Morphological and histological characterization of a new Acropora-eating flatworm: A potential threat to captive acroporid corals

Abstract

The high demand for live corals for decorative and reef-restoration purposes has caused increased interest in land-based coral farming. However coral cultivation is frequently threatened by disease outbreaks and corallivores. Small corallivores, such as the Acropora-eating flatworm (AEFW), can cause the loss of tissues and result in the death of the acroporid corals within a short period. Here, for the first time, this study reports a new Acropora-eating flatworm (new AEFW) that feeds on corals (Acropora). There were significant differences in the size, shape, and appearance between the new AEFW and the polyclad AEFW, Prosthiostomum acroporae, which is the only reported AEFW to date. The new AEFW underwent a life history of direct development. Transmission electron microscope (TEM) observations revealed that the dorsal epidermis of the new AEFW consisted of a single layer of epidermal cells with numerous microvilli and cilia with a typical “9 + 2” pattern in the axoneme structure. The vertical rootlet was approximately three times longer than the horizontal rootlet in the anchored-rootlet system. The basal membrane was well developed with a fibrous microfibrillar layer. The new AEFWs secreted rhabdites that did not have a lamellated structure. All of these morphological and physiological traits suggested that the new AEFWs were different from the polyclad AFEW from the order Polycladida. They were likely from the order Tricladida or Proseriata. Additionally, new AEFWs exhibited acroporid-specific feeding behavior when given seven scleractinian corals. The results indicated that the new AEFWs could be a potential threat to captive acroporid corals.