Page 38 - AlertDiver_Spring2013

36 | SPRING 2013
B y E l l e n P r a g e r , P h . D .
MYSTERY YOUNG
For hours to months at a time, larvae drift across
the seas. They are diminutive, alienlike creatures
typically less than a millimeter across, often looking
very unlike their adult counterparts. In time
and with change they become more familiar in shape and
size, but initially the ocean’s youngest life-forms can be a
mysterious and odd-looking lot.
The paper-thin phyllosoma is a flat, white skeleton of a
creature that more resembles a tiny Halloween decoration or
a squashed spider than a prized delicacy — the spiny lobster.
Another larva, the zoea, is a miniature bug-eyed Pac-Man
sporting a pith helmet, a far cry from the clawed crab it’s
destined to become. Larval sea stars are tricky characters
as well. With a transparent blob of a body and what seem
to be too many arms, this young echinoderm appears to be
mimicking an early-stage squid or octopus.
Tiny jellyfish, baby squid, eels and larval fishes look more like
their grown-up forms, but distinguishing species early in their
development can be almost impossible. Many organisms in the
ocean, particularly bottom-dwelling invertebrates, have complex
life cycles that include large-scale deployment of drifting, nearly
unrecognizable young that transform into adults through a series
of dramatic metamorphoses.
Reproductive Strategies
Over time, several common strategies for bringing forth new
generations in the ocean have evolved. The goal of them all, of
course, is to promote the survival of offspring. Some marine
organisms give birth to what are essentially miniature versions
of themselves; these young are ready to crawl, swim or even
hunt on day one. For example, the young sand tiger shark is
one baby not to be messed with — even in the womb. Sand
tiger shark embryos hatch in utero. The largest tends to hatch
first. Already teethed and hungry, the young pup preys on
its developing siblings. One could say that sand tiger sharks
are the ultimate natural-born killers, having had hunting
experience even before birth. This is an extreme example of
what is called a brooding approach to reproduction, in which
only a few, well-developed young are produced.
A more common strategy is to produce an abundance
of less-developed offspring and broadcast them. In
broadcasting, large numbers of eggs or larvae are released
into the water and become diminutive members of the
wandering, drifting crowd we call plankton.
Advantages of Being a Drifter
Why release thousands to millions of eggs or larvae into the
sea, leaving their travels at the mercy of the wind, waves
and currents? This reproductive strategy allows widespread
dispersal, which can prevent overcrowding back at the
parental home and provide the young with access to new
geographic areas or habitats. In addition, drifting larvae do
not have to compete with adults for food, and genes are
exchanged over a wider range. By releasing an abundance of
young, organisms hedge their bets.
Larvae make for easy meals, especially for relatively small
predators. But even large-mouthed feeders enjoy an egg or
larva feast. During mass spawning events on reefs, in which
corals synchronize the release of eggs and sperm, the nighttime
buffet attracts a wide variety of diners. Worms fill the water,
fish swarm, and even whale sharks swoop in to consume huge
quantities of eggs and developing larvae in mouthful after
giant mouthful. But it’s not just predators that spell doom for
the wandering young. Unfavorable currents, a lack of food
or harmful conditions may all end a young life. By releasing
copious quantities of larvae, chances are that a few will survive.
Odd Forms
Planktonic larvae often look very different from their fully
grown versions because of larvae’s need to stay adrift and
avoid predators. Most larvae are either transparent or
translucent, which helps them go unseen in the ocean’s sunlit
layers. The sharp spine on the crab zoea is a means to deter
some small predators. It also increases the surface area of the
organism, thus helping it stay aloft in the water column.
Spines, spikes, pits, flaps, feathery cilia and a flat shape all
increase an organism’s surface area and improve floatation.
Baby snails, which resemble miniature, flying versions of
Metamorphosis in marine animals
LIFE AQUATIC Courtesy Southeastern Regional Taxonomic Center, S.C. Department of Natural Resources