A detailed observation of the ejection and retraction of defense tissue acontia in sea anemone (Exaiptasia pallida)

Abstract

Acontia, located in the gastrovascular cavity of anemone, are thread-like tissue containing numerous stinging cells which serve as a unique defense tissue against predators of the immobile acontiarian sea anemone. Although its morphology and biological functions, such as defense and digestion, have been studied, the defense behavior and the specific events of acontia ejection and retraction are unclear. The aim of this study is to observe and record the detailed process of acontia control in anemones. Observations reveal that the anemone, Exaiptasia pallida, possibly controls a network of body muscles and manipulates water pressure in the gastrovascular cavity to eject and retract acontia. Instead of resynthesizing acontia after each ejection, the retraction and reuse of acontia enables the anemone to respond quickly at any given time, thus increasing its overall survivability. Since the Exaiptasia anemone is an emerging model for coral biology, this study provides a foundation to further investigate the biophysics, neuroscience, and defense biology of this marine model organism.

Keywords: Acontia; Defense; Ejection; Exaiptasia pallida; Retraction.

Figure 1. Acontia tissue within the body column of Exaiptasia anemone.

Acontia tissue is comprised of long white thread-like organs that have a simple coiling morphology; this tissue is densely lined with nematocysts and form from the mesentery edge near the pedal disk of the anemone. Scale bar: 1 mm.

Figure 2. Acontia ejection and retraction.

(A) relaxed anemone, fully elongated; (B) initial stimulation made the anemone contract but did not release of acontia; (C) the second stimulation caused slight acontia ejection; (D) multiple stimulations provoked defensive state with the profuse release of acontia and upright tentacles; (E) anemone displayed deflated morphology and withdrawn oral disk; (F) start of recovery, anemone mouth remained closed while siphonoglyphs were opened for the intake of water; (G) column elongation and acontia retraction; (H) approximately 40–50 min for the retraction of all acontia until next ejection (Video S1). Scale bar: 2 mm.

Figure 3. Anemone acontia ejection and retraction cycle.

(1) first stimulation caused body contraction and increased water pressure; (2) repeated stimulations prompted anemone defensive state and acontia ejection; (3) deflated oral disk and tentacles for maximum water expulsion; (4) anemone entered recovery state when stimulations ceased; (5) open siphonoglyphs intake water and closed mouth hold water volume, tentacles inflated and oral disk exposed; (6) generation of negative water pressure and elongation of body column pulled acontia back into body, process lasted for 40–50 min.