

and turning. When the shark turns, the free rear tip of the dorsal fin swings back and fourth, which

may break up fin-tip vortices and reduce drag. This function is indicated by the prevalence of

denticles with a crown morphology modified for drag-reduction. Conversely, the free rear tips of

the pectoral fins are commonly tucked alongside the body, rubbing against the dorsal surface of the

shark trunk. As such, denticles in this area require crown morphologies that are streamlined against

snagging, and modified for protection against abrasion.

3.4.2 Functional Aspects of the Fin Denticles of Different Species

The requiem sharks (Carcharhinidae) and hammerhead sharks (Sphyrnidae) are among the faster

swimming taxonomic groups of elasmobranchs (Compagno 1990). This is reflected in the crown

morphologies of the fin denticles, which are specialised for hydrodynamic efficiency. Such

specialisations include longitudinal ridging and highly overlapping, tessellated denticles. Despite

the overall crown modifications for the functional requirement of fast swimming, the investigated

species do not share identical life histories within their taxonomic group. Variations in denticle

crown morphology may therefore correspond to a trade off between protection and hydrodynamic

function, as described by Raschi & Tabit (1992).

The fastest swimming species investigated in this study are likely to be Carcharhinus

falciformis (Silky Shark) and Sphyrna lewini (Scalloped Hammerhead) (Reif 1985a). These species

exhibited crown characteristics that were highly modified for drag reduction such as extensive

parallel ridging, microrelief, and high imbrication (Raschi & Elsom 1986, Raschi & Musick 1986,

Raschi & Tabit 1992). Species that are also considered typically fast swimming, such as

Carcharhinus amblyrhynchoides (Graceful Shark), C. amblyrhynchos (Grey Reef Shark), C. cautus

(Nervous Shark), C. limbatus (Common Blacktip Shark), C. plumbeus (Sandbar Shark), C. sorrah

(Spot-tail Shark), and C. tilstoni (Australian Blacktip Shark), exhibited the above characters to a

The crown morphologies of Carcharhinus amboinensis (Pigeye Shark) and C. leucas (Bull

Shark) differ markedly from all other species investigated. These differences include uneven

denticle spacing, and oblique ridges and cusps. Although the presence of both ridges and cusps is

typically associated with hydrodynamic modification, the uneven ridges and cusps on the denticle

crowns of the fins of these two species may indicate that during swimming these species do not

require as much drag-reduction as other species investigated. This may be due to their life history

patterns as both species are known to spend most of their life in and around estuaries and inshore

environments, the Bull Shark being the only shark investigated that can live in both freshwater and

seawater for extended periods (Last & Stevens 2009, Pillans & Franklin 2004). The diet of the

Pigeye Shark consists of mostly benthic species including elasmobranchs, crustaceans,

cephalopods, and other molluscs (Stevens & McLoughlin 1991), which could indicate a more

benthic foraging lifestyle. Benthic sharks can be more susceptible to mechanical abrasion through

increased likelihood of contact with the substrate and typically show denticle morphologies

modified for protection (Raschi & Tabit 1992).

Carcharhinus plumbeus (Sandbar Shark) exhibits a crown morphology suited to both drag-

reduction and protection. Denticles from the fins of this species possess drag-reduction characters

such as thin, parallel ridges and a high prevalence of bi-crested ridges. Unlike other species

however, the denticles are thicker and often exhibit characters associated with protective function

such as reduced ridge depth and, in particular, the reduction or absence of cusps. The stronger

denticles on the fins of this species may be indicative of a particular requirement for protection

from predators, parasites, or mechanical abrasion due to a benthic foraging lifestyle. Adult

C. plumbeus, like C. amboinensis, are also known to have a diet that includes a wide variety of

mostly benthic species (e.g. benthic teleosts, elasmobranchs, and cephalopods) (McElroy, et al.

2006). However, unlike C. amboinensis and C. leucas, this species is known to undergo extensive

indicative of hydrodynamic function (e.g. thin parallel ridges, bi-crested ridges and tessellated

overlapping crowns) and protective modifications (e.g. thicker crowns and absent or reduced

cusps). Furthermore, a higher prevalence of bi-crested ridges in the denticles of this species may

compensate for reduced cusping by decreasing drag (see section 1.4.1 ‘area C’). The denticles of

C. amboinensis and C. leucas, in contrast, show less parallel ridging and low spacing patterns but

have extensive cusping. Indeed, bi-crested ridges may be a modification for greater drag-reduction

while retaining thicker denticle crowns.

Exhibiting denticle crown morphology that is more adapted for protection is Triaenodon

obesus (Whitetip Reef Shark). This species is typically associated with shallow reef flats and lives

amongst rock and reef formations, showing a narrow home range and strong site fidelity (Robbins

2006). As such, this species would encounter abrasion from benthic structures and would therefore

require morphological adaptations to offset these challenges. This was indeed the case with the

denticle crown morphology of T. obesus. Cusps were absent on all denticles and the denticle crowns

of this species displayed the least ridging of all species investigated.

The denticles of Galeocerdo cuvier (Tiger Shark) exhibited very distinct crown morphology

and could easily be differentiated at all areas on both the dorsal and pectoral fins. This species has

many characteristics that are atypical of the family Carcharhinidae, such as the presence of a caudal

peduncle with lateral ridges, spiracles, and a reproductive mode of aplacental viviparity (Last &

Stevens 2009). The taxonomy of this species is currently being revised to place it in its own

monospecific family (Peter Last, pers com). Consequently, the marked difference in the crown

morphology of the denticles of this species may be due to phylogeny, more so than function.