The blob
This is the first of what will likely be many posts on marine babies with a blobular shape. Most marine larvae look like variations on the blob body plan. The particular blob I will be talking about today is the larva of the common sea star and original keystone species, Pisaster ochraceus.
Adult Pisaster ochraceus, one of the proud parents of all the larvae pictured below.
Pisaster ochraceus is a free-spawning marine invertebrate, meaning that females release eggs and males release sperm into the sea where fertilization occurs. Females release an absurd number of eggs each time they spawn, as many as 40 million. The tiny fertilized eggs develop into little swimming cylinder blobs (gastrula), then into a form of armed blob called a bipinaria larva, and finally as the arms grow and more arms develop it becomes the final larval stage, a brachiolaria. From the gastrula onwards, the larva is wrapped in bands of tiny hairs called cilia that it beats like thousands of miniature paddles to swim and capture food.
Clockwise from top left: Pisaster ochraceus egg, gastrula, bipinnaria, brachiolaria. Images are not to scale.
The larvae feed on microscope single-celled algae called phytoplankton. They spend anywhere from 2-8 months as larvae swimming around and feeding as members of the plankton community before they are big enough and ready to return to the shore and metamorphose into juveniles.
Pisaster ochraceus brachiolaria
During this time out in the plankton they come into contact with multitudes of ravenous predators including fish, jellyfish, and fellow small members of the plankton community. However, this larva does not have any visible defenses like the hard carapace and spines of the crab zoea I covered in my last post. Interestingly, evidence suggests that Pisaster ochraceus larvae may accumulate poisons to discourage predators from eating them [1].
Pisaster ochraceus brachiolaria capturing food.
Pisaster ochraceus brachiolaria swallowing.
When we think about metamorphosis in familiar animals like butterflies and frogs, the entire larva (caterpillar or tadpole respectively) transforms into the adult form. This is not the case in echinoderms, the group of animals that includes sea stars as well as sea urchins, sand dollars, sea cucumbers, brittle stars, and sea lilies. In echinoderms, the adult body forms inside the larval body, almost like a parasite. When it is ready to metamorphose, the adult body will burst out of the larva and attach to the ground. Sometimes the larva then detaches and swims away, but often the tiny new adult (called a juvenile) will resorb the larval tissues, effectively eating itself.
This Pisaster ochraceus brachiolaria ate too much food and is throwing up some of the excess (look for the little circles shooting out from the stomach) while also pooping (look for the stream of stuff leaving the larval body).
Finally, here's a haiku I just composed about this beautiful marine baby:
Hairy blob covered
in multitudes of rowers
pulling for their life
[1] Cowden, C., Young, C. M. & Chia, F. S. 1984 Differential predation on marine invertebrate larvae by two benthic predators. Mar. Ecol. Prog. Ser. 14, 145–149.