Earlier this week, our entire lab had the opportunity to observe some tunicates. It was pretty cool to see some representatives of what are (surprisingly) our closest non-vertebrate relatives. However, some likely missed out on seeing the rarer pyrosomes that were brought in on Tuesday (see Figure 1). Pyrosomes (from pyro = “fire”, and soma = “body”) are pelagic, colonial tunicates and part of the class Thaliacea, an amalgamation of strange chordates. Somewhat like the colonial ascidians, a pyrosome consists of many, individually small zooids all sharing a single tunic and a single large posterior opening through which water exits (although each zooid does have its own excurrent siphon, through which the filtered water is ‘deposited’ into the central chamber of the body before moving out of the shared exit).
At the time that I observed them in the lab, I didn’t have much of a sense of what pyrosomes were, aside from being another tunicate group, nor what might have made them so interesting to our instructors. After a little reading though, it’s clear that the group has a couple of pretty eye-catching features. One of these, as we saw during lecture, is simply the incredible size that the pyrosome colonies can potentially reach. The other, and the one I really want to tell you about, is their remarkable bioluminescent capabilities (their namesake). Equipped with a pair of luminescing organs, one on each side of a zooid’s oral siphon, the “fire bodies” are capable of producing bright, sustained light which can be seen many meters away, up to 100 m in clear water, by one calculation (Mackie and Bone, 1978).
While it’s thought that this luminescence may result from the activity of intracellular, bioluminescent bacteria making their home within the pyrosomes and acting as organelles, differences in the properties of the luminescence observed in pyrosomes and that observed in similar kinds of bacterial symbionts known in fish contradict that notion (Mackie and Bone, 1978). So, it would seem that for now the question of what’s ultimately responsible for the light is still open. What we do know, however, is that the pyrosomes will luminesce in response to light (kind of an odd reaction), electrical stimulation, or mechanical disturbances (e.g. being poked with a fiberglass rod). We also know that once an individual zooid has activated its luminescence, the behavior will essentially spread throughout the colony in a chain reaction, with each of the nearby zooids lighting up in turn (Bowlby et al. 1990). Moreover, not only will adjacent zooids respond to the light signals of their colonial companions, but other nearby pyrosome colonies may respond as well if sufficiently stimulated. When many pyrosomes are present in the same general area it’s possible to observe a vivid array of bright, pale lights produced by the many animals. It was just this sort of observation that led the great Thomas Huxley (“Darwin’s Bulldog”) to remark in 1849: “I have just watched the moon set in all her glory, and looked at those lesser moons, the beautiful Pyrosoma, shining like white-hot cylinders in the water” (Bowlby et al. 1990, citing Huxley). For those lucky enough to be at sea when they’re around, I imagine there are few sights as pleasant as that of the ‘moonlight’ produced by the fire bodies.
For more interesting natural history about pyrosomes in Northern California and the origin of the specimen that came to our lab via Bodega Bay, read the following at the blog, The Natural History of Bodega Head.
Mackie, G. O., and Q. Bone. (1978) Luminescence and associated effector activity in Pyrosoma (Tunicata: Pyrosomida). Proceedings of the Royal Society of London B: Biological Sciences 202: 483-495.
Bowlby, M.R., Widder, E.A., and Case, J.F. (1990) Patterns of stimulated bioluminescence in two pyrosomes (Tunicata: Pyrosomatidae). The Biological Bulletin 179: 340-350.