Clasper pocket: adaptation of a novel morphological feature by lamnoid sharks, which aids with tuna-like swimming.

The evolution of thunniform swimming in sharks involves many morphological modifications that enhance the energetic efficiency of aquatic locomotion. The present study describes a clasper pocket, a male-specific groove to "stow" claspers close to the body, as a novel example of such modifications. According to observations in two lamnoid species, Lamna ditropis and Isurus oxyrinchus, the groove becomes drastically deeper along with clasper elongation through ontogeny. The clasper pocket was found in sharks with a thunniform body plan, so far only observed in Lamnidae and Cetorhinidae. The number of acquisitions of clasper pockets during lamniform phylogeny assessment ranges from one to two, depending on the interpretation regarding the phylogenetic relationships between Lamnidae and Cetorhinidae in the phylogenetic tree is used in the analysis. [ABSTRACT FROM AUTHOR]