other species must also, given the impressive array of chemosensory setae
(aesthetascs) on the antennules of adult males. Indications for a few genera are that
pheromones are not particularly species-specific. For example, Goetze (2008) has
shown that males of both Temora and Centropages will track pheromones from
females of congeneric but definitely distinct species, at least in laboratory containers.
Some of this “heterospecific” trail-following in these partially sympatric pairs
proceeds to mate capture.
Fig. 8.1 Mate-finding behavior in Calanus marshallae. The female slowly sinks,
leaving a vertical trail of pheromone. The male searches horizontally for trails. When
he finds one, he does a lively dance in a space about 12 cm across, then follows the
trail down to the female. He bumps her, she moves, he follows, eventually there is an
embrace and spermatophore transfer.
(^) (After Tsuda & Miller 1998.)
(^) The search process enforces much higher mortality rates for males, which, by being
constantly on the move, encounter not only females but the attack perimeters of
ambush predators like chaetognaths. To reduce this mortality (and save energy)
Calanus males search (Fig. 8.1) only when low levels of female attractant pheromone
are present, indicating females someplace nearby. At least some species of Acartia do
not detect pheromones of potential mates, but follow their shorter trails of
hydrodynamic disturbance from swimming (Kiørboe & Bagøien 2005). Kiørboe and
Bagøien have modeled mating rates by modification of the encounter rate (ER)
equation: ER = β( density)( density). The rate constant, β, is partly a function of
male swimming speed and accounts for both meeting frequency and fractional success
of matings per encounter. Since hydrodynamic disturbances from swimming are
shorter lived, and thus less extensive than pheromone tracks, the Acartia strategy