Depending where you are in the world, you might be very confused about what I am talking about if I said the name zebra shark... In many parts of the world Stegostoma tigrinum is more commonly known as the leopard shark because of it's beautiful yellow and black spotted skin. So how on Earth did this shark get its counterintuitive common name? Who would ever think it look liked a zebra? Well the answer lies in how these sharks change as they grow ...
From Zebra Stripes to Leopard Spots
The term zebra shark exists because these sharks look very different at different life stages. At birth zebra sharks have vertical yellow stripes, which break up dark dorsal saddles in a distinctive zebra-striped pattern. Yet, when these juveniles reach a size between 50 - 90 cm total length, the dark saddles begin to break up into spots, revealing more yellow pigmentation in a spotty-stripe pattern. With advancement into maturity, continued change leads to a yellow background with darker spots of pigmentation, like a leopard. This is known as an "ontogenetic change" - a change which comes naturally during a particular life-stage of an organism (Compagno, 1984; Dahl et al, 2019).
Stripes May Protect Young Zebra Sharks from Predators
But why on Earth do zebra sharks change so much? Radical ontogenetic changes in pigmentation patterns are very common amongst carpet sharks (Order Orectolobiformes), which the zebra shark is a part of. It is not yet known for certain why this occurs (Dahl et al, 2019; Ebert et al 2021).
Some scientists think that zebra sharks may wear their stripes as youngsters, so that they are easily recognisable as a low threat to other types of territorial fish (Dahl et al, 2019; Ebert et al 2021).
Some think the stripes may confuse predators. When the small sharks form a group in their "nursery habitat", the stripes would make it more difficult to pick one out from the crowd. This is known as the "predator dilution effect". If you look at a group of zebras in the zoo, you can see how the stripes can be quite confusing (Dahl et al, 2019; Ebert et al 2021).
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Scientists also think the colours may protect young zebra sharks from predators through "Batesian mimicry". Because the black and white stripes look similar to many venemous sea snakes, predators may avoid eating the small sharks because they look dangerous Dahl et al, 2019; Ebert et al 2021).
On the other hand, some experts also wonder if the change in colours and patterns is a form of communication between zebra sharks - announcing their maturity to the opposite sex. If so, the patterns may play a role in mate choice (Dahl et al, 2019).
A Zebra by Any Other Name...
This species is a perfect example of why scientist use "binomial nomenclature" (some people call this an animal's 'Latin name'). Every species described by science has a two-part name: the first term indicating the genus and the second the species, so that the names tell us something about what group an animal falls into and what other animals it is related to. Every single scientist uses the same name all over the world, in order to reduce confusion about which species is being discussed. You say leopard, I say zebra, but all scientists can recognise Stegostoma tigrinum.
Because of their ontogenetic change in colour and patterns, when zebra sharks were first being described by scientists hundreds of years ago, there was considerable confusion about whether young, sub-adult and mature sharks were the same species. This is understandable given that each life-stage looks so different (Dahl et al, 2019).
Adding to this problem, is that zebra shark pigmentation and patterns can vary enormously between different individuals. These different "colour morphs" created confusion as to whether zebra sharks with varyi (Dahl et al, 2019). colours were actually part of the same species. By 1985 the zebra shark had been known under 15 different scientific names! At this point, a consensus was achieved that the zebra shark would be known as Stegostoma fasciatum (DDahl et al, 2019).
Zebra shark "nomenclature" has actually been contested very recently, after genetic analysis has revealed that a sandy colour morph of the zebra shark (referred to as Stegostoma tigrinum), is in fact the same species as the traditional striped morph S. fasciatum. The sandy zebra shark has subtly different pigmentation patterns to other members of it's species in all three life stages and so it was originally described as a separate species, but the DNA has proved they are one and the same (Dahl et al, 2019).
As this paper was published before the article describing the traditional zebra shark colour morph, Stegostoma fasciatum has now been reverted to Stegostoma tigrinum, to respect the scientists who described these animals first (Dahl et al, 2019).
It's enough to make your head spin! But whatever their name, zebra sharks are undeniably beautiful at every stage of their lives. A shark by any other name would still be as sweet, after all!
References
Compagno LJV (19841). Sharks of the World: An annotated and illustrated catalogue of Shark species known to date. Volume 2. Bullhead, mackerel and carpet sharks (Heterodontformes, Lamniformes and Orectolobiformes). FAO Species Catlogue, Rome. Access online.
Dahl RB, Sigsgaard EE, Mwangi G, Thomsen PF, Jørgensen RD, de Oliveira Torquato F, Olsen L & Møller PR (2019). The sandy zebra shark: A new color morph of the zebra shark Stegostoma tigrinum, with a redescription of the species and a revision of its nomenclature. Copeia, 107:3, 524–541. Access online.
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
By Sophie A. Maycock for SharkSpeak
