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Although the shape of fish scales is useful for determining stock membership, the role of extrinsic (e.g. habitat, food type) and intrinsic (e.g. growth) factors in determining variation in fish scales shape has not been determined. This study examined whether fish scale shape changes as a result of compensatory growth in juveniles of the cyprinid roach, Rutilus rutilus (L.) reared on a fish farm in the UK. This was analyzed using geometric morphometric methods. Sufficient evidence was generated to accept the assumption that food availability and type between different growing-out facilities resulted in compensatory growth and this was sufficient to cause scale shape differences. This was tested using multivariate analysis of variance (MANOVA) analysis with the principal components scores of specimens (PCs) as dependent variables, to investigate whether fish scale shape (where the configuration of landmark coordinates were scaled, translated and rotated) form (where the configuration of landmark coordinates were translated and rotated not scaled) and allometry free (allometrically adjusting scale shape according to length) are related to holding facility (as fixed factor). Cross validated discriminant analysis was used to assess and compare the efficacy of shape, form and allometry free information. Identification rates are much better than chance with allometry free and shape alone, and classification is improved when size is taken into account.

Menezes et al. (2010) show that Mugil cephalus Linnaeus, 1758 is different from Mugil liza Valenciennes 1836, the latter being the mullet found along the Atlantic coast of South America. They also suggest that individuals identified as M. cephalus from the northwest Atlantic could represent a population of M. liza in this region, since they doubt the presence of M. cephalus in waters colder than the ones of the West Indies. In order to clarify the presence of M. cephalus in the northwest Atlantic, this study compares meristic and morphometric measurements of M. cephalus and M. liza from the Gulf of Mexico with those obtained by Menezes et al. (2010) for M. liza from South America and for M. cephalus in the Mediterranean Sea. Results show that there are differences in both morphometric and meristic data between the two species. The morphometric measure that differentiates these species is the distance from the snout to the dorsal fin, which is positioned backwards in M. liza compared with M. cephalus. The body width is consistently greater in M. cephalus than M. liza. The meristic character that discriminates between both species is the number of scales in the longitudinal series that, in M. cephalus, ranges from 38 to 43 while in M. liza between 32 to 39. The information presented in this work confirms the presence of M. cephalus in the Gulf of Mexico and the sympatric presence of M. liza is established, even if its abundance is quite low.

Pacheco-Almanzar et al. Scale Shape Empirical Relationships Tropical Pacific ecoregions show more identification rate (80.4%), whereas the rest of marine ecoregions discriminate less than 53.3%. Possibly, hydrographic features as currents or upwellings circumscribe boundaries between marine ecoregions, and this could produce inherent genetic structure.

The shape of fish scales is, to a considerable degree, species-specific, making it possible to identify species using only one fish scale per specimen. However, to our knowledge, the shape of the rhombic lamination pattern has not been considered to identify species. This study used landmarks and geometric morphometric approaches to address two questions: (1) whether the rhombic lamination pattern of fish scales along the longitudinal axis varies within species and sex and (2) how many fish scales of the rhombic squamation pattern should be considered to obtain an adequate identification. These questions were assessed with a MANCOVA and a cross-validated quadratic discriminant analysis (DA) using the rhombus of one, three, and six scales, and 6, 14, and 26 landmarks, respectively, in order to discriminate between two co-generic species, Mugil cephalus and Mugil curema. Proportions of the total shape variance explained by the total length and the centroid size were 2.3, 11.8, and 10.5% and 4.2, 5.1, and 5.4% for one, three, and six scales, respectively. Thus, analyses were performed on the shape and the form (shape plus size). The MANCOVA and DA analyses were found to be effective in detecting differences in scale pattern shape between species (except in the case of three scales; p = 0.079 for the shape and p = 0.065 for the form), whereas no differences were recorded between size and sex in all cases. Findings indicate that a good identification of species is possible, with no significant differences when using shape or form. The DA provided values of 75.8, 75.0, and 73.4% based on the shape, and of 72.7, 75.8, and 75.0% based on the form, for 1, 3 and 6 scales, respectively. Thus, it is possible to obtain a rapid and reliable identification of species using the rhombus of one scale only without considering the size. This is a useful finding in practical terms since scaling requires data on length. The finding of a suitable discrimination using only the rhombus of one scale raises the possibility of using an ocular adaptor on a camera or mobile phone, allowing many individuals to be easily screened without having to collect scales.

This study gives the first record of Mugil hospes distributed in the Gulf of Mexico. This species previously has been confused with its congener Mugil curema and can be easily differentiated by the shape of the ctenii on their scales.

Mugil curema is a common species in coastal areas to which is conferred reproductive migrations to the open sea with possibilities to spawn in estuarine waters. Previous studies described it as a species complex, which could be explained in terms of reproductive isolation and/or philopatric behavior. In order to test philopatry in M. curema, 10 locations in the Gulf of Mexico, one in the Atlantic Ocean, and three in the Mexican Pacific were sampled. One hundred individuals were analyzed with Cytochrome Oxidase Subunit I (COI), 121 with control region, and 380 with microsatellite loci. The mitochondrial results showed two lineages: Gulf of Mexico (GOM) and Mexican Pacific (MP) separated by a genetic distance of 4.5% with COI and 11.7% with control region. However, shared haplotypes were found between oceans, suggesting evidence of an ancestral connection. In addition, genetic structure was found in MP, suggesting the possibility of cryptic lineages. The microsatellite results showed genetic connection between GOM and MP probably as a consequence of a recent contact. It also showed genetic structure within GOM and MP; genetic subgroups were different by sex for the GOM (FST=0.348 ♂ and 0.275 ♀) and similar for the MP (FST=0.233 ♂ and 0.166 ♀). Based on the results of genetic structure and gene flow the hypothesis of sex-specific philopatry in GOM (♂ philopatric) and both sexes in MP is supported. The divergence found between GOM and MP, and the different philopatric behavior support the hypothesis that they may represent two distinct lineages.

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