‘The Porcelain Crab’
Petrolisthes galathinus feeding on passing plankton
Music, Video, and Aquarium
2010 Coral Morphologic

The porcelain crab’s common name is derived from its propensity to drop claws like a fragile tea cup breaking. When attacked, the would-be predator is usually left with nothing more than a few amputated (and still-twitching) limbs. In a few days the porcelain crab will undergo an ’emergency molt’ of its exoskeleton and begin regenerating its lost appendages.

The porcelain crab shown here, Petrolisthes galathinus, is a common resident of Floridian and Caribbean reefs, living under rubble and coral heads. Turning over loose rocks will often yield a fleeting glimpse of scurrying, purple legs. They can move incredibly fast and generally remain cryptic to the passing scuba diver. While many crab species are territorial and agressive towards members of their own species, these porcelain crabs can be colonial with several dozen porcelain crabs living together under the same rock.

Despite the similar appearances, porcelain crabs are not ‘true’ crabs; they are in fact more closely related to the squat lobster clan (Galatheidae) than the archetypal brachyuran crabs we are all familiar with. Porcelain crabs’ flattened bodies are adapted to their life under rocks and in crevices. One of the defining features of porcelain crabs are the comb-like appendages called ‘setae’ that sweep the water currents in order to collect edible particles that happen to float by. Another pair of specialized appendages scrape the the setae and bring the collected food to their mouthparts. This feeding strategy, with its alternating rhythm, appears robotic in its efficiency.

‘Transmission’
Pseudoceros crozieri or ‘Tiger Flatworm’
Music, Video, and Aquarium
2010 Coral Morphologic

The tiger flatworm (Pseudoceros crozieri) is a stunning species of flatworm that can be found living on rocks and mangrove roots along the shores of the Caribbean. Colonial orange tunicates (Ecteinascidia turbinata) constitute the tiger flatworm’s only food-source. At 35mm in length, it is considerably larger than the previously featured red flatworms. As simultaneous hermaphrodites, the tiger flatworm often travels as pairs and mate regularly. Their pseudotentacle antennae help aid them in finding mates by detecting chemical cues in the water.

Locomotion in this larger flatworm species is accomplished by rippling muscle contractions along the edges of the animal, and aided by a slippery mucous slime. The video is shown in real time.

‘The Lettuce Slug’
Elysia crispata on Halimeda opuntia
Music, Video, and Aquarium
2010 Coral Morphologic

Lettuce sea slugs (Elysia crispata) are a commonly found in protected nearshore Floridian waters where green macroalgae proliferates. They belong to a clan of sea slugs, the sarcoglossans, that are characterized by their ‘sap-sucking’ feeding habits of algae. These slugs slowly patrol mangrove roots and rocks searching for green algae upon which they feed. They store some of the chloroplasts from eaten algae in their tissue, giving it the green coloration. The chloroplasts continue to function, providing the slug with photosynthetic energy. The ruffles along the back of the lettuce sea slug are called parapodia, and help provide more surface area for the chloroplasts to inhabit. They also camouflage the slug amongst the leafy algae that they live amongst. It is very easy to swim past a lettuce nudibranch without ever noticing it.

The scrolled rhinophores (antennae) on the head of the lettuce sea slug help detect the chemical fingerprints of their preferred algal species. If you look carefully, just behind the rhinophores, you’ll notice the small black eye spots that act as rudimentary eyes to detect changes in light and dark.

The macroalgae featured in the film is Halimeda opuntia, (named after its resemblance to the prickly pear cactus Opuntia sp.). It is unique amongst green algae in that it produces a semi-rigid, calcareous skeleton. In fact, the dead ‘leaf’ fragments of Halimeda spp. algae are a more significant producer of coral reef sand than the corals themselves. It is not uncommon to find lettuce sea slugs on Halimeda opuntia algae, as it frequently lives amidst the softer green algae that the lettuce sea slugs prefer.

‘Flatworms’
Convolutriloba retrogemma (flatworms)
Music, Video, and Aquarium
2010 Coral Morphologic

The flatworms (Convolutriloba retrogemma) featured in the video are shown at 3x normal speed. They each range from 2-4mm in total length.

These particular flatworms harbor symbiotic zooxanthellae in their thin tissue and utilize the excess sugars they create as their primary energy source. Packets of zooxanthellae can be seen as the tiny, red-brown dots along the back of flatworm. Their reliance upon this photosynthesis requires that these flatworms bask in sunlight like little photovoltaic cells, and enables them to live without a developed digestive system.

In the wild, this species can be found in the shallow water of protected lagoons and around mangroves. Reproduction is accomplished asexually via fission, in which the flatworms literally split into two. This strategy enables exponential population growth in optimum conditions. They are the preferred prey of several species of larger flatworms and sea slugs; animals that can tolerate their toxic bodily fluids.

While it appears that the flatworms just glide along like magic carpets, they are actually propelled by invisible cilia (flapping filaments) that slide them across a thin layer of mucous laid over whatever surface they happen to be upon.

Upon close inspection of flatworm-to-flatworm interaction, it is apparent that these flatworms do not like making direct contact with each other. If they do, they react as if stung. It seems that this reaction prevents the worms from piling on top of each other in an effort to gain the best solar power. Instead, they jockey for position until they each find a place in which to ‘park’ themselves, like sunbathers on a crowded beach.