Chapter 24 - Vibratile cells and hemocytes in sea cucumbers—Clarifications and new paradigms

Abstract

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.