Prey Capture Ecology of the Cubozoan Carukia barnesi

Abstract

Adult Carukia barnesi medusae feed predominantly on larval fish; however, their mode of prey capture seems more complex than previously described. Our findings revealed that during light conditions, this species extends its tentacles and 'twitches' them frequently. This highlights the lure-like nematocyst clusters in the water column, which actively attract larval fish that are consequently stung and consumed. This fishing behavior was not observed during dark conditions, presumably to reduce energy expenditure when they are not luring visually oriented prey. We found that larger medusae have longer tentacles; however, the spacing between the nematocyst clusters is not dependent on size, suggesting that the spacing of the nematocyst clusters is important for prey capture. Additionally, larger specimens twitch their tentacles more frequently than small specimens, which correlate with their recent ontogenetic prey shift from plankton to larval fish. These results indicate that adult medusae of C. barnesi are not opportunistically grazing in the water column, but instead utilize sophisticated prey capture techniques to specifically target larval fish.

Fig 1. Maximum mean recorded tentacle lengths (mean length mm, n = 6) for Carukia barnesi over a range of medusae bell sizes (niche bell height mm).

Fig 2. Carukia barnesi mean tentacle extension, as a percentage, over 360 minutes, exposed to light (○, dashed line) and dark (△, solid line) treatments.

Error bars represent 95% confidence intervals (n = 6).

Fig 3. The number of tentacle twitches recorded for Carukia barnesi, over one minute intervals (twitch rate per minute), plotted against different tentacle extensions (percent extension).

Video footage was analyzed in approximately 30 minute intervals (i.e. 6 specimens measured 9 times each).

Fig 4. The mean number of tentacle twitches recorded for Carukia barnesi, over one minute intervals (twitch rate per minute) plotted against medusae bell size in millimeters.

Error bars represent 95% confidence intervals (n = 6).

Fig 5. Envenomation of a larval fish (Acanthochromis sp.) that was captured by a twitching tentacle of an adult Carukia barnesi.

a: envenomation site; b: nematocyst cluster; c: bell. The bell size of this specimen is approximately 15 mm in height and the fish is approximately 10 mm in length.