Background on barnacles: The "hitchhikers" of the sea
A barnacle is a type of arthropod (invertebrate) that belongs to the infraclass cirripedia “curl-footed” and is in the subphylum crustacea (crabs, lobsters). Most barnacles tend to live in shallow and tidal waters, except those that live on whales. They are sessile (non-motile) and are suspension feeders. They feed by filtering food particles in water by using a specialized filtering structure. Their protective shell is made of six calcium rich plates. These rings surround the body (carapace). Inside the carapace, the animal lies on its back with its limbs projecting forward. As for sexual reproduction, most barnacles are hermaphroditic. Barnacles will attach to invertebrates and vertebrates including sponges, corals, mollusks, hydrozoans, crustaceans, sea urchins, sea snakes, sea turtles, and of course, whales.2

Barnacle Life Cycle:
The barnacle has two distinct larval stages: the nauplius and the cyprid that occur before they develop into a mature adult. During both larval stages they swim freely in the ocean because they are non-sessile and are not yet attached to a substratum.
Nauplius- When a fertilized egg hatches it is in this stage. It has one eye and does not have a thorax or abdomen. It takes approximately six months until it reaches the cyprid stage.
Cyprid- this stage can last from a few days to a few weeks. The barnacle, during this stage, is looking for a place to settle down and “hitchhike”. The barnacle attaches to a substratum head first by using structures called antennules and secretes a glycoproteinaceous substance that helps cement the two together. The barnacle looks for a host based on its surface texture, wetability, and if there are other barnacles in surrounding areas. After this proteinaceous compound is secreted and the barnacle is attached, the barnacle becomes a juvenile.
Juvenile/Adult- during this stage, the barnacle is sessile and is attached to the substratum. It continues to grow by adding new material to their calcified plates.

The Whale/Barnacle Symbiosis:

The whale barnacle is Coronula diadema although barnacles are host specific so there are different barnacle species for different whale species. Cryptolepas rhachiaecti, for example, is the barnacle that attaches only to gray whales. It is unclear whether the symbiosis is commensal or parasitic. According to some researchers, obligate commensalism describes the relationship best because the barnacles need the whales in order to complete their life cycles and the whales do not benefit or lose. However, some researchers argue that since whale barnacles can accumulate to hundreds of pounds on a single whale, this probably bothers whales. Whales may try to rid themselves of the barnacles by rubbing themselves against the ocean floor or rocks. Interestingly, it seems there are more barnacles on slower whale species while faster whale species (similar to dolphins) have fewer barnacles. Similar to whale barnacles, whale lice also share a symbiotic relationship with whales although whale lice seem to definitely negatively affect the whales in a parasitic way since they actually feed off the whale.

---There are normally hundreds of pounds of barnacles on gray whales. Whales can hold up to about 1,000 pounds of barnacles.
---The “glue” like substance that barnacles secrete to attach to whales is so strong that dentists are studying it for its ability to stick so tight. This glue allows the barnacles to remain permanently on the attachment site. Barnacles are so deeply embedded that they usually form prong-like structures that pierce the skin of the whale. Therefore, it is nearly impossible for whales to rid themselves completely of barnacles.
---Slower whales tend to have more barnacles than faster whales
--- Barnacles seem to only use one sense: touch. They have very sensitive hair fibers on their limbs that help direct them. They also have a single eye, which probably helps differentiate between light and dark.


Larval Development and settlement of a whale barnacle2

In this paper, researchers tried to further study whale barnacles and whether there is a cue from the host that induces larval development. Barnacles from a stranded humpback whale were collected, incubated in filtered seawater for one week and nauplius larvae hatched out. When cyprids were incubated with filtered seawater, no settlement was seen after several days. Interestingly however, when the cyprids were incubated with seawater and a small piece of the whale host’s skin, four out of ten cyprids settled and became juveniles. In the Petri dish, the barnacles were found not on the skin tissue, however, but on the plastic itself indicating that a cue from the host skin allows the barnacle to settle on other substrates.

Whale barnacles and Neogene cetacean migration routes1

Whale barnacle: Coronula diadema
Humpback whale: Megaptera novaeangliae
This paper shows how barnacle fossils are good indicators of whale migratory and breeding routes because whale barnacles are very specific to a whale species. Wherever the whale host is, the barnacle symbiont is also present. During breeding months, barnacles are rubbed off by whales. One researcher observed that whales barnacles are mostly small in the early summer months but become much larger by the end of the summer and are rubbed off by the animal. The fossils that were found were probably rubbed off by migratory whales in the breeding area. Samples of fossils were collected from Ecuador and New Zealand. Since the detachment of whale barnacles has only ever been observed in migratory of breeding routes, researchers believed that the fossils found in Ecuador and New Zealand are evidence that there was once whale migration in the late Neogene and that whales may have used the Mediterranean as a breeding ground during the Pliocene and Pleistocene, unlike today. The diameters of fossils was measured and there was some discrepancy related to fossil location. The diameters of 56 well preserved shells of whale barnacles in Ecuador ranged from 21 to 82 mm in size. This large size indicates that the barnacles were adult when they were detached from whales. The barnacles that were collected from Madagascar, Coronula diadema, were much smaller (2.5-5.0 cm). There are two hypothesis that explain this massive size difference from barnacles in the strata of Punta Canoa and Tablazo formations. The first is the depositional environment was a place where crustaceans preferred to slough off skin and barnacles, so therefore, the barnacles are at all different life cycles and are different sizes. The second theory is that this site was where barnacle-infested whales died so that their mineralized remains stayed situated. Since there were a lot of these barnacles in the Canoa and Tablazo formations, the researchers proposed that during the late Pliocene-Pleistocene, the basin was, as it is today, a breeding area for humpback whales. The whales inhabited these areas for a long time which resulted in significant amounts of Coronula shells. In conclusion, the distribution of fossil cornulids can provide valuable information on the paleobiogeographical and paleoecological studies on migratory whales.


Why and how do barnacles attach to whales?
Since whales are difficult to study (due to size restrictions), there is no significant research on them. Usually barnacles samples are collected from dead, washed up whales. However, if the whale is found dead, there is a chance that the barnacles will be dead too. Research suggests that barnacles reproduce during the whales' breeding season. The whales swim around the warm shallow waters instead of the open ocean. This warmer and shallower water houses a thick "soup" of barnacle larvae. Each barnacle parent can release anywhere from 10,000 to 20,000 spawn which can survive for a few weeks in the water. Research suggests that the larvae in the water detect a chemical signal when the whale swims by that tells them to attach to the whale. Barnacles prefer to attach to whales where the flow of water is consistent. Usually, this means the head or the fins. After the initial attachment that is initiated by the chemical messenger, the barnacle is able to use its antennae to walk around the whale in order to find a favorable location.3 Barnacles probably prefer to attach to whales rather than rocks because a whale's motility provides constant interaction between the water and the barnacle. As well, when the whale swims, the barnacle is able to obtain nutrients from the surrounding water. Since whales are constantly moving around, the water that the barnacles are surrounded by never becomes nutrient depleted.

1. Bianucci et al. “Whale barnacles and Neogene cetacean migration routes”. New Zealand Journal of Geology & Geophysics 49 (2006): 115-120. Print.
2. Nagota, Yasuyuki and Matsumura, Kiyotaka. “Larval development and settlement of a whale barnacle”
Barnacle”. Biol. Lett.2(2006): 92-93. Print.

3. http://scienceline.org/2010/03/how-do-barnacles-attach-to-whales/

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