A branching shape attached to the seabed may look plant-like in a photograph. Appearance alone is a poor guide to the way an organism lives, and marine sponges are a useful reminder of that limitation.
Sponges are animals with bodies organised around the movement of water. NOAA’s introduction to sponges describes how water passing through their porous structure supplies food and oxygen and carries waste away.
Look for a process, not a familiar outline
When reading about a specimen, ask how it obtains food, where it lives and which structures the description identifies. Those questions are more informative than asking which land plant its shape resembles.
A still photograph cannot show the whole process. Openings may be visible, but the water movement and tiny structures responsible for it may require other methods to observe. Do not infer that an attached organism is inactive simply because the image freezes it in place.
For a notebook exercise, place a visual description beside a short account of the documented process. Mark any feature you cannot distinguish in the photograph.
The result is a more careful introduction to unfamiliar ocean life: what the organism looks like remains useful, while what it does receives evidence of its own.