‘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.

‘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.

‘The Arrow Crab’
Stenorhynchus seticornis or ‘Arrow Crab’ guarding a cave entrance
Music, Video, and Aquarium
2010 Coral Morphologic

Take a moment to look into the compound eyes of the arrow crab (Stenorhynchus seticornis). If NASA is looking for a robot capable of navigating rocky planetary terrain, the arrow crab would be a perfect organism to model it after. In the video we look down the sharp, pointed rostrum (‘nose’) of an arrow crab as it appears bobbing in space. In reality, its spindly, spider-like legs are holding it anchored like a sentinel, guarding the opening of a small cave.

Arrow crabs are an abundant species on Floridian reefs, living perched near cracks and crevices in coral heads where they can retreat if threatened. Their pointed rostrum, triangular body, and protruding eyes gives this crab the appearance of a predatory lizard fish that can dash away at a moment’s notice. Instead, the arrow crab is rather slow moving, relying on the fact that the paucity of meat inside the spiny, twig-like exoskeleton of the arrow crab makes it unappetizing to a would-be-predator. This unique anatomical configuration likely explains their abundance in the wild.

Like other decapod crustaceans, the arrow crab has 10 legs (8 walking legs, and 2 pincers or ‘chelipeds’ properly). However, if you look carefully, you’ll notice that this particular crab is missing the last leg on the right side of its body. Fortunately, crustaceans are capable of regrowing amputated legs. Only a few hours after it was filmed, this arrow crab molted, and as if by magic, regenerated its tenth limb.

‘Lima scabra’
The tentacles and mantle of a Lima scabra file clam filter feeding
Music, Video, and Aquarium
2010 Coral Morphologic

Lima scabra is a common resident on Floridian and Caribbean reefs where it can be found wedged in crevices, with only its long tentacles extending out into the water column. Usually these tentacles are crimson red (as seen in the specimen above), although they are occasionally white in color. Lima scabra can grow to about 3.5 (9cm) inches long.

Like most bivalves, Lima scabra is a filter feeder. It siphons water in through its fleshy mantle (seen in the video), and strains any edible particulate matter before pumping the water back it out. It holds itself semi-permanently in place through the use of ‘byssus threads’. The threads are formed by a viscous protein secretion that cures instantly upon contact with seawater. These byssus threads have captured the attention of bio-engineers who seek to replicate their strong adhesive properties for industrial applications. However, if the clam comes under attack from a predator, it is capable of detaching and swimming away. They can move surprisingly quick; swimming in fast, jerky movements, propelled by the repeated snapping-together of its shell.

‘Purple Forest’
Decorator Crab (Microphrys bicornuta) on Asparagopsis taxiformis algae
Music, Video, and Aquarium
2010 Coral Morphologic

This week’s video features an aquascape comprised of the beautiful purple macro algae Asparagopsis taxiformis. However, if you pay close attention to the left 1/3 of the screen, you’ll notice something… moving with claws. Nestled amongst the algae is a perfectly camouflaged decorator crab (Microphrys bicornuta). Keep paying attention… at 26 seconds into the clip you’ll notice a tiny isopod crustacean float by in the current and descend helicopter-style right onto the crab’s back. The unsuspecting isopod has no idea that it has landed upon an algae covered beast. Furthermore, it appears that the crab is not aware of the unexpected visitor until the isopod begins to explore its decorated exoskeleton.  50 seconds into the clip the isopod meets its fate with a few swift snatches of the crab’s claws. Without missing a beat, the crab continues scavenging amongst the rocks and algae. And life on the reef goes on.

Decorator crabs are amazing creatures in that they pick up pieces of their surrounding habitat and place them on their carapace (back, exoskeleton) in order to blend into their surroundings. Decorator crabs that live amongst sponges decorate with sponges, those that live amongst zoanthids use zoanthids, and so on. This instinctual logic is truly remarkable. The crab in the video has attached small pieces of the Asparagopsis upon itself, and as a result is all but indistinguishable from its surroundings.

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