Locating a mate in 3D: the case of Temora longicornis
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Using laser optics to illuminate high-resolution video-recordings, we revealed behavioural mechanisms through which males of the calanoid copepod species Temora longicornis locate females. Males of T. longicornis swam at significantly faster speeds than females along more sinuous routes, possibly reflecting adaptations to increase encounter with females. Upon approaching within 2 mm (i.e. two body lengths) of a female’s swimming path, males accelerated to significantly higher pursuit speeds. Pursuit trajectories closely traced the trajectories of females, suggesting that males were following detectable trails created by swimming females. Males of T. longicornis detected female trails up to at least 10-s old, and tracked trails for distances exceeding 13 cm, or 130 body lengths. Females were positioned up to 34.2 mm away from males (i.e. reactive distance) when males initiated ‘mate-tracking’. It was always the males of T. longicornis that detected and pursued mates. In rare events, males pursued other males. Behavioural flexibility was exhibited by males during mate-tracking. Males generally tracked the trails of ‘cruising’ (i.e. fast-swimming) females with high accuracy, while the pursuits of ‘hovering’ (i.e. slow-swimming) females often included ‘casting’ behaviour, in which males performed sharp turns in zigzag patterns within localized volumes. This casting by males suggested that hovering females create more dispersed trails than cruising females. Casting behaviour also was initiated by males near locations where females had hopped, suggesting that rapid movements by females disrupt the continuity of their trails. Males were inefficient at choosing initial tracking directions, following trails in the incorrect direction in 27 of the 67 (40%) mating pursuits observed. Males usually attempted to correct misguided pursuits by ‘back-tracking’ along trails in the correct direction. Males were observed to detect and track their own previous trajectories without females present, suggesting the possibility that males follow their own trails during back-tracking. Observations of males tracking their own trails and the trails of other males bring into question the specificity of trails as a mechanism promoting reproductive isolation among co-occurring planktonic copepods.