hide behind, soil to burrow in, and other elements of cover. If you walk through a
wood, it is usual to see very few animals. Birds and some insects are moderately
conspicuous, depending upon their capability for flight to move them quickly from
advancing predators. Mammals, reptiles, and most invertebrates, however, are rarely
seen unless special provisions are made. They are there, but they are in cover, hiding
behind, inside, and underneath the obscuring elements of the landscape.
(^) In pelagic habitats, the lack of cover requires different adaptations. One is that many
plankton animals are very nearly transparent and difficult to see at all. Some that are
mostly transparent have slightly variegated color patterns that “break up” against
flickering light from above, leaving no clear image of the animal against its
background. Pigmented or luminescent gut contents are impossible to make
transparent, so feeding often stops during daylight. Animals in several groups, for
example some species of ostracods and several copepod genera (prominently Metridia
and Pleuromamma) leave behind blobs of luminescing mucus as they dart away from
approaching predators. Diel vertical migration to evade visual predators is considered
separately below.
(^) Life in the open requires a constant state of alertness, coupled with the capability to
dart away. Escape darting in copepods is a response to shearing motion in the water
around the antennule, which is sensed by setae extending in multiple directions. Shear
rates on the order of 1.5 s−1 (i.e. Δ[cm s−1]cm−1] evoke escape responses in the
copepod Acartia (Fields & Yen 1997). For ordinary current shear, that would be a
large number, but, for detecting an eddy announcing the approach of a predator, it
represents refined sensitivity. Fields and Yen have demonstrated that these shear
thresholds vary such that species in habitats with high background shear are less
sensitive than those living in quiet water. Time to initiation of escape response is of
order of 1.5 ms in the most anxious copepod species (Lenz et al. 2000), about 100
times quicker than human reflexes. Copepods with the briefest response times have
myelinated axons (Lenz et al. 2000), while those in other families do not and can only
initiate jumps after about 10 ms (still relatively fast). Myelin, a multilayered lipid
wrapping of axons, is found in very few invertebrate groups and is arranged in
different ultrastructural patterns than in vertebrates. It has evolved independently in
copepods. Mechano-sensory dendrites in the bases of setae extending in three or more
directions and at varying distances along the antennules, provide directional
information about the approach of predators (or an experimenter’s vibrating needles),
allowing rapid rotation onto an optimum escape trajectory. Reflex arcs are short and
motor impulses to the swimming legs are carried by giant axons in the ventral nerves,
a feature common in many invertebrates. The muscles of the thoracic legs driving the
acceleration have a fine structure adapted for exceedingly rapid ATP recharge
(Fahrenbach 1963).
(^) Velocity during escape swimming pulses dramatically between the sequence of