In the first of our guest articles, Dr Farrell discusses some identification issues surrounding smooth-hound species in north-east Atlantic waters and the work he has carried out in an attempt to resolve these issues.
Species Identification and Biology
The first and main question that always seems to arise when talking about smooth-hounds is “what species is it?” There are many species of smooth-hound around the world but in European waters there are only three: the starry smooth-hound (Mustelus asterias), the common smooth-hound (Mustelus mustelus) and the blackspotted smooth-hound (Mustelus punctulatus), with the latter usually only found in Mediterranean waters. The starry smooth-hound is found in Northern European waters as well as the Mediterranean and as far south as North Africa. The common smooth-hound is believed to have the same distribution except that it is also found in the Atlantic and Indian Oceans off South Africa.
Both starry and common smooth-hounds are relatively small bottom dwelling sharks that feed primarily on crustaceans such as crabs and prawns. Despite the similarities in appearance of starry and common smooth-hounds, the two species are actually quite different, specifically in terms of their reproductive biology. Both are viviparous which means that they give birth to free swimming pups, unlike for example the lesser spotted dogfish which lays egg cases (commonly known by the public as ‘mermaid purses’). However, common smooth-hounds have a placental connection through an umbilical cord to the pups which transports nutrients from the mother to the pups whereas starry smooth-hounds lack this connection and get their nutrition from a ball of yolk attached to their umbilical cord.
This may seem like a subtle difference, but in biological terms it is highly significant. It means that the two species are unlikely to interbreed and also that the life-history characteristics such as growth rate, age at maturity, reproductive capacity and longevity are likely to be very different. This pattern of aplacental white spotted smooth-hounds and placental unspotted smooth-hounds is known to occur in at least 20 other species of smooth-hounds around the world. Simply put it means that species of Mustelus shark are significantly different and should actually be divided into two groups based on their reproductive biology.
Stars or No Stars?
The debate about smooth-hound identification is not a new one. In fact by studying the historical literature the confusion over smooth-hounds in northern European waters can be traced back over 200 years and in the Mediterranean, smooth-hound research dates back to the Greek philosopher Aristotle (350 BC). The early scientists provided vivid descriptions of their specimens and many included incredibly detailed drawings. By reviewing these it is possible to identify where confusion over the species identification originally arose. Most of this confusion is directly attributable to the variation in appearance of the white ‘stars’ of the starry smooth-hound, as is still the case today.
The above image shows the variation in the appearance of white stars/spots on individual smooth-hound: A – vivid stars, B – faint and C – absent. The presence or absence of stars or spots is often used as a feature for identifying smooth-hounds but could these stars be misleading?
Common smooth-hounds never have any white ‘stars’ on their backs or sides but unfortunately the supposedly species-defining white ‘stars’ of the starry smooth-hound vary greatly in appearance and may be vivid, faint and sometimes even absent (Figure 1). This frequently leads to the misidentification of starry smooth-hounds as common smooth-hounds. The presence or absence of the ‘stars’ should therefore not be used as a criteria for identification of common smooth-hounds.
There are other differences between the species, most notably the pattern of ridges on the dermal denticles (teeth-like scales that cover the skin of sharks) and the pattern of denticles inside the mouth but these can only be seen under a microscope (as shown below) or when you cut open the mouth of the specimen. Neither of which are useful for anglers! Another method is to cut open a pregnant female and check which way the pups are developing but this is obviously not something I would advocate and most anglers would not even conceive of doing something like this, which is why a simple robust genetic method was needed which could be applied to fin clips taken by anglers from live ‘catch and release’ specimens.
This image shows the dermal denticles of smooth-hound observed under a microscope. Image A shows the denticles of the starry smooth-hound which have a clear ridge all the way to the tip; image B shows the denticles of a common smooth-hound which have ridges at the base but not at the tip.
This map shows the distribution of genetic samples collected in this study. Over 800 smooth-hound were tested, some of which were identified by their captor as common smooth-hounds on the basis of the absence of white spots/stars.
The new genetic identification method we developed was the first step in a four year project on Northeast Atlantic smooth-hounds. The test was applied to over 800 smooth-hounds collected over four years from all over the Northeast Atlantic of which 10% had no white ‘stars’ and were thought to be common smooth-hounds. Surprisingly, all were reliably confirmed to be starry smooth-hounds, which raised questions about the actual occurrence of common smooth-hounds in the region. With this in mind, we went back to the historical literature and also studied museum samples in the Natural History Museums in Dublin and Paris. We failed to find even one reliable account of a common smooth-hound north of Portuguese waters, but instead found multiple cases of misidentification and a litany of incorrect labelling of museum samples. So what does all of this mean?
It is difficult to conclusively prove that common smooth-hounds do not occur in Irish waters, unless we catch every single one and genetically test them all (something that is neither possible nor desirable!), so we need to stick to the scientific ‘rule-of-thumb’ of parsimony: the best interpretation of a natural phenomenon is the one that best explains the data and makes the fewest and least complex new assumptions. In this light, since virtually no confirmed catches of common smooth-hounds seem to have occurred over the past 200 years anywhere north of the Bay of Biscay, it appears only scientifically appropriate to suggest that until strong evidence is presented, they should be considered an absent or vagrant species.
This finding allowed us to focus all our efforts on uncovering the biological characteristics of starry smooth-hounds, which is detailed in part 2 of this article.