LOCATING A MATE IN 3D - THE CASE OF TEMORA-LONGICORNIS

Using laser optics to illuminate high-resolution video-recordings, we revealed behavioural mechanisms through which males of the calanoid co pepod species Temora longicornis locate females. Males of T. longicorn is swam at significantly faster speeds than females along more sinuous routes, possibly reflecting adaptations to increase encounter with fe males. Upon approaching within 2 mm (i.e. two body lengths) of a femal e's swimming path, males accelerated to significantly higher pursuit s peeds. Pursuit trajectories closely traced the trajectories of females , suggesting that males were following detectable trails created by sw imming females. Males of T. longicornis detected female trails up to a t 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 mal es (i.e. reactive distance) when males initiated 'mate-tracking'. It w as always the males of T. longicornis that detected and pursued mates. In rare events, males pursued other males. Behavioural flexibility wa s 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 i ncluded 'casting' behaviour, in which males performed sharp turns in z igzag patterns within localized volumes. This casting by males suggest ed that hovering females create more dispersed trails than cruising fe males. Casting behaviour also was initiated by males near locations wh ere females had hopped, suggesting that rapid movements by females dis rupt the continuity of their trails. Males were inefficient at choosin g initial tracking directions, following trails in the incorrect direc tion in 27 of the 67 (40%) mating pursuits observed. Males usually att empted to correct misguided pursuits by 'back-tracking' along trails i n the correct direction. Males were observed to detect and track their own previous trajectories without females present, suggesting the pos sibility that males follow their own trails during back-tracking. Obse rvations of males tracking their own trails and the trails of other ma les bring into question the specificity of trails as a mechanism promo ting reproductive isolation among co-occurring planktonic copepods.