One warm May night while anchored in Bahia Santo Domingo, the gateway to Baja's Bahia Conception, I dragged Paul away from boat work to take a night dive around the wild outside of the peninsula. After rounding Punta Hornito and the fish camp we dinghied beyond Punta Aguja and its flashing white light, slowed around the next tiny point, and dropped anchor in 3 meters on sand. With dive lights in hands we gave ourselves to the mysteries of the black depths and swam away from shore, accompanied by graceful yet goblin-looking banded guitar fish and bullseye electric rays. After passing over a thick band of seaweed we found ourselves hovering over a very special spot: a rhodolith bed; probably the most critical marine habitat that you've never heard of.
Paul's pic of a lizard fish resting on a rhodolith bed
There before us was a visibly endless swath of sea bed consisting of violet-red, delicately branched, golf-ball sized, spherical rhodoliths (rhodo=RED, liths=STONES) made of calcium carbonate, just like the structural skeletons of well-known stony corals. But rhodoliths are not corals, those beloved reef-building animals who have a symbiotic relationship with photosynthetic algae living in their bodies. No, each of these round coralline structures was made slowly over the past 100 years by just red algae alone. Yep. These simple plants make all their food from the sun and use their solar-fueled energy to build these beautifully intricate, knobby, pointy, and leafy, coral-like balls. Each rhodolith contains within it a Dr. Seusian Whoville, containing so much twisty, turny, cavernous real estate that a single ball can be home simultaneously to over 75 individuals, themselves representing over 35 different invertebrate taxa (species). If you investigate a single rhodolith you're sure to find many crustaceans (crabs and their kin), polychaetes (marine worms), mantis and ghost shrimp, molluscs (clams, scallops, snails), and probably some un-named species yet unknown to science. The rhodolith bed itself, with its deep strata of living, dead, and disintegrated red stones provides habitat to burrowing friends like garden eels and electric rays, free-living corals, small fish like gobies and blennies, and is a critical nursery for juveniles of larger species.
Paul's pics of a knobby rhodolith and a foliate, leafy formed rhodolith
Rhodolith beds are scattered all around the world but can only grow in particular environments rich in nutrients like the Sea of Cortez and where there is consistent water currents to keep them rolling and growing round and free of smothering debris. Rhodoliths are greatly understudied as are most of the lives they shelter. The European Union, New Zealand, and Australia have recently begun protecting their biodiversity-supportive rhodolith beds which are greatly susceptible to death due to disturbance from trawling and bottom fishing. Here in Mexico's Sea of Cortez many species rely on these rocking and rolling cradles of coralline algae as nurseries and the call is out for the country to consider locations of known beds when creating marine reserves.
Do you want to get an easy look at a rhodolith bed without night diving with us? You might try the shallows of the southwest passage along Coronado Island. You could snorkel out to the beds that show up as bands of darker water when you sail through. Also at Coronado, poke among the sandy sediment ringing the beach on the island's south side and you may find bleached-white fossil rhodoliths. (Please leave them there.) Actually, the white sands you enjoy at Coronado are mostly made of dead, crumbled rhodoliths! Another easily accessible and frequently visited spot is the southwest edge of Isla Inez, offshore from Punta Chivato. Just be careful not to anchor on them with your dingy!
PS - The drawing is from my sketchbook and gives a sense of the variety and abundance that a single mature rhodolith can support.