Thursday, May 30, 2013

Two types of Aeolidia papillosa larvae

This is a picture of an adult nudibranch Aeolidia papillosa, found on the Charleston docks near OIMB. As with many other nudibranchs, A. papillosa package their eggs a few per egg capsule, the capsules embedded in a gelatinous ribbon, which are deposited as egg masses (pictured below). Early development is encapsulated, and embryos begin to move inside the capsule, before they hatch as shelled veligers (bottom picture).

Many marine invertebrates are characterized by a particular type of development e.g. either lecithotrophic or planktotrophic (see Jon Gienger's blog post, Planktotrophy versus lecithotrophy). Interestingly,  Aeolidia papillosa veligers hatching from the same egg capsule can be polytypic: some released as yolk-laden lecithotrophic larvae, and others as yolk-free planktotrophic larvae (Williams, 1980). Williams (1980) also noted that larvae that hatched without yolk reserves were, paradoxically, larger than those released with yolk reserves, although both types of larvae developed from uniformly small eggs. A simple explanation might be that because these two types of larvae develop within the same egg capsule, it is possible that the yolk-laden (slower developing) larvae are prematurely released from the egg capsule by their yolk-free (faster-developing) siblings.  However, yolk-laden larvae hatched from egg capsules that did not contain any yolk-free larvae. What’s more, smaller larvae were apparently less likely to feed on unicellular algae (e.g. Chlorella, Dunaliella) than their larger siblings.  Both yolk-free and yolk-laden veligers were present in the egg masses I looked at, in addition to yolk-laden trochophores, indicating that larvae were still developing.

One possible explanation for this polytypic development may be bet-hedging (varying strategy to increase the overall chances of offspring survival and success).  Lecithotrophic larvae are expected to survive to metamorphosis better than the planktotrophic under conditions of scarce food, whereas planktotrophic larvae may be more successful when phytoplakton is abundant. Producing both types of larvae may be advantageous when phytoplankton has spatially and temporally patchy distribution (Williams, 1980).

Williams, L.G. (1980). Development and feeding of the larvae of the nudibranch gastropods Hermissenda crassicornis and Aeolidia papillosa. Malacologia 20:99–116.

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