Investigating population structure of Sea Lamprey (Petromyzon marinus, L.) in Western Iberian Peninsula using morphological characters and heart fatty acid signature analyses

Abstract

This study hypothesizes the existence of three groups of sea lamprey Petromyzon marinus L. in Portugal (North/Central group, Tagus group, and Guadiana group), possibly promoted by seabed topography isolation during the oceanic phase of the life cycle. Within this context, our purpose was to analyze the existence of a stock structure on sea lamprey populations sampled in the major Portuguese river basins using both morphological characters and heart tissue fatty acid signature. In both cases, the multiple discriminant analysis revealed statistically significant differences among groups, and the overall corrected classification rate estimated from cross-validation procedure was particularly high for the cardiac muscle fatty acid profiles (i.e. 83.8%). Morphometric characters were much more useful than meristic ones to discriminate stocks, and the most important variables for group differentiation were eye length, second dorsal fin length and branchial length. Fatty acid analysis showed that all lampreys from the southern Guadiana group were correctly classified and not mixing with individuals from any other group, reflecting a typical heart fatty acid signature. Our results revealed that 89.5% and 72.2% of the individuals from the Tagus and North/Central groups, respectively, were also correctly classified, despite some degree of overlap between individuals from these groups. The fatty acids that contributed to the observed segregation were C16:0; C17:0; C18:1ω9; C20:3ω6 and C22:2ω6. Detected differences are probably related with environmental variables to which lampreys may have been exposed, which leaded to different patterns of gene expression. These results suggest the existence of three different sea lamprey stocks in Portugal, with implication in terms of management and conservation.

Figure 1. Location of the river basins from which sea lamprey individuals were collected.

Formation of the three groups (testing hypothesis) based on the geographical location of the river mouth and the proximity to western Iberian oceanic areas with the representation of the seamounts and canyons that contour the three abyssal plains. Acronyms: Iberia AP - Iberia Abyssal Plain; Tagus AP – Tagus Abyssal Plain; Horsheshoe AP – Horsheshoe Abyssal Plain; T – Tore Seamount; ES – Estremadura Spur; G – Gorringe Bank.

Figure 2. Schematic representation of the morphological features recorded for the analysis of geographic variation of sea lamprey in Portugal.

(a) lateral view outline with the representation of the measured morphometric characters: TL, total length; d, disc length; d-a, distance between disc and anus; a-C, tail length; B₇-C, postbranchial length; B₇-a, trunk length; d-D₁, predorsal distance; d-eD₁, distance between disc and posterior end of first dorsal fin; d-D₂, distance between disc and base of second dorsal fin; D₂-C, dorsal part of caudal fin length; lD₁, first dorsal fin length; lD₂, second dorsal fin length; D-D, distance between dorsal fins; H, body depth; d-O, preocular distance; O, eye diameter; O-B₁, postocular length; Hco, head depth; d-B₁, prebranchial length; B₁-B₇, branchial length; d- B₇, head length; d-n, prenostril length; IO, interocular distance; HW, head width; (b) photograph of the oral disc with the representation of the counted meristic characters: AF, anterior field; LF_R, lateral right field; LF_L, lateral left field; PF, posterior field; SO, supraoral lamina; L, lingual lamina; IO, infraoral lamina; TNteeth, total number of teeth; AFteeth, number of teeth in the anterior field; LFteeth, number of teeth in the lateral field; PFteeth, number of teeth in the posterior field; TNrows, total number of rows; AFrows, number of rows in the anterior field; LFrows, number of rows in the lateral field; PFrows, number of rows in the posterior field; IOcusps, number of cusps in the infra-oral lamina).

Figure 3. Plot of the discriminant Z scores and group centroids of discriminate functions 1 and 2 for the three groups of adult sea lampreys based on (a) morphometric characters and (b) fatty acid composition of heart total lipids.

▵ - Group 1: Minho, Lima, Cávado, Douro, Vouga and Mondego basins; ○ - Group 2: Tagus; ▿ - Group 3: Guadiana.