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Background
 Barnacles
are so common a sight in the Bay's tidal zones that few people stop
to wonder what kind of organisms they really are. Could they be mollusks,
with their calcareous, plated shells? It turns out they're actually
arthropods–more closely related to the blue crab than the blue
mussel, with which they sometimes compete for space to attach to the
surfaces of rocks. As Harvard biologist Louis Agassiz wrote in the 19th
century, a barnacle is "nothing more than a little shrimp-like
animal standing on its head in a limestone house and kicking food into
its mouth."
There are two major groups of barnacles: Balanomorpha, or acorn barnacles,
which attach directly to a firm surface, and Lepadomorpha, or goose
barnacles, which live in the ocean and attach gregariously to floating
objects by a long purplish-black, rubbery stalk. This group of barnacles
has fewer plates than acorn barnacles and those plates are smooth. The
common goose barnacle (which in entirety resembles a goose's extended
neck) can grow to a length of six inches.
Acorn barnacle species live in intertidal regions and are exposed
to the air for a significant part of each day. They secrete a shell
of overlapping plates of calcium carbonate with a gap at the top covered
by valve-like "doors" that open when the animal is submerged,
and close when it is exposed to the air. The base of this igloo-like
structure attaches to a firm surface such as a pier, rock, or the hull
of a boat. When the tide is low, the barnacle closes its valves to retain
moisture and does not emerge from its "igloo" until it is
again submerged, at which time six pairs of feathery, chitinous legs
called cirri emerge and sweep planktonic material (its source of nourishment)
into its cavity.
Four principal species of acorn barnacles inhabit the Bay: the little
gray barnacle, (Chthamalus fragilis); ivory barnacle,
(Balanus eburneus), the bay barnacle (Balanus improvisus)
and the white barnacle, (Balanus subalbidus).
- The little gray barnacle is conical with a flattened top, roughly
1/4 inch tall and 3/8 of an inch wide at the base. Its limey plates
are grayish white. This species often congregates on rocks in small
groups or alone, usually near the high intertidal zone along the coast
from Cape Cod to Florida and Texas. It also attaches to the blades
of marsh grasses. Like other acorn barnacles it eats only when submerged,
which in this case occurs only at high tide.
- The ivory barnacle is a larger species that prefers the waters
of the lower Bay but can be found in bays and estuaries from Maine
to South America. Roughly 1 inch high by 1 inch wide, this species
is also conical with a flat top, with two pairs of limey plates overlapping
one of two pairs of unpaired plates. It inhabits rocks and pilings
in the lower bay and rivers, near the low-tide line and in shallow
water. It prefers very brackish, almost fresh water.
- The bay barnacle is white, about 1/4 inch high and 1/2 inch wide,
similarly constructed, preferring to attach to rocks, pilings oyster
shells and other hard-shelled animals in brackish estuaries, in a
wider range of depths, from shallow water to 120 feet deep. Its habitat
stretches from Oregon to Ecuador, and from Nova Scotia to Florida
and Texas.
- The white barnacle, while similar to the bay barnacle, tends to
prefer less saline waters than the bay barnacle.
Life Cycle
- Barnacles are hermaphroditic, containing both male and female organs,
but to reproduce must be fertilized or fertilize another individual.
- To accomplish this a barnacle must extend a sperm tube as far as
several inches from its shell and through the valves of another attached
barnacle. Fertilized eggs grow within the barnacle shell and later
are released into the water as larvae.
- Late spring–between May and June–in the Bay region finds
the water rich with such larvae, which are often consumed in large
numbers by schools of juvenile finfish.
- Barnacle larvae are distinguished by two stages, the nauplii stage,
in which the larvae appears to be triangular and resembles the larvae
of a copepod, with two small spiky appendages.
- A few days later the nauplii stage gives way to the cypris larval
stage, in which the larva resembles small seeds, and is unable to
take in or digest food. Over the next days the cypris swims in search
of a suitable place to attach, and often chooses an area that is already
inhabited. Scientists conjecture that the older barnacles may release
a chemical signal into the water that attracts larvae to the same
locations. The cypris larvae secretes an adhesive material that enables
it to attach to the stationary object with its head. Gradually the
animal secretes the plates that protect its body.
Although the gnarled appearance of barnacles seems to imply that the
animals can permanently withstand any shift in its fortunes, they are
vulnerable to periods of continual dryness and will also perish in extreme
cold temperatures or other harsh conditions. Barnacles also succumb
to predation by bryozoans and sponges, which grow over them and smother
them, or whelks, which cover the conical barnacle and force open its
valves.
Illustration courtesy: Ken Forrest /
VIMS
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Last modified: 06/06/01
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