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.

Fig 1. Location of sampling sites in Australia (Murray River), New Zealand (Mokau and Waikawa rivers), Argentina (Chubut and Santa Cruz rivers) and Chile (Temuco).

Fig 1 was created using ArcGIS 10.6 based on our own collection sites, geographic information and shapefiles available at NIWA and geographic resources freely available online: https://www.arcgis.com/home/item.html?id=170b5e6529064b8d9275168687880359.

Fig 2. Lateral view of an adult lamprey depicting morphometric measurements.

d: oral disc length; d-o: snout to eye length; d–b₁: prebranchial length; b₁‑b₇: branchial length; e: eye height; Ab: maximum branchial depth; At: maximum trunk depth; Lt: trunk length; Lta: tail length; Ld₁: length of first dorsal fin; hd₁: height of first dorsal fin; d₁-d₂: space between dorsal fins; Ld₂: length of second dorsal fin; hd₂: height of second dorsal fin; d₂-c: space between the posterior end of the second dorsal fin and origin of caudal fin; Ld₂-c: length from origin of the second dorsal fin to the tip of the caudal fin; TL: total length. Not shown is oral disc width (w).

Fig 3. Coloration of fresh run Geotria lampreys.

a) G. macrostoma displaying the iridescent blue markings on the outer edge of the eye and over the pineal gland (arrow) and the two blue-green longitudinal stripes characteristic of both Geotria species, b) G. australis lacking iridescent blue markings on the outer edge of the eye and over the pineal gland.

Fig 4. Immature and mature G. macrostoma.

a) fresh run G. macrostoma displaying the blue-green coloration to the dorsal region, the silver/white ventral surface and the iridescent blue markings along the trailing edges of the dorsal fins and along the entire edge of the caudal fin as well as the fleshy tip of the tail, b) G. macrostoma after six weeks in fresh water lacking the blue-green coloration to their dorsal region and the loss of iridescent markings to the fin tip, c) sexually mature female G. macrostoma (assumed post-spawning), d) sexually mature male G. macrostoma (assumed post-spawning). Scale bars = 2 cm.

Fig 5. Immature and mature G. australis.

a) fresh run individual, b) after approximately four weeks in fresh water, c) post-spawning female (top) and post-spawning male (bottom). At sexual maturity the second dorsal and caudal fins in both sexes are contiguous. Scale bars = 2 cm.

Fig 6

Dentition of immature G. macrostoma (a-d) and G. australis (e-g): IO: infraoral lamina; LC: lateral circumoral; LL: longitudinal lingual lamina; OF: oral fimbria; OP: oral papilla; RP: radial plate; SO: supraoral lamina; T: labial teeth; TL: transverse lingual lamina.

Fig 7. Dentition of a mature male G. macrostoma.

Scale bar = 1 cm. The transverse lingual lamina is tricuspid showing two lateral and one central tooth.

Fig 8. Plot of oral fimbriae length as a percentage of oral disc width for immature NZ G. australis from the Waikawa River and immature G. macrostoma from the Santa Cruz River.

For both anterior-lateral and posterior regions, the mean ± 95% confidence interval is displayed. Error bars that do not overlap depict significant differences between regions and lamprey species (p<0.05).

Fig 9. Plots of the 21 morphometric characters and condition factor.

For each character, the mean ± 95% confidence interval is displayed. All characters were significantly different among the five lamprey populations (NZW: Waikawa River, New Zealand; NZM: Mokau River, New Zealand; Au: Murray River, Australia; ArS: Santa Cruz River, Argentina; ArC: Chubut River, Argentina). Error bars that do not overlap depict significant differences between lamprey populations (P < 0.05). Abbreviations: TL: total length; CF: condition factor; W: width. All measurements are in millimetres and weight is measured in grams. Note d₂-c (space between the posterior end of the second dorsal fin and origin of the caudal fin) is not graphed as it is absent in G. macrostoma.

Fig 10. Factor plot showing canonical scores of population means for the first two discriminant functions (canonical roots).

Root 1 (R1) was positive so the higher the score the larger the snout to eye length of the lamprey relative to total length, and root 2 (R2) was positive, so the higher the score the larger the first dorsal fin length. (NZW: Waikawa River, New Zealand; NZM: Mokau River, New Zealand; Au: Murray River, Australia; ArC: Chubut River, Argentina; ArS: Santa Cruz River, Argentina; Ch: Temuco, Chile; ArS A: mature adult lamprey from the Santa Cruz River, Argentina; Ch A: mature adult lamprey from four Chilean rivers).

Fig 11. Prebranchial and branchial regions of six G. macrostoma from the Santa Cruz River showing the variation in size from snout to eye and size of the oral disc.

Scale bar = 2 cm.

Fig 12. Oral disc of immature adult holotype of Geotria saccifera (BMNH 1886.11.18.112) showing the prominent ridges of epithelium flanking the labial teeth (arrow).

Photographed by Phil Hurst, Photographic Unit, © The Natural History Museum, London.

Fig 13. Oral disc of mature adult holotype of Geotria allporti (BMNH 1871.8.18.51) without any ridges of epithelium flanking the labial teeth.

Note the serration along the inner edge of both lateral flanges of the supraoral lamina reported by Günther [45] in the original description. Photographed by Phil Hurst, Photographic Unit, © The Natural History Museum, London.