The Biological Bulletin, Vol 189, Issue 1 29-35, Copyright © 1995 by Marine Biological Laboratory

IMMUNOLOGY

Microfilament Contraction Promotes Rounding of Tunic Slices: An Integumentary Defense System in the Colonial Ascidian Aplidium yamazii

E. Hirose and T. Ishii
Biological Laboratory, College of Agriculture and Veterinary Medicine, Nihon University, Fujisawa, Kanagawa 252, Japan

In Aplidium yamazii, when a slice of a live colony (approximately 0.5 mm thick) was incubated in seawater for 12 h, the slice became a round tunic fragment. This tunic rounding was inhibited by freezing of the slices, incubation with Ca2+-Mg2+ -free seawater, or addition of cytochalasin B. Staining of microfilaments in the slices with phalloidin-FITC showed the existence of a cellular network in the tunic. Contraction of this cellular network probably promotes rounding of the tunic slice. In electron microscopic observations, a new tunic cuticle regenerated at the surface of the round tunic fragments; the tunic cuticle did not regenerate in newly sliced specimens nor in specimens in which rounding was experimentally inhibited. Based on these results, an integumentary defense system is proposed in this species as follows. (1) When the colony is wounded externally, contraction of the cellular network promotes tunic contraction around the wound. (2) The wound is almost closed by tunic contraction. (3) Tunic contraction increases the density of the filamentous components of the tunic at the wound, and it may accelerate the regeneration of tunic cuticle there.