How big can Europan fish get?

Question

Assumptions:

• Jupiter's moon Europa has a salty subsurface ocean in contact with a rocky mantle.
• The ocean is populated by hydrothermal vents produced by tidal heating. Global heat flow is similar to that of Earth (0.089 W/m^2). Lower gravity means lower buoyancy which means less heat transport - so vents are more numerous than on Earth, but less hot.
• Europa’s vents release roughly the same nutrients as those on Earth.
• The ocean is enriched in oxygen due to radiolysis and crustal ice circulation.
• Chemotrophic bacteria analogous to Earth’s chemotrophs managed to form around the vents around 4 billion years ago.
• An ecosystem has developed through billions of years of evolution, in which bacteria are the primary producers.

Given the available energy and rough estimates for the chemistry of Europa’s ocean, how big can Europan “fish” get? Are “space squid” a realistic part of the Europan ecosystem, or is there only enough energy and food for, say, tiny krill?

I’m looking for a science-based answer based on the structure of ecosystems. Is there enough energy and food to allow for Earth-analogous trophic levels and thus big fish? If so, just how big can these fish get? If not, what’s the size limit in such an ecosystem?

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Related but non-duplicate questions:

• This question asks about the size limit for sea creatures in general
• This question asks if a climate stable enough to support an ecosystem could exist on an icy moon; does not ask about the life within it
• This question asks about a possible Europan civilization; does not address evolution or ecosystem development
• This question asks broadly what Europan life would look like; does not factor in energy or resources
• This question also asks broadly what Europan life would look like; it focuses on "what features would evolve" as opposed to "is there enough energy for big fish"

Answer 1

One factor to consider is what kind of life we are talking about. If one considers a colony organism to be one creature instead of myriad smaller creatures, this could allow a rather large form.

Without that, the larger creatures seem likely to be apex predators. Further, a largely immobile lifestyle would tend to support a larger size. I could believe a large creature that sits near a vent and eats lots of smaller things, possibly moving from time to time to a different vent. It might be considered to "farm" the vents. The questions at this point might become: how many can be supported by the environment/planet/moon?

Answer 2

Great Question!

I’m going to say relatively small

The ultimate issue is that vents provide exponentially less energy than the sun, which means that the food chain can’t get as big due to this inherent constraint. This doesn’t mean you can’t have fish at all, just that you should expect it to be small, like the actual fish that live amongst black smokers, as they have to contend with very small amounts of food.

Eelpouts are one of the most numerous species around deep sea vents, and the biggest get about a meter long, and likely get a lot of their food from marine snow, which is ultimately photosynthetic in origin.

https://en.m.wikipedia.org/wiki/Eelpout