Essays by David Denning
Photographs by David Denning and Bruce Russell
Larvae of Marine Animals
WE'RE NOT TALKING MERMAIDS HERE!
These are marine animals in their early stages, within a few weeks after hatching from the egg. Can you guess what the adult animal looks like? Follow the links to see the adult form.
Larva #1 (Size - 1.2mm)This veliger larva lives in the ocean for several weeks after hatching from a microscopic fertilized egg. You can see the motion of cilia, hair-like structures on the edge of a wing-like velum . Can you guess what the adult animal looks like? What advantage might there be for an animal to have a larva that drifts about in the ocean? Size: 2mm.
Larva #2 (Size - 2mm)Many marine animals send their larvae out into the ocean to drift and feed. You can see the packed gut of this nauplius larva , showing that it successfully harvested microscopic plants from the sea. The green color is chlorophyll pigment from the algae food. Notice that the green changes to yellow near the anus, indicating digestion and absorption are completed in this region of the gut.
Larva #3 (Size - 2mm)Two important features help to distinguish this larva - it is segmented, and it has sets of bristles. The head are four eyes and a ring of cilia, hairlike structures used for movement and for pushing fluids through and around the body. An earlier larval stage of this animal called the trochophore larva lacks the bristles. This larva has already spent several weeks in the ocean as a planktonic animal, and it will feed and swim there for several more weeks before settling to the bottom to find a permanent home
Larva #4 (Size - 1.6mm)Like Larva #3, this larva appears to be segmented, but it lacks the distinctive bristles. Notice the cilia (beating hairs for movement) on the outer surface, and the pair of widely-separated eyes. This larva developed from a yolky egg after it was fertilized. It uses the yolk for nutrition, so it doesn't actually feed during its 2-3 week life in the plankton. What then would be the advantage of a planktonic larva for this species?
Larva #5 (Size - 1.4mm)Many animals undergo a dramatic metamorphosis at the end of their larva stages. The most familiar example of this is the change from catapillar to adult butterfly . The larva shown here is one of severral larval stages in the life-history of a very familiar marine animal. Notice its symmetry - it has a right and left side that are mirror images of each other - the condition called bilateral symmetry. Check the symmetry of the adult animal.
Larva #6 (Size - 2.3mm) Recognize your relative? Of all the marine larvae shown on this page, this larva is the most closely related to humans and other vertebrate animals. Yet the animal that results when this larva metamorphoses into its adult form will have no backbone (it is an invertebrate), and it will have a primitive form of feeding with a filtratioin basket.