Morphometric and physical characteristics distinguishing adult Patagonian lamprey, Geotria macrostoma from the pouched lamprey, Geotria australis

Abstract

The pouched lamprey, Geotria australis Gray, 1851, has long been considered monotypic in the Geotriidae family with a wide southern temperate distribution across Australasia and South America. Recent studies have provided molecular and morphological evidence for a second Geotria species in South America; Geotria macrostoma (Burmeister, 1868). The aim of this study was to determine morphometric and physical characteristics of adult G. macrostoma that further differentiate this re-instated species of Geotriidae from G. australis. The diagnostic features discriminating immature adult G. macrostoma from G. australis when entering fresh water, are distinct differences in dentition, oral papillae and fimbriae counts and differences in coloration. In addition, G. macrostoma display greater growth of the prebranchial region and oral disc and has a deeper body depth and higher condition factor. All current ecological knowledge of the genus Geotria is based on Australasian populations, which may not be applicable to G. macrostoma. To ensure the conservation and protection of the Patagonian lamprey as a re-identified species, further investigations are needed to understand its life history, biology and ecology throughout its range.

On the invalid resurrection of the lamprey genus Exomegas Gill, 1883

“…Crucially, the examination of numerous G. australis from Australasia and Chile and G. macrostoma from Argentina shows that the TLL of these lampreys possesses three prominent cusps at the completion of metamorphosis and up to the commencement of the upstream spawning migration and thus implicitly including the marine feeding phase (de Buen, 1961; Hubbs & Potter, 1971; Potter et al ., 1980; Neira, 1984; figure 6b,f in Baker et al ., 2021, the former reproduced here in Figure 1a,c). The possession of a large central cusp on the TLL, as well as the two large cusps to either side, is crucial for the feeding process in Geotria .…”

“…In some instances, the authors have observed an even earlier transition from three to two cusps on the TLL. Early upstream migrants of G. australis from the Waikawa River (46.59° S, 169.14° E), New Zealand (figure 6e in Baker et al ., 2021, reproduced here in Figure 1b), and the lower Murray River, Australia (35.53° S, 138.81° E), and early and late upstream migrants of G. macrostoma from the Chubut (43.45° S, 65.91° W) and Santa Cruz rivers (50.05° S, 69.01° W), Argentina (figure 6d in Baker et al ., 2021; Figure 1d), already show a bicuspid state, indicating that reduction or loss of the middle cusp may occur just before entering the fresh water, at the beginning of the spawning run. A reduction in the size of the middle cusp on the TLL or replacement of the tricuspid TLL by a bicuspid TLL has already been observed in upstream migrants of G. australis from Australia, New Zealand and Chile by Maskell (1929) and De Buen (1961) and in G. macrostoma from Argentina and Islas Georgias del Sur (South Georgia Island) by Lahille (1915) and Potter et al .…”

“…In addition, Gray (1851) described the dentition of the TLL of G. australis as a “tongue with two elongate, conical, arched teeth, with a triangular plate on the lower side of the base,” and plate 1, figure 3, in that paper (a drawing of the oral disc of G. australis ) shows this configuration. Similarly, the holotype of Geotria saccifera Regan, 1911, a junior synonym of G. australis from Otago, New Zealand (BMNH 1886.11.18.112), possessed two large, sharp cusps at either side of one finer, shorter cusp (see figure 12, a photograph of the oral disc of the holotype of G. saccifera , an immature male, in Baker et al ., 2021).…”

“…The information provided in this study corroborates that the reduction or disappearance of the middle cusp of the TLL at the beginning of or during the spawning run constitutes a key feature of Geotria . The authors hope that this contribution, together with the information presented in their previous studies and references therein (Baker et al ., 2021; Riva Rossi et al ., 2020), will help clarify the long‐standing uncertainties regarding the classification of South American lampreys and firmly establish the taxonomic status of G. macrostoma in Argentina, the Malvinas (Falklands) and Islas Georgias del Sur (South Georgia Island).…”

“…These two Geotria species can be distinguished by the position of the cloaca and the distance between the second dorsal and caudal fins in larvae as well as immature adults (Riva Rossi et al, 2020). In addition, Baker et al (2021) provided detailed morphometric and morphological data that further differentiate the immature adults of G. macrostoma and G. australis, including differences in the extent of growth of the prebranchial region and oral disc, the morphology, size and arrangement of teeth, the number of oral papillae and fimbriae and colouration. Diagnostic characters of the genus Geotria are based primarily on the dentition and include a supraoral lamina with four cusps; a transverse lingual lamina (TLL) with three or two cusps depending on the stage in post-metamorphic life (see later); two longitudinal lingual laminae, each with four or five unicuspid teeth; and one enlarged, darkly pigmented oral papilla on either side of the oral disc.…”