Barnacles (marine mammals)


Barnacle is the common name for over 900 marine species of the subclass Cirripedia. Barnacles are unique among crustaceans in being sessile; they attach to a variety of inanimate and animate objects. Barnacles live in polar regions of the world, as well as tropical and temperate waters. The principal superorder is Thoracica, which is composed of stalked (order Lepadomorpha) and sessile (order Balanomorpha) barnacles. At least 20 barnacle species have some association with marine mammal species; those barnacles also belong to Thoracica. Barnacles attaching to marine mammals are often referred to as ectoparasites. In actuality, the barnacles do not parasitize the whale, but use their host as a substrate from which they feed on plankton in passing water. Epizoic is a more appropriate term describing this lifestyle. Barnacles attached to marine mammals have been described as an example of phoresis bordering on commensalism.

I. Life History

Barnacles have been described as “nothing more than a little shrimp-like animal, standing on its head in a limestone house and kicking food into its mouth.” The barnacle’s life cycle includes six free-swimming planktonic stages (nauplius) progressing by molts into a cypris (final larval stage), which searches for a place to settle. After settling, the cypris secretes cement from glands located in the base of the first antenna to anchor itself and metamorphoses to grow and molt. After attachment, the barnacle then begins the secretion of calcareous plates that become its home. Through an opening in the plates, six pairs of feathery, leg-like appendages (cirri) can emerge and spread out to sweep through the water, like a net, to entrap planktonic organisms. Most barnacles are hemaphroidites, i.e., each individual possesses reproductive structures of both sexes. The breeding season of barnacles that cling to whales is synchronous with that of the whales’ breeding season.

II. Sessile Barnacles

The balanomorphs or sessile barnacles are stalkless; the barnacle “shell” attaches directly to a surface. Marine mammals have interactions with Balanus, Cetopims, Chelonibia, Coro-nula, Cnjptolepas, Platylepas, Ttibicinella, and Xenobalanus. Three of these, Xenobalanus. Platijlepas, and Tubicinella superficially resemble stalked barnacles, but are actually aberrant sessile barnacles and can be considered pseudo-stalked. Because of their superficial resemblance to acorns of oak trees, coronuline (Coronula sp.) barnacles are called aconi barnacles.

III. Stalked Barnacles

The stalked, pedunculate, or goose barnacle is considered to be the more primitive. This barnacle is mounted on a muscular, flexible stalk (peduncle) that is attached to a firm base, and the upper portion of the animal is the major part of the body, bearing shell plates. Conchodenna, Lepas, and Pollicipes are all commensals of marine mammals.

IV. Barnacles and Marine Mammals

Barnacles appear to settle in greatest numbers on slower-moving cetaceans, as evidenced by the great number of barnacles found on gray whales (Eschrichtius robustus) and other mysticetes, compared to delphinids. Bottlenose (Tursiops trun-catus) and striped (Stenella coendeoalba) dolphins involved in mass mortality events on the U.S. Atlantic coast and in the Mediterranean, respectively, had an abundance of barnacles that might have been due to the reduced movement of sick dolphins and/or impaired immune functioning of the skin.

Cryptolepas rhachianecti is consider to be host-specific to gray whales but has been found on a killer whale (Orcinus orca) that stranded in southern California and on belugas (Del-phinapterus leucas) housed in San Diego Bay. Xenobalanus is almost always found on the trailing edges of the dorsal and pectoral fins and on the tail flukes of cetaceans (Fig. 1). Platylepas may be mistaken for Xenobalanus; there is one record of Platylepas hexasttjlos (found associated with Xenobalanus) from a bottlenose dolphin. Tubicinella major has been found buried cryptically among callosities of southern right whales (Eubalaena australis). A Ttibicinella sp. has been collected from the flank of a stranded northern bottlenose whale (Hyperoodon ampullatus). Coronula is a large barnacle that attaches almost exclusively to the skin of mysticetes and is rarely found on odontocetes. C. reginae and C. diadema are common epizoites of humpback whales (Megaptera novaean-gliae). Acorn barnacles can be found attached to flukes, flippers, ventral grooves, genital slit, and head of humpback whales (Fig. 2). One humpback was reported to have as much as 450 kg of acom barnacles attached to it. Humpback males scrape each other with their barnacle-encrusted flippers on the breeding grounds. The two Conchoderma species (C. auritum and C. virgatum) are the only true stalked barnacles recorded from cetaceans, with the exception of one record of Pollicipes poly-merus that was found associated with those two species on a humpback whale. The stalked barnacle Conchoderma requires a hard surface for attachment. Conchodenna auritum is identified by its rabbit-eared appendages. C. auritum may be found at a site where teeth are exposed and unprotected, such as on enipted teeth of adult male beaked and bottlenose whales, or because of a malformation (including bone injury) in the jaw (Fig. 3). Conchoderma is less commonly found on baleen plates and has been once recorded from the penis of a stranded sperm whale (Physeter macrocephalus). C. auritum is also often found attached to Coronula (most commonly to C. diadema and much more rarely to the barnacle Pollicipes). C. virgatum is never attached directly to a cetacean, but is epizoic on other barnacles, usually C. auritum, as well as the parasitic copepod Pennella, which may attach to cetaceans. Cetopirus complanatus closely resembles C. reginae; there are only two well-documented records from southern right whales. Lepas usually occurs on floating objects, yet L. pectinata and L. hilli have been found between the teeth of some Mediterranean striped dolphins.

Xenobalanus attached to the dorsal fin of a bottlenose dolphin.

Figure 1 Xenobalanus attached to the dorsal fin of a bottlenose dolphin.

Humpback whale with acorn barnacles (Coronula) and Conchoderma auritum attached to the barnacles. Also visible are white-rim scars from acorn barnacles that have dropped off.

Figure 2 Humpback whale with acorn barnacles (Coronula) and Conchoderma auritum attached to the barnacles. Also visible are white-rim scars from acorn barnacles that have dropped off.

Conchoderma barnacles attached to the teeth/jaw of a pantropical spotted dolphin (Stenella attenuata).

Figure 3 Conchoderma barnacles attached to the teeth/jaw of a pantropical spotted dolphin (Stenella attenuata).

There are few published records of barnacles on pinnipeds. Three species of Lepas (L. pacifica, L. australis, and L. hilli) and Conchoderma auritum have been recorded from the dorsal body surface of various seal species. Manatees may acquire barnacles when in salt water; these barnacles die in fresh water and drop off, leaving scars. The common barnacle found embedded in the skin of West Indian (Trichechus manatits) and West African (T. senegalensis) manatees is Chelonibia ma-nati. Platylepas hexastylos has been found on dugongs (Dugong dugon) and West Indian manatees. Balanus amphitrite, B. eburneus, B. reticulatus, B. trigonus, and B. improvistis attach to Chelenibia on manatees, but not directly to the skin. When population of preferred prey are reduced, sea otters (Enhydra lutris) in California and Alaska will eat Balanus nubulis.

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