Garra, Freyhof & Yoğurtçuoğlu & Jouladeh-Roudbar & Kaya, 2025

Freyhof, Jörg, Yoğurtçuoğlu, Baran, Jouladeh-Roudbar, Arash & Kaya, Cüneyt, 2025, Handbook of Freshwater Fishes of West Asia, De Gruyter : 208-209

Dollo’s law and the lower lip in Garra .

In evolution, “Dollo’s Law” states that once a complex trait has been lost, it cannot evolve again in the same way. Although the law has fallen into disrepute, it is still one of the most important rules in evolution. This is interesting because species in the former genus Hemigrammocapoeta ( G. culiciphaga , G. caudomaculata , G. festai , G. nanus ) have a well-developed “lower lip”. Molecular characters place these in Garra and treat Hemigrammocapoeta as a synonym. This hypothesis has been rejected because garras have only a very short remnant of the lower lip in the corner of the mouth, usually called the labellum. If Hemigrammocapoeta were part of Garra , the lower lip would have re-evolved in these species, breaking Dollo’s law. The gular disc on the ventral side of the head of garras has often been regarded as a modified lower lip. But the gular disc is a structure completely independent of the lips or mouth. It develops from tissues of the gular region, an area behind the mouth, between the lower jaws, often extending to the end of the ventral head. In Garra , as in many other algae-eaters ( Capoeta , Chondrostoma ), the lower jaw is covered by a keratinous sheath. In contrast to Capoeta or Chondrostoma , there is a skin fold behind the lower jaw, separated from it by a groove, which some authors have called the lower lip. This skin fold is not homologous to the lower lip and is called the subtoral fold. Species in the former genus Hemigrammocapoeta also have a complete subtotal fold behind the lower jaw, so their “lower lip” cannot be homologous to the subtotal fold. Hemigrammocapoeta has a well-developed “lower lip” separated in the middle by a short, pointed papillate structure. This structure has a small transverse split in the middle and may represent the reduced torus and labrum of the gular disc. As the structures of the gular disc in Garra are not split in the middle, the “lower lip” of Hemigrammocapoeta is most likely not homologous with any of them. Their “lower lip” is probably homologous to the labellum, which is usually restricted to the corner of the mouth in Garra . Like other Cyprinidae , the enlarged labellum/lower lip is continuous with the upper lip. As the labellum in Garra is the remnant of the lower lip, the “lower lip” of Hemigrammocapoeta represents a true lower lip. Do these species violate Dollo’s law by re-evolving the lower lip? Not in our opinion, because Dollo’s law is about the loss of complex characters, and (1) the lower lip was not completely lost, and (2) a shorter or longer lower lip is a simple change of a character state, not a complex one.

Further reading. Kottelat 2020 (gular disc).