Body Markings of the Whale Shark: Vestigial Or Functional?

BODY MARKINGS OF THE WHALE SHARK: VESTIGIAL OR FUNCTIONAL? By STEVEN G. WILSON Department of Zoology, The University of Western Australia, Nedlands, WA 6907, Australia *Current address: Department of Zoology, University of New Hampshire, Durham, NH 03824, USA and R. AIDAN MARTIN ReefQuest Centre for Shark Research, P.O. Box 48561, 595 Burrard Street, Vancouver, BC V7X 1A3, Canada ABSTRACT The whale shark’s distinctive body markings are similar to those of other orectolobiform sharks. These markings likely conceal their sluggish, bottom-dwelling relatives through disruptive colouration. It is argued here that the whale shark’s body markings similarly function to camouflage them in their pelagic environment. The whale shark’s countershaded colouration eliminates the optical appearance of relief against its visual background. Disruptive patterns resembling elements common in its environmental background break up the whale shark’s outline. Other possible functions for the whale shark’s markings are considered, including: radiation shielding, intraspecific communication (species recognition, sex recognition, postural displays, schooling coordination), and interspecific communication (aggressive mimicry). However, they are either discounted or evidence substantiating these functions is found to be lacking. INTRODUCTION globe (Compagno 1984; Colman 1997; Eckert and Stewart 2001), Prior to 1986, there had been including Ningaloo Reef, Western only 320 reported sightings of Australia. Consequently, there is the whale shark, Rhincodon increasing research interest into typus (Orectolobiformes, the biology of whale sharks. In Rhincodontidae), worldwide addition to being the world’s (Wolfson 1986). Today they can be largest living fish and possessing a reliably encountered and studied distinctly unique body form, one in several locations around the 118 of the whale shark’s most striking relevance (Myrberg 1991). In features are its body markings, the essence, logical inference and function of which has only been circumstantial evidence can be briefly speculated about in the meaningfully applied to the literature. The purpose of this interpretation of morphology. paper is to review the possible Such intuitive methods can relevance of these markings to the provide ethologists with testable life history and biology of the hypotheses that potentially whale shark. explain a suite of observations Despite their being among the under a logically consistent most abundant large animals on theoretical model. However, earth, our knowledge of the interpretations divorced from behaviour of sharks in the wild is their normal environmental con- almost non-existent. Myrberg text (including social dynamics, (1976; 1991) and Gruber and prey behaviour, oceanographic Myrberg (1977) have recognised the conditions, etc.) should be applied difficulties arising from studies of cautiously until such time that sharks in captivity and in the they can be supported by direct field. For most species, captive observation and experimentation. environments cannot be created to adequately simulate natural settings to ensure ‘natural’ DISCUSSION behaviour. Field studies are often even less favourable. Many species BODY MARKINGS are fast-moving or far-ranging, Dorsally, the whale shark’s basic while some are dangerous to colouration is blue, grey, or observers (Johnson and Nelson brown, while ventrally it is white 1973; Myrberg et al. 1972), leading to (Last and Stevens 1994). Overlying logistical and methodological this dark dorsal background is a constraints that combine to make distinctive checkerboard pattern the cost and effort of research composed of pale spots, vertical prohibitive (Myrberg 1991). As a bars and horizontal stripes (Figure result, scientific knowledge of the 1). At birth, all orectolobiform behaviour of sharks lags decades sharks have patterns of bars, often behind that known about large in the form of wide bands or terrestrial animals. saddles, and spots. However, in As these problems will likely most species these markings fade persist into the foreseeable future, or change with age (Dingerkus alternative ways of interpreting 1986). With the exception of the the adaptive roles of shark whale shark, all orectolobiform structures and features must be sharks are primarily benthic. considered. One accepted Benthic or bottom-dwelling approach to the study of shark sharks often possess bold body behaviour is based strictly on markings that most likely provide anatomical considerations, sup- camouflage through disruptive plemented with inference about colouration (Bass 1978). Within the their functional and behavioural order Orectolobiformes, 119 Figure 1. Adult whale shark, TL ca. 10.0 m. Figure 2. Neonate whale shark, TL ca. 0.58 m. 120 Stegostoma is regarded as the dasypogon, less than1mTLare primitive sister taxon of orange-brown in colour and Rhincodon (Dingerkus 1986; typically inhabit water shallower Compagno 1988). The zebra shark, than 10 m and are thus often Stegostoma fasciatum, is thus the encountered by divers. Larger whale shark’s closest extant individuals become increasingly relative. Juveniles possess vertical pale, the largest at 3 to4mTLare yellow bars and spots that break yellowish-white, and typically the background colouration into inhabit ever deeper water as they dark brown saddles (Compagno grow (down to a depth of at least 1984). When zebra sharks are 40 m) (R.A. Martin, pers. obs.). between 50 and 90 cm TL, these Similarly, young nurse sharks, saddles break up into small spots, Ginglymostoma cirratum, are which become more evenly spaced pinkish brown with small brown as the animal grows. or blue spots, while adults are a uniform chocolate brown and ONTOGENETIC CHANGE IN typically inhabit much deeper BODY MARKINGS water than juveniles (Carrier 1991; R.A. Martin, pers. obs.). Whale The ontogenetic change in sharks are born with markings pigmentation pattern in most similar to those of adults (Figure orectolobiform sharks raises some 2). Since whale sharks are not important issues. The change may thought to change their pelagic be related to the different habitats habitat substantially from used by neonates/ juveniles and neonate to adult, there may be no adults. Neonate/juvenile zebra selective pressure to change their sharks seem to live primarily in pigmentation pattern as they age. water deeper than about 50 m (R.A. Martin, pers. obs.). Such Cott (1940) uses the ‘ontogeny depths may represent a relatively recapitulates phylogeny’ argu- predator-free refuge for the young ment to account for the presence and this pattern partially explains of stripes and spots in the young why juveniles are so rarely of many open country predatory collected, and even more rarely cats, including as lions, pumas and seen by recreational divers, lynxes. The functional relevance compared with adults. The bold of these markings is questioned in zebra-pattern may function as cubs that are sheltered in dens or disruptive colouration, visually holes. Cats inhabiting wooded concealing the pups from surroundings, such as leopards, potential predators by breaking pumas and ocelots, generally up their bodies into a series of retain or intensify these patterns irregular shapes against the in adulthood. Cott (1940) suggests backdrop of benthic cover. The that stripes and spots may adults of this species are most represent a primitive pattern in commonly encountered by divers cats, accounting for their presence in sandy areas around reefs at in the young of open country depths of less than 30 m. Young species on ancestral rather than ecological grounds. This rather tassled wobbegongs, Eucrossorhinus 121 rigid application of Haeckel’s Law sharks observed at Ningaloo Reef could also be applied to the exhibit healed bite marks and/ or presence of bars and spots in have pieces missing from their orectolobiform sharks that lose fins (S.G. Wilson, pers. obs.). these patterns. But it raises the Neonate whale sharks have been question of why these markings found in the stomachs of a blue are retained in adult whale sharks. shark, Prionace glauca (Kukuyev 1996), and a blue marlin, Makaira VESTIGIAL mazara (Colman 1997). Are the whale shark’s distinctive To rapidly reach a size large pattern of body markings vestigial enough to avoid predation, many with no modern function? Their sharks put their energy into pigmentation pattern bears evi- somatic growth, delaying repro- dence to its phyletic relationship duction until relatively late in life with similarly marked benthic (Stevens and McLoughlin 1991). ancestors. However, the whale Whale sharks reach sexual shark is a pelagic fish that would maturity when ca. 9.0 m TL and not be viewed against a benthic aged in their late 20s (Wintner substrate. In classical Darwinian 2000). A female whale shark natural selection, retention of a harpooned off the coast of Taiwan characteristic would be favoured in 1995 contained 301 embryos if it enhances survival and/ or (Joung et al. 1996), more than reproductive success or is at least double the number of embryos not deleterious in any way. reported in any other species of Alternatively, do the whale shark. Such high fecundity may shark’s body markings reflect an represent a mechanism to com- adaptation to its modern pelagic pensate for the delay in repro- lifestyle? The examination of this duction made to achieve a large question forms the substance of size. Large litters would also offset the remainder of this discussion. high neonate and juvenile mortal- Other possible explanations

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