Are sea sponges kosher?

Question

The sea sponge is scientifically classified as an animal, therefore maybe it would be considered unkosher, as seafood. On the other hand, it seems much more similar to a vegetable of some kind, so would be kosher to eat.

So, are sponges kosher?

Answer 1

The Yerushalmi (Shabbos 7:2) mentions sponges (ספוג) in a group of items where cutting them causes them to grow back even more, and therefore declares that someone who does so has performed two categories of work, "reaping" and "planting." These melachos refer to plants, not animals, so I would think that indicates that the sponge is viewed as a plant (probably because it doesn't move around).

That said, I haven't found this Yerushalmi cited as halachah.

Another possible data point, though, might be that in the course of explaining when a sponge may or may not be used on Shabbos, Shulchan Aruch (Orach Chaim 320:18) cites Aruch, who explains that the sponge sits atop the head of a large fish (a whale, maybe?) and covers its eyes when it sticks its head out of the water, so that it doesn't see ships and wreck them. Mishnah Berurah there (320:45) alternately describes a sponge as "a wool-like material found on beaches." Both of these descriptions suggest that they saw sponges as inanimate or vegetable matter rather than an independent life-form.

Answer 2

Although this does not answer the question, I came across the following quote from R. Yosef Albo, Sefer ha-Ikkarim 3:1:

Coral is intermediate between inanimate matter and plants. We also find the sea sponge, which only has the sense of touch, and is an intermediate between plant and animal stages. We also find the monkey to be intermediate between animals and man.

This idea is repeated, for example by R. Moshe Isserles (the Rema) in his commentary מחיר יין, on מגילת אסתר, א:ו, and is ultimately based on Aristotle's classifications.