Brilliantly bright babies
As I mentioned in my first post, crab babies can have the most extraordinary colorations. Despite their tiny size, these larvae are often brilliantly colored, especially in the clear waters of the tropics. In that first post I talked about the first larval form in the life cycle of crabs, called a zoea. Today I will focus on the second and final crab larval form, the megalopa.
Pitho laevigata megalopa
Megalopae look much more like crabs than do zoeae. They have claws (chelae) and a more flattened crab-shaped carapace. Just as with the barnacle cyprid I discussed in my last post, the job of this megalopa is to find a good habitat to settle and metamorphose into a juvenile crab. However, unlike barnacle cyprids, most megalopae must continue feeding during the arduous search for a place to settle. The megalopa in the image above is around 2-3 mm long, which is pretty big for the plankton, so they should be able to eat most other organisms they encounter in the plankton, probably placing them near the top of the plankton food web. Megalopae are one of very few marine larval forms that we can easily see by naked eye. In fact, my first encounter with a megalopa was in 2012 when I was kayaking in the San Juan Islands, noticed something small swimming in the water, and picked a tiny megalopa out of the water. I would like to say the megalopa excited me more than the orcas we saw a few minutes later, but that would be a lie.
Pitho laevigata megalopa.
The particular megalopa I am featuring today is the eggshell urn crab, Pitho laevigata. This is a small, tropical spider crab that lives as an adult on seagrass and coral rubble in shallow subtidal reef flats. The adults are pale green to gray in color, making them difficult to find within seagrass and algae. Interestingly, the megalopae also have a striking green coloration, along with bright red and dark black patches not found in the cryptic adults. This species develops remarkably quickly, reaching the megalopa stage merely 4 days after hatching. I reared Pitho laevigata to megalopae at the Galeta Marine Lab in Panama during my research on crab larval coloration, feeding them nauplius larvae of brine shrimp (also known as sea monkeys). Brine shrimp are bright orange in color, so diet could in part explain the coloration of my Pitho laevigata megalopae. It would be interesting to feed crab larvae differently colored food to see how it affects their coloration; to my knowledge this has never been tested.
Adult Pitho laevigata clinging to coral rubble
Larvae of this species have never been described in the literature so the images in this post show an animal that probably less than a handful of people have ever seen, likely fewer and maybe just two (myself and my research assistant). Science has advanced to the point where it is newsworthy when an entirely new species is discovered and described. But we know very little about the larval stages of marine animals and there is so much more to describe. Just focusing on crabs, which have been well studied in part because they taste pretty good, we have only described the larvae of about 14% of all crab species (Clark and Cuesta 2015). It is pretty amazing to think of the diversity of animal forms that have still never been seen by human eyes!
Huge megalopa
showing its true colors while
brightening clear seas
Clark PF, Cuesta JA (2016) Larval systematics of Brachyura. In: Castro P, Davie PJF, Guinot D, Schram FR, Vaupel Klein JC von (eds) Treatise on Zoology - Anatomy, Taxonomy, Biology. The Crustacea. Brill, p 981–1048