I am looking for examples from the literature, or personal observations.

My idea is that the "U-shape", common in some intertidal filter feeders (e.g. some intertidal polychaetes, some burrowing shrimps), might facilitate the flushing of water inside the burrow (unidirectional flow) during flood tide, after wastes and CO2 accumulated during low tide, while the burrow was exposed to air.

This would be a reasonable adaptation for burrowing gobies living in anoxic sediments, e.g. intertidal mud.

Furthermore, the burrows of intertidal gobies should be exposed to air for some time during low tide, therefore there is a risk of desiccation. A U-shape would allow to sit in the deeper and wetter portion of the burrow (just like a J-shaped one), where the fish might even bury itself to find more moisture (a vertical burrow would probably make burying quite difficult for a fish).

One more point: turrets around the openings (one should be higher than the other one, following the above rationale), of such a burrow would allow a passive flushing (while underwater, with a current flowing on top - flood, ebb tide) due to a Bernoulli effect, just like the feeding mechanism of some sponges. Therefore if mud-intertidal gobies do exist, do the burrows of these gobies also happen to have turrets?

For other reasons, I suspect a good group to search to reply to this question would be the Gobionellinae ... any idea?