that the predominant meals were Oithona similis, a ∼1 mm copepod found in 37% of
worms (14 to 73% in seven samples of 100). The sampling was done with paired
bongo nets of 183 and 333 μm mesh, and the chaetognaths were taken from the coarse
mesh sample that included almost no Oithona, so net feeding on at least that species
was likely nil (it was up to 50% of meals for chaetognaths from the 183 μm mesh
samples). From the fine-mesh sample came an estimate of the relative abundance of
Oithona and other species, allowing an evaluation of how selective the chaetognaths
had been in taking their dominant prey over other candidates. An electivity index
ranging from −1 (rejection) to +1 (exclusive diet),
(^) averaged −0.05, not significantly different from zero. A few other prey were taken,
with Metridia copepodites having positive average electivity. Eukrohnia juveniles,
living mostly below the seasonal thermocline, ate lots of Oithona but selected against
them (E = –0.33), favoring Oncaea (E = +0.73), a copepod of similar size that clips
on to bits of mucus, jellyfish surfaces, and the like. Sagitta and Eukrohnia larger than
14 mm switched progressively to the copepodites of Calanus and Neocalanus,
swallowing animals as wide and slightly wider than their heads, with electivity for
Oithona becoming negative. Like snakes, arrow worms can stretch their mouths and
bodies around prey somewhat wider than they are. Prey size increases with body size,
perhaps most importantly mouth size (Fig. 9.2; Pearre 1980).
Fig. 9.2 A demonstration of the increase in prey size with predator size, head width
here, for the chaetognath Parasagitta elegans. The spread of prey-grasping spines
increases with head width. Both the trend for increase and the scatter are typical of
many (not all) predators. The dashed line is 1 : 1, and the best-fit exponential function
shows that head width increases faster than prey size.
(After Pearre 1980.)