First record of Baseodiscus mexicanus ( Bürger , 1893 ) ( Nemertea : Heteronemertea ) from Japanese waters

The heteronemertean Baseodiscus mexicanus (Bürger, 1893) was previously known exclusively from the eastern Pacific, except for fragmentary records from Palau and Indonesia, which suggested a possible amphi-Pacific tropical distribution. A single specimen of B. mexicanus collected in Okinawa, Japan, represents the first record of this species from Japanese waters and provides additional evidence for the occurrence of the species in the western Pacific. Baseodiscus mexicanus is strikingly similar in appearance to a sympatric moray eel, Gymnomuraena zebra (Shaw, 1797) (Actinopterygii: Anguilliformes: Muraenidae), possibly indicating a relationship of mimicry between them. 1 Hokkaido University, Faculty of Science, Department of Natural History Sciences. Kita-ku, N10, W8. 060-0810. Sapporo, Hokkaido, Japan. 2 University of the Ryukyus, Graduate School of Engineering and Science. Senbaru 1, Nishihara, Nakagami. 903-0213. Okinawa, Japan. * Corresponding author. E-mail: kazi@mail.sci.hokudai.ac.jp Hiroshi Kajihara 1*, Ryuta Yoshida 2 and Daisuke Uyeno 2 First record of Baseodiscus mexicanus (Bürger, 1893) (Nemertea: Heteronemertea) from Japanese waters Baseodiscus mexicanus (Bürger, 1893) is a large heteronemertean in the family Valenciniidae (or Baseodiscidae). Its known distribution is largely restricted to the eastern Pacific: west coast of Mexico (Bürger 1893; Joubin 1905; Coe 1940; Salcedo Martínez et al. 1988; Hochberg and Lunianski 1998), Panama (Coe 1905; 1940; 1944), Colombia (Hochberg and Lunianski 1998), Galapagos Islands (Coe 1944; Hochberg and Lunianski 1998), and Chile (Friedrich 1970). Reports from the western Pacific are confined to a field guide by Colin and Arneson (1995) containing a photograph taken in a seagrass bed at Lighthouse Reef, Palau, and a museum catalogue by Chernyshev and Volvenko (2008), listing a specimen collected in 1975 from Batu Moncho Bay, Komodo Island, Indonesia (Figure 1). Individuals of B. mexicanus are usually 20–80 cm, but occasionally up to 2–4 m, in length and are readily identifiable by a distinctive coloration pattern consisting of a brownish-green, maroon, deep red, mahogany, or brownish-violet background with numerous white rings encircling the body at irregular intervals (Coe 1940). Living among shells and corals (Coe 1944), it is apparently the largest and most abundant nemertean found in the tropical eastern Pacific (Coe 1905). During fieldwork conducted by SCUBA diving off Cape Manza, On’na-son, west coast of Okinawa-jima, Japan, a specimen of B. mexicanus (Figure 2A) was collected from a crevice in coral growing on a rock mass (26°30’16” N, 127°50’39” E) at a depth of 3–4 m, on 31 March 2012. The specimen was fixed in 95% ethanol and deposited in the Ryukyu University Museum, Fujukan, in Okinawa, Japan (voucher RUMF-ZN-00001). In life, the fully extended specimen was about 4 m long. The background body color was dark brownish purple, with numerous white bands (Figure 2A). In our specimen, the rounded head is demarcated from the body by a transverse cephalic furrow encircling the neck, from the entire part of which numerous secondary furrows extend anteriorly. Numerous, small, black ocelli are distributed along the lateral sides of the head. There are neither horizontal cephalic slits nor a caudal cirrus. The mouth opens mid-ventrally just posterior to the first white band on the trunk (Figure 2B). Figure 1. Map showing the known distribution of Baseodiscus mexicanus (Bürger, 1893). 1, Bürger (1893); 2, Coe (1905); 3, Joubin (1905); 4, Coe (1940); 5, Coe (1944); 6, Friedrich (1970); 7, Salcedo Martínez et al. (1988); 8, Colin and Arneson (1995); 9, Hochberg and Lunianski (1998); 10, Chernyshev and Volvenko (2008); 11, present study. Figure 2. Baseodiscus mexicanus (Bürger, 1893), RUMF-ZN-00001, photographs of living specimen (anaesthetized). A. General appearance. B. Magnification of head, ventral view. A B

First record of Baseodiscus mexicanus (Bürger, 1893) (Nemertea: Heteronemertea) from Japanese waters Baseodiscus mexicanus (Bürger, 1893) is a large heteronemertean in the family Valenciniidae (or Baseodiscidae).Its known distribution is largely restricted to the eastern Pacific: west coast of Mexico (Bürger 1893;Joubin 1905;Coe 1940;Salcedo Martínez et al. 1988;Hochberg and Lunianski 1998), Panama (Coe 1905;1940;1944), Colombia (Hochberg and Lunianski 1998), Galapagos Islands (Coe 1944;Hochberg and Lunianski 1998), and Chile (Friedrich 1970).Reports from the western Pacific are confined to a field guide by Colin and Arneson (1995) containing a photograph taken in a seagrass bed at Lighthouse Reef, Palau, and a museum catalogue by Chernyshev and Volvenko (2008), listing a specimen collected in 1975 from Batu Moncho Bay, Komodo Island, Indonesia (Figure 1).Individuals of B. mexicanus are usually 20-80 cm, but occasionally up to 2-4 m, in length and are readily identifiable by a distinctive coloration pattern consisting of a brownish-green, maroon, deep red, mahogany, or brownish-violet background with numerous white rings encircling the body at irregular intervals (Coe 1940).Living among shells and corals (Coe 1944), it is apparently the largest and most abundant nemertean found in the tropical eastern Pacific (Coe 1905).
During fieldwork conducted by SCUBA diving off Cape Manza, On'na-son, west coast of Okinawa-jima, Japan, a specimen of B. mexicanus (Figure 2A) was collected from a crevice in coral growing on a rock mass (26°30'16" N, 127°50'39" E) at a depth of 3-4 m, on 31 March 2012.The specimen was fixed in 95% ethanol and deposited in the Ryukyu University Museum, Fujukan, in Okinawa, Japan (voucher RUMF-ZN-00001).
In life, the fully extended specimen was about 4 m long.The background body color was dark brownish purple, with numerous white bands (Figure 2A).In our specimen, the rounded head is demarcated from the body by a transverse cephalic furrow encircling the neck, from the entire part of which numerous secondary furrows extend anteriorly.Numerous, small, black ocelli are distributed along the lateral sides of the head.There are neither horizontal cephalic slits nor a caudal cirrus.The mouth opens mid-ventrally just posterior to the first white band on the trunk (Figure 2B).

A B
This is the first record of B. mexicanus from Japanese waters (new Japanese name: zebura-himomushi [= "zebra ribbon-worm"]), providing further evidence for an amphi-Pacific tropical distribution.Although there are no reports from the central Pacific, a similar, banded species, B. cingulatus (Coe, 1906), has been described and reported from Hawaii (Coe 1906;1934;1947).Baseodiscus cingulatus, however, differs from B. mexicanus in having a light-colored body with many narrow, reddish-brown rings.
Baseodiscus mexicanus exhibits a remarkable resemblance to the zebra moray eel, Gymnomuraena zebra (Shaw in Shaw and Nodder, 1797) (Actinopterygii: Anguilliformes: Muraenidae) (Figure 3), possibly indicating a relationship of mimicry.Near Cape Manza in Okinawa, B. mexicanus and G. zebra occur in exactly the same habitat, with the latter species much more abundant (Uyeno, pers.obs.).The distributions of these two species broadly overlap; the moray eel is distributed from the Red Sea and eastern African coast to the Society Islands, north to the Ryukyu and Hawaiian Islands, and south to the Great Barrier Reef (Fricke 1999); it also occurs in the central eastern Pacific: along southern Baja California, Mexico; and from Guatemala to northern Colombia, including the Galapagos (McCosker and Rosenblatt 1995).Future investigations of the ecology and toxicology of these animals may shed light on a possible coevolutionary relationship between them.