Pigmentation and Acid Storage in the Tunic: Protective Functions of the Tunic Cells in the Tropical Ascidian Phallusia nigra

Abstract

The tunic morphology of the tropical ascidian Phallusia nigra was studied with regard to its protective functions by means of light and electron microscopy. The tunic has a smooth surface free of epibionts. Beneath the tunic surface, phagocytic cells containing pigment granules (pigmentary tunic phagocytes) are densely distributed and form the pigmentary layer. These pigments may protect the body from irradiation. When poorly pigmented specimens growing in a dim location are transplanted to a bright location, pigmentation is induced and the tunic becomes black. In this process, tunic phagocytes probably produce pigment granules and migrate to the pigmentary layer. The tunic under the pigmentary layer is packed with highly vacuolated cells (tunic bladder cells). Vital staining with LysoSensor, a fluorescent pH indicator, demonstrated that the vacuoles contain strong acid. Acidic fluid leaks out when the tunic is injured. Leaking acid could serve to disinfect the injured area, be an irritant to potential predators, and/or be involved in anti-fouling. Additional key words: phagocytosis, pigment granules, light, protection, anti-predation, antifouling An integumentary tissue has a variety of functions to protect the body from the environment. It can be the most important defense system in many sessile organisms. The only sessile chordates are ascidians, whose bodies are wholly covered with a leathery or gelatinous matrix called the tunic. Although the tunic is a kind of extracellular matrix situated outside the epidermis, free cells called tunic cells are distributed within it, and the presence of tunic cells makes it a mesenchyme-like tissue. Many types of tunic cells have been described in ascidians: they are involved in phagocytosis (De Leo et al. 1981; Hirose et al. 1994), conduction of impulses (Mackie & Singla 1987), tunic contraction (Hirose & Ishii 1995), bioluminescence (Chiba et al. 1998), photosynthetic symbiosis (Hirose et al. 1996), and allorecognition between colonies (Hirose et al. 1997). The ascidian tunic is, therefore, an attractive material for investigating the diversity of integumentary functions required for adaptation to the environment. In the tropics, sessile organisms can expand potena E-mail: euichi@ sci.u-ryukyu.ac.jp tial habitat through protective systems against strong sunlight (cf. Jokiel 1980; Bingham & Reyns 1999) as well as other environmental pressures, such as predation, infection, and competition for space. Phallusia (= Ascidia) nigra SAVIGNY 1816 is a large solitary ascidian inhabiting shallow tropical waters. Its tunic and mantle contain black pigment that may protect the body from sunlight. Moreover, P. nigra is known to concentrate vanadium and to have an extremely acidic tunic fluid, and these properties may be effective for anti-fouling as well as anti-predation (Stoecker 1978, 1980a,b). The tunic morphology and chemical analysis have been reported for this species with special reference to anti-fouling function (Stoecker 1978), but ultrastructural studies had not been carried out. I describe here the morphology of the tunic cells in P. nigra and examine their possible protective functions in adaptation to the tropical environment. There are two types of tunic cells: one type contains phagosomes and pigment granules, and the other type stores strongly acidic fluid in large vacuoles. These features may be concerned with protection against infection (or scavenging), light, and predation, respectively. This content downloaded from 157.55.39.248 on Sun, 12 Mar 2017 18:08:46 UTC All use subject to http://about.jstor.org/terms Pigmentation and acid storage in ascidian tunic