|photo courtesy of: the Butterfly site|
|photo courtesy of: National Geographic|
Animation of sea star development, showing multiple larval stages.
courtesy of: Daniel Brown
Having a complex, multi-stage life-cycle, like that of the sea star, may seem like more trouble than its worth to us direct developers, but there are many benefits to metamorphosis which can outweigh the costs. Most marine larvae hatch from eggs which are broadcast in large numbers into the water column. The larvae then join the ranks of zooplankton drifting at the whim of the currents. As plankton, they do not compete with the adults of their species for space or food, and they also may drift a long ways before settling down, thus decreasing the risk of both inbreeding and local extinction. These and other benefits help explain why so many species have a larval stage, especially in the marine environment, but the mechanisms underlying how metamorphosis occurs are where things start to really get interesting!
Whether it be a frog or a butterfly on land, or a mussel or jelly in the sea, all metamorphosis is hormonally controlled. The signaling pathways that lead to metamorphosis in insects and amphibians are well-studied, and better understood than those of most marine species. Tadpole metamorphosis is controlled by hormone signaling via the HPT axis. External stimuli (such as temperature, light, or water level) first trigger receptors to send a message to the hypothalamus, causing it to secrete corticotrophin releasing factor (CRF), which then stimulates the pituitary gland to secrete thyroid stimulating hormone (TSH), which causes the thyroid to release two forms of thyroxine, or thyroid hormone (T4 and T3). Thyroxine is then responsible for triggering growth of new body parts, and breaking down those in the larval form that are no longer needed.
| Tata, 2006.|
Bugula neritina is a sessile (non-moving), colonial invertebrate that belongs to the Phylum Bryozoa. It is native to the Mediterranean, but has now spread world-wide, and can often be seen in purple or brown clumps on piers or boats in local bays. Bugula have ciliated, non-feeding larvae which are released from a brood chamber, before swimming in the water column for anywhere from a few hours to several days, depending on how quickly they encounter a suitable place to spend the rest of their lives.
|Bugula sp. larvae photo courtesy of: angelfire.com|
|Bugula neritina adult colony photo courtesy of: reefedu.com|
The molecular mechanism underlying metamorphosis in this species was recently investigated by Wong et al., and they found drastic increases in expression of proteins involved with energy metabolism and protein synthesis during metamorphosis (not surprisingly), as well as upregulation of proteins specific to the Wnt signaling pathway. This pathway is well-studied in many organisms, and appears to be implicated in Bugula metamorphosis as well. (For more detailed information on this, see the article by Wong et al. listed in the references!) Based on the patterns of gene expression seen in this study, it seems that many larval structures are already pre-programmed to turn into specific adult structures.
|Bugula neritina metamorphosis Wong, et al., 2012|
As an aside:
“I cannot make you understand. I cannot make anyone understand what is happening inside me. I cannot even explain it to myself.”
-Franz Kafka, The Metamorphosis
Hopefully, you have not come away from this blog feeling like poor Gregor in Kafka's famous short story! (Hopefully, you also have not just recently awoken to find yourself a large beetle...) I imagine some caterpillars and brachiolaria larvae may be able to empathize with Gregor, however.
Lynch, William. 1947. The behavior and metamorphosis of the larva of Bugula Neritina . The Biological Bulletin 92:115-150.
Oguro, C., M. Komatsu, and Y. Kano. 1976. Development and Metamorphosis of the sea-star, Astropecten scoparius valenciennes. Biological Bulletin 151: 560-573.
Pechenik, J. 1999. On the advantages and disadvantages of larval stages in benthic marine invertebrate life cycles. Marine Ecology Press 177:269-297.