OSTRACOD RESEARCH

at the Lake Biwa Museum, Japan


Robin James Smith

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Genus Cytherissa Sars, 1925

Superfamily: Cytheroidea Baird, 1850
Family: Cytherideidae Sars, 1925

One species has been reported from Japan:

1. Cytherissa lacustris (Sars, 1863)


Left side view (top) and dorsal view of Cytherissa lacustris, length 0.9 mm. Many specimens from Lake Biwa are heavily encrusted with a thick, brown layer.


External view of a female left valve of Cytherissa lacustris, length 922 microns.


External view of a female right valve of Cytherissa lacustris, length 912 microns.


Additional figures in: Meisch 2000 (carapace and appendages); Smith & Janz 2008 (carapace).
Habitat: Found below 25 m in Lake Biwa. Typical species of cooler parts of lakes.
Mode of life: Benthic.
Reproduction mode: Asexual in Japan and most other areas. Males only known from Lakes Baikal and Hovsogul.
Known distribution: Lake Biwa, Shiga Prefecture (Smith & Janz 2008). Elsewhere, Eurasia and North America (Meisch 2000).

References:
Meisch, C. 2000. Freshwater Ostracoda of Western and Central Europe. In: Schwoerbel, J., Zwick, P., (Eds.), Süßwasserfauna von Mitteleuropa 8/3. Spektrum Akademischer Verlag, Heidelberg, Berlin 522 pp.

Smith, R. J. & Janz, H. 2008. Recent species of the Family Candonidae (Ostracoda, Crustacea) from the ancient Lake Biwa, Central Japan. Journal of Natural History, 42, 2865 - 2922.


Key characters Cytherissa lacustris
Carapace - ventral valve overlapLeft valve overlaps right along ventral margin
Carapace - margin structure - septaNo septa present
Carapace - margin structure - ridges on inner calcified lamellaNo ridges
Carapace - dorsal view - female with brood pouchBrood pouch not visible
Carapace - lateral view - ventral marginVentral margin approximately straight to concave
Carapace - lateral view - valve heightLeft and right valves similar height
Carapace - internal viewAnterior calcified inner lamella wider than posterior one
Carapace - ornamentation - denticlesNo denticles near ventral margin
Carapace - ornamentation - sulcusNo sulcus
Carapace - ornamentation - surfaceSurface pitted
Carapace - width/length ratio (w/l)Between 0.4 and 0.7 (medium species)
Antennule - segmentsFive moveable segments
Antennule - Rome organRome organ absent
Antenna - exopoditeExopodite a spinneret seta
Antenna - swimming setaeNo swimming setae present
Fifth limb - morphologyWalking leg
Sixth limb - segmentsFour segments
Sixth limb - setae on first segmentFour setae
Sixth limb - h3 seta on final segmentNo h3 seta
Seventh LimbWalking leg, larger than sixth limb
Seventh Limb - segmentsFour segments
Seventh Limb - first segmentFour setae
Caudal ramusReduced to only one or two setae or absent
Caudal ramus - attachmentWith no caudal ramus attachment