Vibratile cells and hemocytes in sea cucumbers—Clarifications and new paradigms

Immunity in echinoderms, often considered the ancestral
deuterostome cellular defense system, relies primarily on cells
called coelomocytes. These cells have been found in the perivisceral
and hydrovascular cavities, the lumen of the haemal
system, and associated with the tissues of various internal organs
(i.e. marginated). Coelomocytes consist of various cell types that
display different morphologies, of which sea cucumbers show the
greatest diversity among echinoderms, with seven currently
accepted types: phagocytes, spherulocytes (morula cells), vibratile
cells, hemocytes, progenitor cells, crystal cells, and fusiform cells.
Among them, two types are the topic of debate regarding their
occurrence and distribution: vibratile cells and hemocytes. We
aimed to address this controversy through an investigation of
coelomocytes in several sea cucumber species (across orders
Holothuriida, Apodida, and Dendrochirotida) and various other
species of echinoderms (including sea urchins, crinoids, brittle
stars, and sea stars) by light microscopy and scanning electron
microscopy. We conclude that vibratile cells have been erroneously
considered coelomocytes but are in fact spermatozoa (or less
developed male gametes), at least in holothuroids. Hemocytes,
which to date were strictly described in the perivisceral fluid of
species in the order Dendrochirotida and Molpadida, were
detected in the hydrovascular fluid of one apodid (Chiridota
laevis) and three holothuriid species (Holothuria forskali, H. atra,
and Bohadschia argus). We suggest that both the misidentification
of vibratile cells and oversight of hemocytes in certain groups may
largely be attributed to the collection method, which most often
involves puncturing blindly through the body wall using a syringe
instead of dissecting the holothuroids to carefully sample coelomocytes
along the organs and inside the various cavities.