The common name doesn't refer to this jelly's mood (although how you'd assess a jelly's mood is a question you might ponder while watching these critters float by). Rather, it comes from the set of four white canals that you can see under the bell of the jelly. You can see the canals so well because this jelly is fairly transparent. One assumes this is to make it less visible to predators. I'm guessing, however, that animals that feed on cross jellies sometimes get mixed up and ingest clear plastic bags instead -- one more reason to take steps to keep our trash out of coastal ecosystems.
I don't know that transparency helps the cross jelly sneak up on the zooplankton on which it feeds. (Do you need to sneak up on zooplankton? Are zooplankton aware of the approach of a hungry jelly in time to launch evasive maneuvers?)
This jelly doesn't have the prominent, frilly mouth arms you might see on some other jellies. Instead, its bell is ringed with hundreds of fine tentacles that are relatively short given the size of the jelly's bell. To my eye, it makes the cross jelly look like a relative of the "scrubbing bubbles" in the Dow bathroom cleaner commercials of my childhood.
The Monterey Bay Aquarium notes that research indicates that cross jellies can detect chemicals, including oil, in the water they swim in, and that they may be able to "smell" food in the water and propel themselves toward it. I'm guessing that a jelly nervous system is interestingly different from a human one, and would be interested to know how "smell" differs between cross jelly and human.
Indeed, to the extent that researchers are prepared to say that a jelly can detect anything, whether it be a whiff of tasty zooplankton or a patch of oil, they must be observing some kind of response from the jelly to the thing being detected. What kinds of observable responses do these jellies display?
(Do you need to sneak up on zooplankton? Are zooplankton aware of the approach of a hungry jelly in time to launch evasive maneuvers?)
zooplankton is such a huge and varied category of life that the answers include 'yes', 'no', 'maybe', and (most common) 'we do not know anything at all about that weird critter'.
I'm guessing that a jelly nervous system is interestingly different from a human one, and would be interested to know how "smell" differs between cross jelly and human.
Beautiful post and photos, SG! Cnidaria do have a fundamentally different nervous system organization from more complex metazoans. One big difference is that they do not possess a central nervous system. Rather, their neurons are organized in a "nerve net", where sensory information is processed and leads to motor outputs only locally in the body. However, the fundamental cellular properties of cnidarian neurons are remarkably similar to those of more complex metazoans.