Taxonomy, morphology and distribution of the common dolphin, Delphinus delphis (short-beaked form) and Delphinus capensis (long-beaked form), in West African waters = Taxonomía, morfologia y distribución del delfín común, Delphinus delphis (delfín de morro corto) y Delphinus capensis (delfín de morro largo), en aguas del Noroeste Africano

Abstract

[eng] Distinguishing population units of small cetaceans continuously distributed in a widespread area is challenging, but critical for their conservation and management. The common dolphin (genus Delphinus) has a wide distribution range that has led to the differentiation of a number of morphotypes which, until today, remain of unclear taxonomic adscription. In many areas, two morphotypes were initially distinguished and later separated into two species: the long-beaked common dolphin, or Delphinus capensis, and the short-beaked common dolphin, or Delphinus delphis. The general aim of the present thesis is to investigate the taxonomy, morphology, distribution, and habitat use of the common dolphins (genus Delphinus) occurring in the eastern sub-tropical Atlantic Ocean (NW Africa). This area is amongst those most productive in the world. It is characterized by a quasi-permanent upwelling zone that promotes high biological productivity and a highly diverse and abundant fauna of marine mammals. It has been proposed that the short- and long-beaked forms of common dolphins occur sympatrically in this area, a scenario that provides a unique opportunity to discriminate between the two morphotypes and investigate whether differences are of taxonomic relevance. For the present thesis we investigated the local marine trophic network and the relationship between apex predators, as well as the distribution (offshore vs. inshore) and niche segregation (trophic level exploited) of the two common dolphin morphotypes. Additionally, differences in morphology and habitat use between NW Africa common dolphins and those from other areas, particularly from the northeastern and southeastern Atlantic Ocean, were investigated. Because research of intra and inter specific population variability requires a multiplicity of approaches, different methods were used, including stable isotope analyses of carbon (δ13C) and nitrogen (δ15N) in bone, and measurement-based and 'landmark'-based (geometric) morphometric analyses of the skull. The results obtained revealed that most of the marine mammal species distribute in the outer continental shelf and the upper slope, and that common dolphins are rarely distributed inshore, displaying a typical oceanic behaviour. With the exception of baleen whales, common dolphins showed the lowest trophic level of all marine mammals analyzed, despite the fact that there was large variability between individuals in the exploitation of food resources, possibly to adapt to local environmental variations. The presence of both short- and long-beaked morphotypes of common dolphins was confirmed off the coast of NW Africa, with the short-beaked form inhabiting waters closer to coast and feeding at a lower trophic level than the long-beaked form. Morphological analysis of the skull revealed that variation in relative beak length in common dolphins from NW Africa was larger than in other Delphinus populations worldwide; as opposed to other regions, relative rostrum size followed a clinal variation and intermediate ratios were found. There were significant differences between common dolphin populations and species analyzed in the size and shape components of the skull. The skull of the short-beaked morphotype from NW Africa was shorter but broader than that of the long-beaked. However, despite the differences, there is a closer morphological similarity between the short-beaked morphotype of NW Africa and Delphinus delphis, and between the long-beaked morphotype of the same region and Delphinus capensis. We conclude that taxonomic splitting of common dolphins in this area into two putative species should be postponed until further research is conducted because skull differentiation could be related to niche segregation and not to speciation. Furthermore, the above results indicate that the taxonomic model described for the Northeast Pacific should not be generally applied to other areas where the sort- and long-beaked morphotypes co-habit.

[spa] Distinguir unidades poblacionales de pequeños cetáceos distribuidos de forma continua en un área extensa es fundamental para su conservación y gestión. En el amplio rango de distribución de los delfines comunes se han establecido varios morfotipos de adscripción taxonómica incierta, identificados por la longitud relativa de su morro. En muchas áreas, dos morfotipos fueron distinguidos y más tarde separados en dos especies: el delfín común de morro largo, Delphinus capensis, y el delfín común de morro corto, Delphinus delphis. El objetivo general de la presente tesis es investigar la taxonomía, morfología, distribución, y uso del hábitat del delfín común (género Delphinus) en el este del Océano Atlántico Subtropical (NW África). Se ha propuesto que las formas de delfín común de morro corto y largo ocurren en simpatría en esta área, lo que permite una oportunidad única para discriminar entre los dos morfotipos e investigar si las diferencias son de importancia taxonómica. Los resultados han demostrado una gran variabilidad entre los individuos en la explotación de los recursos alimentarios, posiblemente para adaptarse a las variaciones ambientales locales del ecosistema. La presencia de ambos morfotipos de delfines comunes se confirmó en la costa NW de África; la forma de morro corto habita aguas más cercanas a la costa y se alimenta en un nivel trófico inferior a la de morro largo. El análisis morfológico del cráneo reveló que la variación en la longitud del morro es más grande que en otras poblaciones, con el morfotipo de morro corto presentando un cráneo más corto pero más amplio que él de morro largo. Como era previsible, existe una similitud morfológica más cercana entre el morfotipo de morro corto del NW de África y Delphinus delphis, y entre el de morro largo y Delphinus capensis. La división taxonómica de delfines comunes en dos especies, en esta área, debe posponerse hasta que se realicen más investigaciones, ya que la diferenciación craneal puede estar relacionada con la segregación de nicho y no con la especiación. Además, el modelo taxonómico descrito para el Pacífico Nordeste no debe aplicarse indiscriminadamente a otras áreas donde ambos morfotipos cohabitan.