animal Type
Maximum Size

4.5 cm

(1.8 inches) bell diameter


200–700 m

(660–2,300 feet)



in twilight (mesopelagic) zone


Small crustaceans, jellies, and fishes


Pacific, Atlantic, and Indian Ocean


Meet a deep-sea escape artist.

Between the surface and deep seafloor lies the midwater. This is a world with no boundaries, just open water as far as the eye can see. Imagine life in this big expanse. Predators patrol these waters. How would you survive if there is no place to hide? When danger approaches, the silky jelly (Colobonema sericeum) uses speed and clever tricks to avoid being eaten.

Stinging cells called nematocysts cover the tentacles that dangle from Colobonema’s bell. Those stinging cells do not just capture prey—they make the silky jelly an unappetizing meal too. Although the tentacles have a bright blue color when we shine our lights on them, they cannot actually make their own blue light. The bell of the jelly, however, can radiate brilliant blue flashes of light. This light show potentially startles visual predators. It may also be a burglar alarm that attracts unwanted attention to any threat.

The silky jelly is one of the fastest swimming jellies, using coordinated jet propulsion to flee danger. Our observations of Colobonema in the depths of the Monterey Canyon have helped visualize that escape in finer detail. While at rest, Colobonema spreads out curly tentacles to capture food. But when threatened, the jelly’s bell rapidly contracts and its shape quickly transforms. The rounded bell becomes almost tubular in appearance and Colobonema’s coiled tentacles straighten and elongate. Now, a mad dash to safety begins. A single swimming burst can propel the silky jelly more than five body lengths forward.

As they make a quick exit, they drop sticky tentacles to confuse potential predators. We have seen silky jellies with tentacles of varying, uneven lengths. This suggests that Colobonema can grow back their lost tentacles.

We have recorded an extensive archive of deep-sea video that offers insight into more than just animal behavior. Our researchers use this trove of data to understand long-term trends in populations of midwater animals. Comparing thousands of observations of deep-sea jellies reveals how their populations ebb and flow over time. Silky jellies are abundant in Monterey Bay, but only at specific depths. During warm El Niño events, they become scarce. As climate change alters the ocean, what will the future look like for Colobonema? We worry they will get squeezed into a smaller and smaller range.

Animals of the deep have built remarkable strategies to thrive in the ocean. But even an escape artist like Colobonema cannot outrun the impacts of climate change. We must act quickly to ensure the future of these denizens of the deep. Understanding how our actions affect the ocean and Earth’s climate is critical. Share what you have learned and help us grow our community of ocean champions.


Video Clips


Haddock, S.H.D., L.M. Christianson, W.R. Francis, S. Martini, C.W. Dunn, P.R. Pugh, C.E. Mills, K.J. Osborn, B.A. Seibel, C.A. Choy, C.E. Schnitzler, G.I. Matsumoto, M. Messié, D.T. Schultz, J.R. Winnikoff, M.L. Powers, R. Gasca, W.E. Browne, S. Johnsen, K.L. Schlining, S. von Thun, B.E. Erwin, J.F. Ryan, and E.V. Thuesen. 2017. Insights into the biodiversity, behavior, and bioluminescence of deep-sea organisms using molecular and maritime technology. Oceanography, 30: 38–47. PDF.

Meech, M.E., C.E. Mills, S.H.D. Haddock, and R.W. Meech. 2021. Two swimming modes in Trachymedusae; bell kinematics and the role of giant axons. Journal of Experimental Biology, 224: 1–12.

Widder, E.A., S.A. Bernstein, D.F. Bracher, J.F. Case, K.R. Reisenbichler, J.J. Torres, and B.H. Robison. 1989. Bioluminescence in the Monterey Submarine Canyon: Image analysis of video recordings from a midwater submersible. Marine Biology, 100: 541–551.