Turbellarian taxonomic database

Waminoa brickneri


Waminoa brickneri is a flat, disc-shaped acoel, 3–4 mm in diameter and 1 mm thick,
found closely appressed to the external surfaces of stony and soft corals (Fig. 1A). 
It occurred in groups of non-overlapping individuals causing the corals it infested, 
especially lightly colored ones, to appear spotted. The body was circular in profile 
except for a small medial notch at the posterior margin where the reproductive organs 
lay. The animal appeared bronze-colored with small white speckles, deriving its 
coloration from abundant dinoflagellate endosymbionts and scattered white pigment 
spots. Fixed specimens were more ovate in shape (Fig. 1B), narrowed at the anterior 
end and broader posteriorly, measuring 2.2–3.2 mm long, and lacked the pigment spots. 
Two types of endosymbionts were harbored in the parenchyma—a more abundant smaller 
type measuring about 4–8 µm and a larger type measuring about 11–16 µm (Fig. 1C). 
Both types of endosymbionts lay scattered throughout the parenchyma and both were 
clearly identifiable as dinoflagellates by their permanently condensed chromosomes. 
By differences in the pyrenoid and by comparison with symbionts in W. litus and other 
acoel species, the smaller type appeared to be a species of Symbiodinium, while the 
larger one appeared Amphidinium-like (Barneah et al. 2004).
The epidermis was generally transparent, fully ciliated, and richly glandular 
(Fig. 2B,C; 3A).  Especially abundant were rhabdoid glands in the dorsal body wall 
and lateral sides. Like rhabdoid glands in other acoels, these glands had narrowed 
apices that gave them a tooth-like appearance in sectioned material, and their 
rhabdiform secretory granules appeared striate. The ventral body wall was dominated 
by mucous glands having more amorphous secretory bodies; these mucous glands also occurred 
scattered in the dorsal body wall, but no rhabdoid glands occurred on the ventral side.  
Both rhabdoid and mucous glands stained metachromatically with toluidine blue, indicating 
presence of glycosaminoglycans; the mucous glands stained more darkly red, indicating 
more acidic polysaccharides. A third type of gland appeared to be a serous gland 
containing small spherical secretion granules staining dark blue with toluidine blue.  
It occurred sparsely scattered in both the dorsal and ventral body walls and more 
abundantly supraterminally at the anterior end; glands concentrated around the male 
gonopore appeared to be of the same type (Fig. 3C). 
The mouth was ventral in the posterior third of the body length. The digestive syncytium 
contained no recognizable contents—specifically, no nematocysts, which would indicate 
feeding on the coral tissues, could be found.
Although we found freshly hatched juveniles with a statocyst and paired eyes (Fig. 1C), 
mature specimens of W. brickneri appeared to lack both.  Also lacking was a frontal organ. 
The brain appeared to be bilobed and lay insunk just under the epidermis close behind 
the anterior tip of the body (Fig. 2); the two main masses consisted of central neuropile 
and nucleated rind and were connected by a short medial commissure. Behind it a short 
distance was a globular mass of unknown function consisting of close-packed, palely 
staining spherical cells with large nuclei.
The copulatory organs produced a slightly thickened ridge medially in the terminal 
quarter of the body (Fig. 1A). The seminal vesicle was most prominent, appearing white 
from contained sperm, and lay just in front of the posterior notch in the body margin. 
The seminal bursa, though unpigmented, continued the ridge anterior of the seminal 
vesicle, and the mouth and central digestive syncytium continued it anterior of that 
toward midbody.
The paired ovaries extended from about the second quarter of the body length posteriorly 
behind the mouth and were ventrally situated (Fig. 2). The vagina was ciliated and 
opened on the ventral body wall immediately anterior to the male gonopore (Fig. 3A). 
It led to a seminal bursa consisting of an ill-defined syncytial mass with scattered 
and clumped sperm; extending from the lateral and anterior edges of the bursa were 
multiple sclerotized bursal nozzles. We could identify at least 4 of these small 
(3 µm diameter, 12 µm length) nozzles in sections of one specimen and at least 8 each 
in sections of two other specimens.
Paired testes lay dorsal and lateral to the ovaries (Fig. 2), and led posteriorly 
to the seminal vesicle. The seminal vesicle was walled by thin, loosely concentric 
muscles and reached to an indistinct penis papilla (Fig. 3).  The center of the 
seminal vesicle in all specimens contained a mass of light-staining tissue interpreted 
as penis glands. Dark-staining, small-granuled serous glands surrounded the male 
gonopore. Flanking the seminal vesicle were prominent false seminal vesicles continuous 
with the tracts of sperm descending from the testes (Fig. 2). 
The partial 18S rDNA sequence of Waminoa brickneri, which we deposited in GenBank under 
accession number AJ875221, emerged as most closely related to “Acoel sp. strain Asp2” 
(AJ012526) and “Acoel sp. strain Asp3” (AJ012525) in a BLAST search of GenBank sequences. 
These two acoel species are described there as being supplied by the Coral Reef Research 
Foundation, Palau, and the latter one specifically as a “striped acoel, found on Porites 
coral.”
Preliminary analysis of sequence data (unpublished) showed the sequence of W. brickneri 
to group most closely to sequences of members of the family Convolutidae 
(e.g., Amphiscolops sp.), not the Haploposthiidae in which Waminoa is currently 
classified.