A Leopard that Changes its Spots
Updated: Jun 15
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 places, Stegostoma fasciatum is 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 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.
But why on Earth does this happen? Radical ontogenetic changes in pigmentation patterns are nearly universal amongst carpet sharks (Order Orectolobiformes), which the zebra sharks is a part of, but it is not yet known why this occurs. It has been hypothesised that zebra sharks may wear their stripes as neonates so that they are easily recognisable as a low threat to territorial fish. It may also confuse predators when the babies form large groups in their "nursery habitats". On the other hand, it has been suggested that the adults use the pigmentation to announce their maturity and that the patterns play a role in mate choice. It is quite a mystery and more work is required for us to be certain.
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 tells 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 we both understand Stegostoma fasciatum.
Because of this ontogenetic change in colour pattern, when zebra sharks were first being described by science there was considerable confusion about whether young, sub-adult and adult sharks were the same species. This is understandable given that each life-stage looks so different!
Adding to this problem, is that zebra shark pigmentation and patterns can vary enormously between individuals (known as different "colour morphs"). This also also created confusion as to whether zebra sharks of different 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.
Yet, zebra shark "nomenclature" has recently been contested. This has arisen because 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. As this paper was published before the article describing the traditional zebra shark colour morph, it is now thought Stegostoma fasciatum should be replaced by Stegostoma tigrinum, to respect the scientists who described it first (Dahl et al, 2019).
It's enough to make your head spin!
Whatever their name, zebra sharks are undeniably beautiful at every stage of their lives. In fact, they are my personal favourite... A shark by any other name would be as sweet, after all!
Compagno LJV (2002). 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.