captured particles.
(^) Extraction of particles from their watery background by crustaceans occurs by
several qualitatively different mechanisms. Euphausia, typical of filter-feeding krill,
comes closest to the simple screening of the salps. The anterior surfaces of the long
inner branches (endopods) of the paired thoracic legs bear long, anteriorly directed
setae which close the spaces between successive legs (Fig. 7.2), and the setae bear a
series of setules, which in turn bear secondary setules, creating tightly woven filter
screens beneath the body on either side. In addition, each leg bears short combing
setae on the medial surfaces, and these comb through the filter setae on the leg
behind, from leg surface to seta tip. According to Hamner (1988), water is drawn
between the thoracic leg screens at an opening below the head as the animal thrusts
the legs to the sides. The short, flap-like outer branches of the legs (exopods) lie over
the screened spaces between the endopods, preventing the basket from filling through
the filter screens. Then the leg tips close the filter at the bottom, pushing water
vortices (and particles) upward into the basket, which then closes from the sides,
squeezing water out through the screens that retain particles. As the legs are pulled
back again, the combing setae move particles toward the seta tips, which are cleaned
by other combs on the mouthparts. Food is shoved into the mouth under the labrum
(the bulky upper lip), ground by the mandibular blades, and ground again by meshing
teeth in the crop.
Fig. 7.2 Thoracic legs of particle-feeding euphausiids bear long anteriorly directed
setae, forming a filter basket (a). This fills from the front, beneath the head, when the
legs are opened (b) (flow is shown by the dye stream moving from a pipette tip at the
right). During opening, the filter surface is covered by the exopods (outer legs) to
keep water from moving in through the screen.
(^) (After Hamner 1988.)