Chesapeake Bay Program - Barnacle

<body> <p>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 arthropodsmore 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." </p> <p> 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.</p> <p> 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. <br> <br> Four principal species of acorn barnacles inhabit the Bay: the <b>little gray barnacle</b>, (<i>Chthamalus fragilis</i>); <b>ivory barnacle</b>, (<i>Balanus eburneus</i>), the <b>bay barnacle</b> (<i>Balanus improvisus</i>) and the <b>white barnacle</b>, (<i>Balanus subalbidus</i>).</p> <ul> <li> 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. </li> <li> 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. </li> <li> 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. </li> <li> The white barnacle, while similar to the bay barnacle, tends to prefer less saline waters than the bay barnacle.</li> </ul> <p></p> <p><b>Life Cycle<img src="../images/barnacle1.jpg" width="374" height="174" align="right" alt="Barnacle illustration by Ken Forrest / VIMS"></b></p> <ul> <li>Barnacles are hermaphroditic, containing both male and female organs, but to reproduce must be fertilized or fertilize another individual. </li> <li> 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.</li> <li> Late springbetween May and Junein the Bay region finds the water rich with such larvae, which are often consumed in large numbers by schools of juvenile finfish. </li> <li> 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. </li> <li> 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.</li> </ul> <p> 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. </p> <table border="1" cellpadding="0" cellspacing="0" bgcolor="#C6C68C" width="100%"> <tr bgcolor="#C6C68C" valign="top"> <td><strong>Other Sites of Interest:</strong> <ul> <li><a href="http://www.mbl.edu/leuckart/L86.GIF" target="_top">Great barnacle illustration</a> - Marine Biological Lab</li> <li><a href="http://www.mov.vic.gov.au/crust/barnbiol.html" target="_top">Biology of Barnacles</a> - Museum Victoria / Australia</li> <li><a href="http://www.enature.com/guides/show_species_fg.asp?beautyID=19347&curGroup=Seashore_Creatures&recNum=SC0002&fromWhere=speciessearch" target="_top">Ivory barnacle</a> - eNature.com</li> <li><a href="http://www.micscape.simplenet.com/mag/artjan99/barnac.html" target="_top">Barnacles</a> - Living on the edge - Microscopy UK</li> <li><a href="http://science.whoi.edu/labs/pinedalab/Animations/Animated%20barnacle.html" target="_top">Animated barnacle</a> - GREAT animation! - Woods Hole</li> </ul> </td> </tr> </table> </body>