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Weekly “What is it?”: Mystery Lump

Dozens of these oddly-shaped organisms washed onshore around Pensacola Beach and Perdido Key after Hurricane Sally in September 2020. Photo credit: Carrie Stevenson, UF IFAS Extension

Every now and then, somebody sends in a “stump the expert” question, requiring further research, study, and a little experimentation to figure it out. I love these. Biology geek-wise, few things are more exciting than the idea of discovering something new, or at least rare. So, when our friend, Master Naturalist, and National Park Ranger Bob sent photos of—then hand-delivered—this biological oddity, the quest was on. I polled the ecologists around me, sought confirmation from an invertebrate expert at the University of West Florida, sliced it open, and checked to see if it would float. I even surveyed my family, as my husband grew up here fishing and spending time in the water, and my teenage son is an avid naturalist. I guess my approach was wrong with the family, because when I held up the paper sack holding a “mystery organism,” they were all a little terrified. Once I convinced them to trust that the bag’s contents would not bite, they were equally mystified. We observed it looked like a large stone arrowhead, covered in green and white filamentous algae, but it was lightweight and weathered.

Final verdict? A sponge. A weathered, algae-covered, hurricane-tossed sponge.

The open spaces in the drying sponge allowed coastal grasses to grow right through them! Photo credit, Robert Pitts, National Park Service

Most of us are familiar with the wild sponges harvested by divers, particularly if you’ve ever visited Tarpon Springs and learned the fascinating history of the free-diving Greek community there. But you really don’t see many sponges in Pensacola, especially strewn around the beach. Due to our cool winter temperatures, fewer sponge species populate the northern Gulf than in tropical areas.  In addition, heavy sedimentation from river outflow and high rates of summer algae growth can smother young sponges. Therefore, most sponges in this area live on deep, offshore reefs or on midrange depths of artificial reefs, of which the northern Gulf has many. Our park ranger friend had been seeing pieces of these sponges drying in the wrack line and grasses of Ft. Pickens and both sides of Big Lagoon since last fall. This led us to believe that Hurricane Sally broke up the offshore sponge beds, or those growing on reefs, and dropped pieces all along our beaches.

While the outside appearance was deceptive, cutting the sponge open revealed its internal structure and confirmed its identity. Photo credit: Carrie Stevenson, UF IFAS Extension

As for basic sponge biology, they are exceedingly simple animals—essentially a colony of single-celled organisms—that rely on filter feeding. They are sessile, meaning they settle and stay in one location, and their structure is provided by a “skeleton” of microscopic calcium or silica-based crystalline spicules. Sponges have large chambers and canals for moving water in and out, which result in the open pore spaces we recognize in dried sponges. While alive, these spacious chambers provide habitat for other small organisms, including worms, shrimp, algae, and bacteria. Many sponges form symbiotic relationships with bacteria; sponges provide protection and light access for bacteria, while bacteria provide chemical defense and nutrients for the sponges.

As for identifying a specific type of sponge, there are thousands, but the most typical genus in our area is Haliclona, with 6 species found in American waters. The most common varieties in the Gulf are H. rubens (a red sponge) and H. viridis (yellowish-green). However, with a hurricane, a different tropical species could have been washed in from afar.

Next time you find something unusual washed up at the beach, let me know! I work with a whole team of curious biologists who can help figure it out.