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the adults, have a winglike sheet of tissue that keeps
them adrift. Larvae additionally need a means to
detect and capture organic particles for food; although
larvae are not considered true swimmers, many are
weakly motile for this purpose.
The Metamorphosis
The larvae that successfully run the ocean’s gauntlet morph
into juveniles and eventually adults. They may become
full-fledged swimmers or sink to the bottom to live on
(and perhaps attached to) the seafloor. Just finding the
right spot to homestead can be problematic. Some benthic
invertebrates, such as barnacles and oysters, are considered
gregarious settlers — they need to settle in previously
established communities of their own kind. Other organisms
need a specific type of substrate or chemical cue or have to
avoid others of their own kind due to cannibalistic appetites.
When, where and how organisms transition from larvae to
juvenile to adult varies. In some cases it is just a matter of
increasing in size, but in others it is an extreme life change
where little, if any, resemblance to the larval form remains.
Larval fish frequently look similar to their adult forms.
One type of fish, however, shows a surprising dissimilarity.
The larval flatfish appears very fishlike, with a typical body
shape and tail and an eye on either side of its head. An adult
flatfish, such as a flounder, sole or halibut, however, has
both eyes on the top of its body (or is it the side?). During
the transition from larva to adult, these fish have a truly
awkward adolescent stage in which one of the eyes moves
over the top of the head to the other side. The fish then lies
down on the bottom on its eyeless side. Some species of
flatfish are right-side blind, while others are left-side blind; it
depends on which eye did the traveling.
In many benthic invertebrates the metamorphosis
from floating larva to bottom dweller includes a series of
morphological and biochemical changes. The most dramatic
changes may take place while drifting, or they may occur once
the organism settles on the bottom. In corals, much of this
transformation takes place on the seafloor. Once a suitable
landing site is detected (often with chemical cues), the coral
larva swims down. In some species of coral, the presence of
crustose coralline algae provides the chemical signal required.
Coral larvae in contact with a suitable bottom first contract
and then lay down an organic matrix that’s followed by the
beginnings of their stony calcium-carbonate skeletons. The
metamorphosis continues as the coral develops its first skeletal
cup and a mouth surrounded by a ring of tentacles containing
nematocysts (stinging cells). For coral larvae that are the result
of broadcast spawning, at this stage they take on their algal
partners, the symbiotic zooxanthellae that live within their
tissues. The budding of clones from the initial coral-polyp
settler generates a growing coral colony. If a substrate is not
suitable, coral larvae sometimes get a second chance — a free-
floating reprieve — by contracting, detaching and returning to
the life of a drifter in search of a good home.
For echinoderms such as sea urchins, brittlestars, sea
cucumbers and sea stars, the metamorphosis from larva to
familiar-looking juvenile takes place both in the water column
and on the bottom. Sea stars, for example, may transform from
barrel-shaped blobs to armed octopus mimics while adrift in
the water and then attach to the seafloor and morph again.
Eventually a crawling, free-living sea star comes into being.
From far left: the phyllosoma larva of the spiny
lobster (Panulirus argus); the tadpole-like
larva of a tunicate; the zoea larva of the
spider crab (Maja squinado); the unusual
larva of the Luidia sarsi sea star
PHOTOS THIS PAGE BY Richard Kirby