If you have ever been stung by a jellyfish or coral you’ve already encountered one of these – a nematocyst. It is the stinging capsule found within specialized cells of Cnidaria. Nematocysts have poison filled harpoons that rapidly fire to attach to and incapacitate prey, or to deter predators.
This 3D model, available for download at Thingiverse if you would like to print it yourself, is an homage to anatomical relief models that probably adorned your doctors office and junior high science classroom. It shows the nematocyst in three stages of discharge. First, the inverted harpoon is packed within the capsule. The harpoon is inside-out at this point. Second, a stimulus has led the capsule to pressurize, swinging open the operculum (the trap door at the top), and begin everting the poison filled thread. As it extends, it turns right-sideout, exposing hooked barbs as it goes. Third, the thread is fully extended. At this point, it would be lodged in the flesh of whatever brushed up against it.
This model was designed by Daniel Newman, a certificate student in the Natural Science Illustration program at the Rhode Island School of Design, while doing an internship facilitated by Amy Bartlet Wright in my lab. His original concept sketches can be viewed above, along with the printed models.
Riley Thompson made this animation about the fascinating lifecycle of narco babies.
We usually don’t think of babies that grow inside their mothers as parasites, but sometimes the lines get very blurry. This is especially true in Narcomedusae, a group of poorly known jellyfish found throughout the world’s oceans. Some species of Narcomedusae (affectionately called narcos by the people that study them) can grow inside their own mother, who provides nourishment and a safe environment for her. The narco babies can then leave their mother, find another jellyfish of an entirely different species, attach to its flesh, and thrive on the nourishment and safe environment it provides. The physiological interaction of baby and host is similar in both cases – the host provides, the baby takes. But in one case the host is providing for its own offspring, in the other it is providing for somebody else’s offspring.
Thanks to Rebecca Helm and Fabien Lombard for their help translating the wonderful paper on narco life cycles: Bouillon, J. (1987) Considérations sur le developpement des Narcomeduses et sur leur position phylogénétique. Indo-Malayan Zoology 4 : 189-278.
We have a front and a back and two legs. We walk around on our two legs. When we need to change the direction we are moving in, we first turn our body to face the new direction and then use our same two legs to keep going. It works for us.
But what about a round animal that also has an odd number of limbs? This is the question that Henry Astley, a graduate student in Tom Robert’s lab here at the Brown UniversityDepartment of Ecology and Evolutionary Biology, set out to answer. Like their relatives the starfish, brittlestars have five arms. Unlike starfish, which crawl around with the thousands of sticky tube feet that line the bottoms of their arms, brittlestars get around by moving their whole arms. They can move much more quickly than starfish, scurrying under a rock or sprinting across the ocean bottom.
Surprisingly, nobody had previously described the details of how brittlestars get around with their arms. Do all five arms play an equal part all the time, or do only some of the arms move at once? Do they have a favorite front and back, or can any arm serve as the front or back?
In his paper published this week (“Getting around when you’re round: quantitative analysis of the locomotion of the blunt-spined brittle star, Ophiocoma echinata“), Henry answers all these questions. It turns out that most of the work of getting around is only done by two arms at a time. These arms move in a rowing motion, much like a sea turtle crawling along the beach, while the other arms stay out of the way. My favorite part of the story though, is how brittlestars turn. Rather than rotate their body to face a new direction, as we do, they just chose a new front and back and row with a different pair of arms. Not only do they not have a favorite front and back, they constantly change their front and back to change direction.