In his 1558 Historia Piscium the Swiss scholar Conrad Gessner described a species of fish which was at that time new to science as an “addition” to another species. His scientific discovery highlights how species were identified and depicted.
The illustration below, included on one of the final pages of the Swiss scholar Conrad Gessner’s 1558 Historia Piscium, is remarkable for two reasons: the species depicted had never been described in a scholarly publication before, and it is the only one in over 750 aquatic species discussed by Gessner to be described as an addition to another species . The story behind this shows us a scientific discovery that highlights how species were identified and depicted.
“Additio ad alausam” Conrad Gessner, Historia Piscium, 1558, p. 1259
So similar they can be called by the same name
Gessner describes this addition to the “alausa clupea”, included in the appendix to his work, as so similar to this that either they are both the same species or they are so similar they can be called by the same name. This notwithstanding, the illustrations do not look much alike at all. The two depicted species clearly differ in body shape, type of scales, and judging by the row of vague black dots on the addition’s back, even in colouration. Why then does Gessner describe these as similar?
“Alausa clupea” Conrad Gessner, Historia Piscium, 1558, p. 21
Gessner’s descriptions of these species can shed light on this. He describes the Alausa clupea as similar to a herring but bigger and broader, about a cubit in size, roughly 45 cm. Based on this description and the depiction it seems likely this is an Allis shad, a close relative to the herring. 
An Alosa alosa or Allis shad
As for the addition, Gessner provides the local names “Meienfische” and “Alfe” for this species. Like the Allis shad it is described as similar to the herring but much bigger, with a body as wide as a wide carp, a high back, a length five times its width, and four black spots on either side of its back. Based on the description, the depiction, and the provided names, we can assume this second species is a Twaite shad, a very close relative of the Allis shad.
An Alosa fallax or Twaite shad
These species really do look a lot alike apart from a few small, barely noticeable differences. Gessner was well aware of this. He described how both species could be found in his native Basel and this provided him with ample opportunity to study and compare these. In fact the Allis and Twaite shad are so closely related they can interbreed. This explains why Gessner thought these so similar he describes one of these as an addition to the other. But if this is the case, why do Gessner’s illustrations look nothing alike?
Animal identification and depiction
Gessner’s descriptions of these species can shed light on this, here he emphasises a short list of physical characteristics these fishes do not share. The Allis shad has smaller scales than the Twaite shad, it has a wider body, and the Twaite shad has more profound black dots on its side.
This emphasis is what determined the style of Gessner’s woodcuts. Gessner began to study the Allis shad and the Twaite shad after he had received a drawing and description of the latter from his acquaintance Johan Kentmann who was studying fish in the River Elbe. Gessner based the woodcut illustration of the Twaite shad that would be printed in his publications on this drawing, but emphasised the little differences with the Allis shad. To this he added an emphasised woodcut of the Allis shad.
As we have seen, this resulted in two very different illustrations depicting very similar fish. Rather than provide us with realistic portraits of species, Gessner is telling us what to look out for when we try to identify them. The taxonomical depictions used by biologists today, are based on the same principle.
A discovery unacknowledged
Convinced by these physical differences, in his later publications Gessner described these fish as species within the same family. For the following century and a half however Gessner would remain the only one to believe so, the Twaite shad was not again described as a distinct species until Lacépède did so in 1803.
La Cepède, B. G. E. de 1803. Histoire naturelle des poissons. Tome cinquième. - Paris. (Plassan)
It took a long time for people to realise that Gessner’s “addition” really did exist. Generally the two species do not share a habitat and only one or the other can be found in the same place. As a result authors described only one, and not both of these species and assumed Gessner had made a mistake. Today Lacépède is acknowledged as the researcher who first identified the Twaite shad, while Gessner’s discovery has been forgotten.
 “additio ad alausam”
 Also see: Sophia Hendrikx. Identification of herring species in Conrad Gessner's ichthyological works, a case study on taxonomy, nomenclature, and animal depiction in the sixteenth century. In: Paul J. Smith and Karl A.E. Enenkel (Eds.), Zoology in Early Modern Culture. Intersections of Science, Theology, Philology and Political and Religious Education, Leiden: Brill, 2014.
 Alosa alosa
 Alosa fallax
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