Mitrocoma cellularia (A. Agassiz, 1865) : Cross Jelly
Phylum Cnidaria / Class Hydrozoa / Order Leptomedusae / Family Mitrocomidae

The cross jelly is among the more common hydromedusae to appear in the waters of Monterey Bay and other West Coast locations. Typical of members of this group, the bell is transparent, thus making it difficult to see without careful observation. The margin of the bell (diameter up to 90 mm) is lined with several hundred fine, white tentacles. The most conspicuous part of this jelly are the 4 radial canals to which are attached the white gonads. They form the relatively conspicuous “cross” that gives the common name to this species. The small stomach has 4 frilly lips. A strong bioluminescence is produced around the bell margin. Fall and winter months are the best times to find Mitrocoma in Monterey Bay. At times they may form large aggregations that are easily visible from the surface. The third photo shows a swarm that has invaded near-surface waters of a kelp forest at Point Lobos in central California. Cross jellies are found in nearshore waters from Alaska to central California.

Recent research has shown that cross jellies can actually detect chemicals in the water – they respond with changes in swimming behavior and extensions of tentacles when presented with traces of blended food. This may be implicated in their tendency to form large aggregations since they may be able to detect concentrations of prey. Other species presumably have similar chemical sensing abilities.

Mitrocoma cellularia, video courtesy of Patrick Anders Webster, Pt. Lobos CA, January 2014

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