OKINAWAN
OPISTHOBRANCH OF THE WEEK

Janolus mirabilis Baba & Abe, 1970 (6mm)
Photo by Atsushi Ono

Opisthobranch of the Week Data

Taxonomy:

Frequency on Okinawa: Collection Data:
[brackets indicate range for all Okinawa-collected specimens of the species]


Species Account:

        The above featured arminoid nudibranch, Janolus mirabilis, was photographed by Atsushi Ono from the waters of Zamami Island in the Kerama Islands, one of a series of islands in the Kerama Islands Group, which are located 30 ~ 40 kilometers west of the Okinawa Capitol, Naha. This featured animal was discovered by Atsushi during January, 1999, in 5m of water. As of this date I've not personally seen Janolus mirabilis in the waters of Okinawa's main island and Atsushi reports (pers. comm.) seeing over twenty individuals, but none were collected. The animal is considered to be uncommon in the Kerama Islands.

        The initial specimen of J. mirabilis was collected from the shore of Abugashima, Toyama Bay (Japan Sea coast of Honshu Island) on August 17, 1958. I've added a second page with a copy of a drawing illustrating the holotype specimen (copied from Baba & Abe, 1970). There is an additional image of another specimen from the Kerama Islands on the page as well. This second image also shows a large white spot in the center of the dorsum, as mentioned in Marshall & Willan (1999: 135).

        The following description of Janolus mirabilis (the Japanese name of this little arminoid is Karajishi-umiushi) is taken from Baba & Abe (1970):

        Living animal about 7 mm in length. Rhinophores distinctly perfoliated above. There is a crest between the 2 rhinophores. Branchial papillae falling off easily. According to a coloured sketch made from life these papillae are covered with acutely pointed tubercles near the tip, and they stand in a single row on the antero-lateral margins of the body. A small number of papillae are found also on the postero-lateral margins of the body. The bare space of the back is covered with minute tubercles. Anus near the rear end of the mantle in the median line. Genital orifice on the right side below the anterior end of the pericardial prominence. Nephroproct not determined. Foot broad with rounded corners. No tail crest.
        General integument of body translucent ashy yellow, and the back is tinged with dark brown. Rhinophores dark brown below, opaque white at tip. Inter-rhinophorial crest black. Branchial papillae tipped with opaque white; the liver diverticulum within the papillae yellowish. Tip of oral tentacles whitish. Tail and sole spotted here and there with opaque white.
        Jaws very large in proportion to the size of the body, yellowish brown in colour, and not provided with serial denticles. Radula greatly reduced in size. Formula about 20 X 5.1.5. Teeth colourless. The central with 1 denticle on each side of the median cusp. The first 3 laterals with 7 - 10 serrulations each, the rest smooth. Genitalia not determined.
Remarks: The present new species, though more or less astonishing in the family Janolidae in the external appearance of the animal, was referred to Janolus by the combination of the following characters of the genus: (1) the presence of an inter-rhinophorial crest, ( 2 ) the possession of tuberculated branchial papillae, and especially ( 3 ) the absence of serial denticles on the edge of the jaw plates. In the previously recorded species of Janolus the radular teeth are sometimes smooth and sometimes denticulated.

        Marshall & Willan (1999) have an excellent description of Janolus mirabilis and the following is taken from that publication:

This species is recorded from Australia for the first time. It is a very small arminoid nudibranch with a broad body and large foot. The tail is exceedingly long taking up one third of the body length. The exceptionally large cerata are arranged in two clusters, the smaller cluster (with about eight cerata) at the posterior end of the body, and the larger one (about 15 cerata) at the anterior end of the body which encircles the rhinophores like a ruff. The large space between these two clusters and the foot has insignificant papillae. The cerata are club-like and bulbous with numerous acutely pointed papillae arranged in rings, and each one ends in a long spiked tip. The rhinophores are large with five or six large lamellae and there is a tall narrow intervening crest (caruncle). The anus is in the dorsal midline amongst the posterior ceratal cluster. The basic body colour is translucent pale brown with flecks of white and a large white patch in the centre of the dorsum as well as a white streak in front of the rhinophores. The foot and tail are transparent and spotted with vivid white. The cerata are pale brown with some dark brown patches at their base and their tips are transparent white. The rhinophores are brown.

Janolus mirabilis moves quite rapidly with a jerking motion, flicking its cerata backwards and forwards as it moves. The rhinophores, however, are not flicked forward during locomotion.

Janolus mirabilis is a rare species as there is only one other known specimen from Japan, the holotype. The two widely separated clusters of cerata are unique and caused Miller & Willan (1986: 388-389) to question whether the cerata in the intervening space had been autotomised. There was no indication of this in our specimens from Heron Reef and, in fact, the cerata did not autotomise when the animals were handled. The drawing of the living animal given by Baba & Abe exactly matches the animals found on Heron Reef.

The diet and spawn are unknown.

        There is an excellent photograph of one of the two collected Heron Reef specimens in Marshall & Willan (1999), and Julie Marshall has several images of this same arminoid nudibranch posted on the Sea Slug Forum.

        Here are links to the species, both described and unknown, featured on these pages from Okinawan waters, as of early-September, 2014:

  • Janolus mirabilis (the above featured animal)
  • J. savinkini
  • J. toyamensis
  • J. sp. 1
  • J. sp. 2
  • J. sp. 4
  • J. sp. 5
  • J. sp. 6
  • J. sp. 7
  • J. sp. 8
  • Etymology:

    Literature Cited:


    Page Date: 14 Jun '04
    Page Modification Date: 01 Sep '14
    Digitally manipulated photo
    Copyright © 2014 Robert F. Bolland