How big can Europan fish get?

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?

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"

posted almost 5 years ago

Zxyrra‭

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