Morphological variation among widely dispersed larval populations of anadromous southern hemisphere lampreys (Geotriidae and Mordaciidae)

Abstract

Larvae of all three southern hemisphere anadromous parasitic lampreys were collected from rivers in Australia, New Zealand and South America. Body intervals were measured, trunk myomeres counted and the frequency of pigmentation in different body regions recorded. Morphometric data were subjected to multiple group principal components analysis (MGPCA) which took into account changes during growth. The components (together with myomere counts) and the pigmentation data were both subjected to discriminant analysis. Ordination and rank correlation tests revealed no evidence for either latitudinal clines or a continuum of circumpolar change amongst larval lamprey populations. Clustering of population centroids clearly distinguished between Mordacia lapicida (Gray) from Chile and M. mordax (Richardson) from south‐eastern Australia. Populations of Geotria australis Gray divided into groups representing three geographical regions, namely Argentina, Chile and Australasia (Western Australia, Tasmania and New Zealand). Ammocoetes from Argentina were the most divergent, possessing a more posterior cloaca, taller dorsal fins, a greater gap between dorsal fins, and distinctive pigmentation on the head and caudal fin. Within the Australasian group, Western Australian and New Zealand populations clustered closer than either did with those from Tasmania. The cluster analyses for larval populations of G. australis suggest that, during their marine trophic phase, the adults of this species originating from Argentinian and Chilean rivers follow different migratory routes, whereas those from Western Australia, New Zealand and, to a lesser extent, Tasmania intermix.

Revalidation of the Argentinian pouched lamprey Geotria macrostoma (Burmeister, 1868) with molecular and morphological evidence

“…Recent studies based on morphological data found great differences between representatives of Geotria from Argentina and those from Australasia and Chile [5,20,21,44], and recently these differences have been corroborated with genetic data by Nardi [41] who reported a different species of Geotria inhabiting rivers in Patagonia, at the southern tip of Argentina [41]. In the present study, the recent collection of downstream migrating juveniles and sub-adult lampreys from the largest Atlantic basins from Patagonia allowed us to verify these morphological and genetic divergence reported in these previous studies and, based on this information, determine whether one or two species of Geotria exist in Argentina [e.g., 33].…”

“…For morphological analysis, the 125 sub-adult individuals collected at the Santa Cruz River and the 39 sub-adult individuals collected at the Chubut River were anesthetized, photographed on their left side, and examined for the main external characters cited as diagnostic in the literature of Geotria taxonomy (e.g., the position of the cloaca and the distance between the second dorsal and caudal fins in sub-adult individuals) [6,7,18,20,21] (Fig 2). Since the status of type specimens of Petromyzon macrostomus or Geotria macrostoma is unknown (collected specimens were deposited at the Museo Argentino de Ciencias Naturales-MACN-but holotypes were not designated), the state of these characters in fresh specimens were matched and confirmed with those reported in the original descriptions of the Argentinian lamprey provided by Burmeister [22], Berg [23,25,26], Smitt [30], Lahille [33], Sielfeld [37] and also with morphological descriptions of Geotria australis from Chile [20], New Zealand [17], and Australia [6].…”

“…The taxonomic status of Geotria still remains unresolved, largely due to the misinterpretation of the morphological changes that occur during its sexual maturation [8,[17][18][19]. These changes affect body size, the number and arrangement of the teeth, the height and position of the dorsal and caudal fins, the size of oral disk relative to the head, and the development of an exceptionally large gular pouch in males [8,18,20,21]. Based on this morphological variation, several taxonomic rearrangements and nomenclatural acts have been proposed, but all of them are now considered as Geotria australis [5].…”

Lampreys: Biology, Conservation and Control

“…The main morphological characters used to describe the ammocoetes of the various species are the number of trunk myomeres, the shape of the caudal fin, and the patterns of pigmentation on various parts of their body surface and tongue precursor (Vladykov 1950;Potter and Osborne 1975;Neira et al 1988). In contrast, the most important characters for describing the adults of the various species are those involving the dentition on the suctorial disc and piston (Figs.…”

“…6). Eventually, the Southern Hemisphere lampreys were considered to be represented (Neira et al 1988), it is important that further studies be undertaken to ascertain whether Geotria comprises two or more closely-related species rather than a single species. At the family level, there had also been disagreement, for example, as to whether G. australis should be allocated to a family on its own or included with that comprising all Northern Hemisphere species (Potter and Strahan 1968).…”