A case of atrial siphon duplication in Styela clava (Tunicata: Ascidiacea)

Abstract

The clubbed tunicate, Styela clava, is a solitary stolidobranch ascidian native to the north-west Pacific Ocean. It was originally described by Herdman (1881) from specimens dredged from Osaka Bay off Kobe, Japan (34°35'N 135°10'E) from 8 fathoms and 50 fathoms. This species is a major invasion menace to all temperate marine ecosystems and has successfully invaded both coasts of North America, Europe, and Australasia (Carlisle, 1954; Abbott & Johnson, 1972; Clarke & Therriault, 2007; Davis & Davis, 2007) posing a threat to shellfish aquaculture by smothering target species and fouling gear (LeBlanc et al., 2007). Styela clava occurs mainly in low intertidal to shallow subtidal zones and is common on rocks, floats, and pilings in protected waters. It is a sessile suspension feeder and normally has two anteriorly directed siphons: one oral (incurrent) and one atrial (excurrent). This note describes an abnormal specimen of S. clava with a single oral siphon and two atrial siphons. A normal S. clava is pictured in Figure 1A. The abnormal specimen (Figures 1B & 2) was discovered on a Pacific oyster culture tray in Deep Bay, Vancouver Island, British Columbia, Canada (49o27.60'N 124o43.80'W) in November 2007. It had three functional siphons and measured 4.5 cm in total length. The specimen was brought to the laboratory and held in a 20-L tank in a static seawater system at 11oC. In order to determine which siphon(s) was incurrent, we introduced Red Food ColourTM dye (McCormick Canada, London, Canada) to the aquarium and monitored the water currents. The specimen showed no adverse reaction to the dye, although it briefly retracted the