In tropical waters, some colonial ascidians harbor cyanobacterial symbionts such as Prochloron (reviewed by Lewin and Cheng 1989; E. Hirose et al. 2009a; Hirose 2014). The host ascidians always belong to the family Didemnidae, which is likely the largest family in the class Ascidiacea (e.g., Kott 2004; Shenkar et al. 2011). Although photosymbiotic didemnids are sometimes more common than any other ascidians in shallow coral reefs, they are often overlooked because of their small size and cryptic habitats. In contrast, they have been attractive sources of bioactive compounds for researchers in the biochemical and pharmaceutical sciences (e.g., Schmidt et al. 2012). To date, about 30 species in the four didemnid genera (Didemnum, Diplosoma, Lissoclinum, and Trididemnum) have been recorded as host species worldwide (e.g., Kott 2001). However, the ranges of distribution of individual species are less understood, as few faunal records of photosymbiotic ascidians exist (e.g., Kott 2001; Monniot and Monniot 2001). To identify these species, it is often necessary to examine zooid morphology under a microscope. Therefore, reexamination may be necessary for some records in older publications.

The Pulau Pari Technical Management Unit for Human Resources Development on Oceanography Competency is a marine laboratory located on Pari Island (Thousand Islands, Indonesia). This laboratory of the Indonesian Institute of Sciences (LIPI) is one of the key stations for marine science in the Java Sea. Therefore, acquiring biodiversity data in this area is essential. Here, we report the photosymbiotic ascidian fauna observed in the shallow coral reef in the vicinity of this laboratory.

Samples were collected by snorkeling in the shallow subtidal zone down to approximately 2 m or less at low tide in the back reef, reef flat, and reef crest off Pari Island (5°52'S, 106°36'40"E) on 28–30 November 2013 (Fig. 1). Ascidian colonies were photographed in situ prior to collection. Specimens were anesthetized using menthol and 0.37 M MgCl₂ for approximately 2 h and then fixed with 10% formalin–seawater. Fixed colonies were dissected under a binocular stereomicroscope. Zooids and spicules were photographed using a microscope equipped with differential interference contrast optics. In some photomicrographs of the thoraxes, several images were combined to increase the depth of field using the post-processing image software Helicon Focus Pro 4.2.8 (Helicon Soft, Ltd., Kharkov, Ukraine). Cyanobacterial symbionts were identified based on the colour in live specimens and the cytomorphology under a light microscope. Ascidian taxa were mainly identified following Kott (2001) and Hirose and Su (2011). The work by Shenkar et al. (2014) was also consulted for synonyms. Specimens were deposited in the Museum Zoologicum Bogoriense, Research Institute for Biology, Indonesian Institute of Science (LIPI), Indonesia.

Nine photosymbiotic ascidian species were found in the subtidal zone of the coral reef off Pari Island. Symbiont cyanobacteria within all ascidian species were identified as Prochloron didemni that is the only taxonomically valid species. Depending on the host species, Prochloron cells were distributed in the common cloacal cavities, in the tunic, or in both the common cloacal cavity and the tunic. Although the Prochloron cells in the cavity are morphologically different from those in the tunic (Cox 1986), they are indistinguishable genetically (Münchhoff et al. 2007; Hirose et al. 2012).

Didemnum molle Herdman, 1886 Fig. 2A

Diplosomoides molle Herdman, 1886

Leptoclinum molle (Herdman, 1886)

Lissoclinum molle (Herdman, 1886)

Didemnum sycon Michaelsen, 1920

Specimen.

MZB. Asc. 00001

Habitat.

Coral limestone at reef crest.

Remarks.

Colonies were dome-shaped. Several morphotypes in colony shape and color exist in this species (i.e., brown, gray, white, large, and small type). These morphotypes can also be distinguished by the partial sequence of the cytochrome oxidase subunit I (COI) gene (M. Hirose et al. 2009; Hirose et al. 2010a). Brown-type colonies were found in the present survey. Prochloron cells were distributed within the common cloacal cavity. Testis and/or oocyte were found in some zooids.

Figure 2.

Photosymbiotic ascidians with tunic spicules. Colonies in situ and tunic spicules (inset) of Didemnum molle (A), Trididemnum miniatum (B), Lissoclinum patella (C), Lissoclinum punctatum (D), and Lissoclinum timorense (E). Tunic cells contain Prochloron cells in the tunic of Lissoclinum punctatum (F). Scale bars = 20 µm.

Diplosoma gumavirens Hirose & Oka, 2009 Fig. 3A, B

Specimens.

MZB. Asc. 00006

Habitat.

Shaded side of dead coral branches in reef flat.

Remarks.

Colonies were oval cushions and entirely green due to Prochloron cells distributed within the common cloacal cavities. A blue ring of structural color encircled each branchial siphon. Retractor muscle emerged from halfway along esophageal neck of a zooid. On each of the right and left halves of the branchial sac, there were five stigmata in the first (top), second, and third stigmatal rows and four stigmata in the fourth row (bottom). Here, we describe the pattern of stigma number as 5–5–5–4. This record is the first of this species from outside of the Taiwan–Ryukyu area.

Figure 3.

Photosymbiotic ascidians without tunic spicules (Diplosoma species). Colonies in situ and thorax of zooid of Diplosoma gumavirens (A, B), Diplosoma simile (C, D), Diplosoma simileguwa (E, F), and Diplosoma virens (G, H). en endostyle; es esophagus; rt retractor muscle. Scale bars = 0.1 mm.

All photosymbiotic ascidians described here have also been recorded in the Ryukyu Archipelago, Japan (Hirose 2013 and references therein). Among the nine species, Diplosoma simileguwa and Diplosoma gumavirens were originally described from the Ryukyus in 2005 and 2009, respectively (Oka et al. 2005; E. Hirose et al. 2009b), and this report is the first to record these species outside of the Taiwan–Ryukyu area. The present records significantly expand our understanding of their range of distribution. The other seven species have also been described from the Great Barrier Reef (Kott 2001); thus, these species are widely distributed in the Indo–West Pacific. Among the five photosymbiotic ascidians recorded from Singapore, i.e., Diplosoma simile, Lissoclinum bistratum, Lissoclinum punctatum, Lissoclinum timorense and Trididemnum cyclops, (Su et al. 2013), two species, Lissoclinum bistratum and Trididemnum cyclops were not recorded in the present survey. These species are likely distributed in the Java Sea, considering that they are commonly found in West Pacific coral reefs. The recognition and identification of species are often important in field studies dealing with biocoenosis, and we hope that the present report will be helpful in future surveys and field courses in this area. Additional species, including undescribed species, are potentially distributed around Pari Island, considering its location within a biodiversity hot spot. Therefore, additional extensive surveys are necessary to characterize the photosymbiotic ascidian fauna in this area.