Author: Brittany Raffa
Date: March 2008
A study published in the March 14 edition of the journal Science documents the latest discovery about sand dollars. Scientists have previously known that sand dollar larvae can clone themselves and split into two new sand dollars, but no one had connected such an act to a survival technique. Biologists at the University of Washington now have evidence that four day-old sand dollar larvae create clones of themselves within 24 hours of being exposed to fish mucus, a signal that predators are present.
"This is the first case that we know of any larvae to clone in response to predator cues," said Dawn Vaughn, a biology graduate student who is lead author of the study.
The method by which male and female sand dollars usually reproduce is by releasing sperm and eggs into the water, which then join to become larvae. The larvae from the sand dollar species Dendraster excentricusfloat as part of the plankton at various levels in open water. Plankton is made up of animals and plants that either float passively in the water or have limited means of swimming so they are carried by the current. As the larvae are carried they feed slowly and grow. After six weeks, the larvae reach a size of around three-hundredths of an inch and then dwell on the ocean floor to finish developing to adulthood.
The risks the larvae encounter are in large part due to being neither fast nor agile as they float, leading to a high mortality rate as fish easily dine on a quick meal. Vaughn took four-day-old larvae and placed one each into shot glasses, along with water, algae for food, and mucus taken off a Dover sole, a common predator of sand dollar larvae.
Within 24 hours, many of the single larvae placed in shot glasses with the fish mucus had become two. This was done in two ways.
Some of the larvae simply split in two. However, it was more common that the larvae grew a bud that then detached and developed into smaller larvae. The cloning process occurs too slowly to save a larva if a predator is right by and about to attack. But if the larva detects a predator that has just entered the area, it often has enough time to clone itself. None of the larvae placed in shot glasses without fish mucus split.
By cloning, the larvae double their odds of survival by doubling the number of larvae. "From their perspective, if both the original larva and the clone survive that's great then there are two of you," explained Vaughn. Researchers speculate that a second advantage could be that fish have a harder time spotting the smaller larvae, which as a result of being split in two are now half their original size, making it less likely that the larvae will be eaten.
But there is probably a tradeoff. If not, the larvae would always clone themselves, not merely in dire circumstances. The tradeoff is that the reduced size of the larvae could affect survival later in the sand dollar's life. This is because the larger sand dollars usually experience increased predation when a sand dollar has grown out of the larvae stage and into the juvenile stage. Sand dollars that have gone through the larval cloning process are smaller as juveniles than those that have not been cloned when they settle to the sea floor. Vaughn plans to study this aspect in the next phase of her research.
Vaughn observed that the larval sand dollars' response did not seem to rely on which fish species the mucus came from. Instead, it appears the larvae reacted to the bacterial breakdown of the mucus, and then associated that with the presence of a predator. In Vaughn's words, "They might respond to a fish species that would not eat them."
This discovery demonstrates the sweeping vastness of the field of marine biology. The echinoderms, the phylum that the sand dollar belongs to, date back 535 million years ago to the Cambrian period; yet despite being in existence for so long, it is only now that we are beginning to understand one of its defense mechanisms. The findings of Vaughn's next study on the effect of size in the survival of juvenile sand dollars will indeed be awaited with anticipation.
Written by Brittany Raffa
Reviewed by Neil Majithia
Published by Pooja Ghatalia