Osteología comparada del esqueleto caudal de Achirus lineatus y Achirus mazatlanus (Pleuronectiformes: Achiridae)

El esqueleto caudal constituye una importante herramienta para evaluar las relaciones ancestro-descendencia de los peces teleósteos. Por tal motivo, se realizó un análisis osteológico comparativo de dicha estructura, a partir de ejemplares de las especies Achirus lineatus (Linnaeus, 1758) y A. mazatlanus (Steindachner, 1869), obtenidos de colecciones científicas de referencia y recolectados in situ en Veracruz y Baja California Sur (México). Los organismos fueron identificados con claves taxonómicas especializadas y procesados mediante la técnica de aclaración y tinción diferencial, para evaluar las posibles similitudes o diferencias en las estructuras que componen al complejo caudal. Achirus lineatus y A. mazatlanus se caracterizan por la presencia de una fórmula caudal compuesta por 1 epural + 5 hipurales + 1 parahipural, además de una espina neural y dos espinas hemales; así como por la presencia de 16 radios principales y la ausencia de los radios procurrentes y el uroneural. Asimismo, se observo que en la especie A. lineatus, los hipurales se encuentran más cercanos uno del otro; sin embargo, en lo general, no se aprecio una clara diferencia en los elementos que componen la aleta caudal de las especies bajo estudio. Por consiguiente A. lineatus y A. mazatlanus son consideradas como especies fraternas

Morphometric Convergence and Molecular Divergence: the Taxonomic Status and Evolutionary History of Gymnura crebripunctata and Gymnura marmorata in the Eastern Pacific Ocean

To clarify the taxonomic status of Gymnura crebripunctata and Gymnura marmorata, the extent of morphological and nucleotide variation between these nominal species was examined using multivariate morphological and mitochondrial DNA comparisons of the same characters with congeneric species. Discriminant analysis of 21 morphometric variables from four species (G. crebripunctata, G. marmorata, Gymnura micrura and Gymnura poecilura) successfully distinguished species groupings. Classification success of eastern Pacific species improved further when specimens were grouped by species and sex. Discriminant analysis of size-corrected data generated species assignments that were consistently accurate in separating the two species (100% jackknifed assignment success). Nasal curtain length was identified as the character which contributed the most to discrimination of the two species. Sexual dimorphism was evident in several characters that have previously been relied upon to distinguish G. crebripunctata from G. marmorata. A previously unreported feature, the absence of a tail spine in G. crebripunctata, provides an improved method of field identification between these species. Phylogenetic and genetic distance analyses based on 698 base pairs of the mitochondrial cytochrome b gene indicate that G. crebripunctata and G. marmorata form highly divergent lineages, supporting their validity as distinct species. The closely related batoid Aetoplatea zonura clustered within the Gymnura clade, indicating that it may not represent a valid genus. Strong population structuring (overall ΦST = 0·81,P \u3c 0·01) was evident between G. marmorata from the Pacific coast of the Baja California peninsula and the Gulf of California, supporting the designation of distinct management units in these regions

Evolution and development of the synarcual in early vertebrates

The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occurs in vertebrate taxa such as the fossil group Placodermi and the Chondrichthyes (Holocephali, Batoidea). Although the synarcual varies morphologically in these groups, it represents the first indication, phylogenetically, of a differentiation of the vertebral column into separate regions. Among the placoderms, the synarcual of Cowralepis mclachlani Ritchie, 2005 (Arthrodira) shows substantial changes during ontogeny to produce an elongate, spool-shaped structure with a well-developed dorsal keel. Because the placoderm synarcual is covered in perichondral bone, the ontogenetic history of this Cowralepis specimen is preserved as it developed anteroposteriorly, dorsally and ventrally. As well, in the placoderm Materpiscis attenboroughi Long et al., 2008 (Ptyctodontida), incomplete fusion at the posterior synarcual margin indicates that both neural and haemal arch vertebral elements are added to the synarcual. A survey of placoderm synarcuals shows that taxa such as Materpiscis and Cowralepis are particularly informative because perichondral ossification occurs prior to synarcual fusion such that individual vertebral elements can be identified. In other placoderm synarcuals (e.g. Nefudina qalibahensis Lelièvre et al., 1995; Rhenanida), cartilaginous vertebral elements fuse prior to perichondral ossification so that individual elements are more difficult to recognize. This ontogenetic development in placoderms can be compared to synarcual development in Recent chondrichthyans; the incorporation of neural and haemal elements is more similar to the holocephalans, but differs from the batoid chondrichthyans

Biology, ecology and conservation of the Mobulidae

The Mobulidae are zooplanktivorous elasmobranchs comprising two recognized species of manta rays (Manta spp.) and nine recognized species of devil rays (Mobula spp.). They are found circumglobally in tropical, subtropical and temperate coastal waters. Although mobulids have been recorded for over 400 years, critical knowledge gaps still compromise the ability to assess the status of these species. On the basis of a review of 263 publications, a comparative synthesis of the biology and ecology of mobulids was conducted to examine their evolution, taxonomy, distribution, population trends, movements and aggregation, reproduction, growth and longevity, feeding, natural mortality and direct and indirect anthropogenic threats. There has been a marked increase in the number of published studies on mobulids since c. 1990, particularly for the genus Manta, although the genus Mobula remains poorly understood. Mobulid species have many common biological characteristics although their ecologies appear to be species-specific, and sometimes region-specific. Movement studies suggest that mobulids are highly mobile and have the potential to rapidly travel large distances. Fishing pressure is the major threat to many mobulid populations, with current levels of exploitation in target fisheries unlikely to be sustainable. Advances in the fields of population genetics, acoustic and satellite tracking, and stable-isotope and fatty-acid analyses will provide new insights into the biology and ecology of these species. Future research should focus on the uncertain taxonomy of mobulid species, the degree of overlap between their large-scale movement and human activities such as fisheries and pollution, and the need for management of inter-jurisdictional fisheries in developing nations to ensure their long-term sustainability. Closer collaboration among researchers worldwide is necessary to ensure standardized sampling and modelling methodologies to underpin global population estimates and status