Opisthobranch of the Week Data
Costasiella usagi is considered to be uncommon in Okinawan waters; I've not seen the animal here on Okinawa although I've collected a series of related, undescribed members of the species from Okinawa's main island (included in list below). Atsushi mentions (pers. comm.) seeing over ten individuals of C. usagi but has collected only two individuals from the Keramas *. In addition, Atsushi mentions the species is found on the green alga, Avrainvillea erecta, and it's found growing on open sandy bottoms. Previously on these pages I've featured an additional animal described by Ichikawa and photographed by Atsushi (C. kuroshimae).
Ichikawa (1993) described this species from five specimens found off Ishigaki Island (the main island in the Yaeyama Archipelago; an island chain about 420km southwest of Okinawa's main island).
The following description is taken from Ichikawa (1993):
The large eyes lie close together between the bases of the simple rhinophores. The anterior corners of the foot are tentaculiform and the tail is long and pointed. The head and the tail are not pimented [sic], and the rhinophores are black. The median cerata are grey, while the outer ones are black. All the cerata possess a white end with a black tip. Moreover, all the cerata bear longitudinal striations reaching from the base of each ceras to the white apical portion. The cerata are arranged in about 10 rows. The ventral side of the foot is white.
Smaller specimens have the median cerata coloured green, instead of grey.
None of the species in the genus Costasiella described until now have differently coloured median and outer cerata. The present new species differs from . . . C. paweli, in colour pattern and the absence of a mark on the neck, and is distinguishable from all other species of the genus in having unique colouration and striation of the cerata.
Kathe Jensen (2002) reporting in the Sea Slug Forum, mentions that she has a series of Costasiella species from Singapore. She apparently at one time was to have described these along with Jon Sigurdsson, but that the paper didn't get started due to her following statement:
" . . . this awful paper by Ichikawa appeared. In this paper she describes 6 species of Costasiella based on photos and serial sections. There are so many characters not mentioned in these descriptions that it is impossible to compare these species with others."
Previously I've featured several species of Costasiella on these pages and as of the current date there are several described and undescribed species of Costasiella, reported from Okinawan waters. The featured animals are as follows:
C. kuroshimae C. paweli C. usagi (the above featured animal) C. vegae C. sp. 1 C. sp. 2 C. sp. 3
* Keramas = the Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha.