| Phagocytosis represents the first line of internal defense throughout the animal kingdom. In the black sea cucumber Holothuria leucospilota, phagocytosis is carried out by amoebocytes, the most abundant type of coelomocytes (blood cells). The present study is the first to quantity phagocytosis by flow cytometry in any marine invertebrate. Fluorescent beads and yeast cells were used as targets for phagocytosis. It was found that particles with size of 1 {dollar}mu{dollar}m in diameter were cleansed most efficiently. The percentage of phagocytosing amoebocytes and mean number of beads ingested by each amoebocyte were positively correlated with bead/cell ratio and incubation time until a plateau point was reached. Phagocytosis in such a massive way is aided by some phagocytosis-enhancing molecules. We identified and purified a 20 kDa protein, which was functionally analogous to vertebrate opsonin. We suggest that this is a new member of opsonin-like molecule family among invertebrates. This is the first time an opsonin-like molecule is reported in sea cucumbers. Our preliminary results showed that both amoebocytes and spherule cells can produce this molecule. The inhibitory effect of heavy metal on phagocytosis was also investigated. Phagocytotic function of amoebocytes was related to ambient concentration of zinc, excessive exposure of zinc can damage both the structure and phagocytotic function of the amoebocytes, some amoebocytes were killed and lysed, and those remaining intact were less effective in phagocytosing foreign particles. These results suggest that under zinc stress, the defense system of this animal can be compromised. |
