Belongs within: Thoracica.
Contains: Tetraclita, Balanidae, Pyrgomatidae.
The Balanomorpha are the clade of barnacles containing the familiar acorn barnacles. Members of the clade are characterised by a symmetrical wall formed of 1, 4, 6 or 8 plates, and with the operculum usually formed from paired terga and scuta. At least the first two pairs of cirri are modified as mouth appendages with the third pair also being somewhat modified in Coronuloidea and more extensively modified in Balanoidea.
Barnacles among the coral
Published 5 December 2007
Barnacles are among the oddest animals on the planet. Genealogically speaking, they’re crustaceans, but with such a highly derived morphology that, except for the jointed cirri (actually derived legs), you’d be hard pressed to find an obvious character marking them as such. Much is made of how barnacles spend their lives functionally upside-down (the legs are protruded out to filter food particles from the water, after which they are funneled downwards towards the mouth), their enviable reproductive organs, and how much pain a patch of them can cause while walking below the high-tide mark on a rocky coast. The most familiar barnacles are the rock-inhabiting pyramidal forms, but others seek out different habitats.
Barnacles living on corals have been assigned to three families, the entirely coral-living Pyrgomatidae, the genus Armatobalanus in the Archaeobalanidae and the genus Megabalanus in the Balanidae, but Anderson (1992) agrees with previous authors that the Pyrgomatidae and Armatobalanus form a single clade, with Armatobalanus representing the more ancestral form from which the more specialised Pyrgomatidae originated. Armatobalanus has a wall constructed of six plates as in other families of barnacle, while Pyrgomatidae show a trend towards fusion of the wall plates, with at most four and sometimes a single plate in the wall. The range of variation from more generalised to more specialised forms seen by Charles Darwin during his major revision of the world’s barnacles was a significant factor in confirming Darwin’s acceptance of the concept of transmutation of species, and the Pyrgomatidae are no exception. Coral-inhabiting barnacles run the gamut, from taxa that are merely resident on the coral and have a fairly typical barnacle morphology such as Armatobalanus, to the derived Hoekia monticulariae with fused opercular plates, vestigial cirri and enlarged mouthparts that feeds directly on the coral overgrowing it.
The greatest threat to any coral-living animal is being overgrown by the coral itself. In Armatobalanus, this is prevented using purely mechanistic means—as the cirri are extended from the aperture, they actively scrape away any overgrowing coral, and enlarged maxillipeds that also protrude from the aperture flick away the resulting debris. In the Pyrgomatidae, a frill has developed that protrudes from the aperture when open on either side of the cirri, and probably secretes a growth inhibitor that excludes the coral (the presence of some sort of chemical defense is indicated by the fact that dead barnacles are rapidly overgrown). Some species of the less derived pyrgomatid genus Cantellius, while possessing the apertural frill, also retain the teeth on the cirri and enlarged maxillipeds of Armatobalanus species. More derived pyrgomatid species show a trend towards reduction in size of the aperture, which Anderson (1992) suggests may be because that reduces the size of the perimeter the barnacle needs to keep clear of coral. The downside of aperture reduction is that it requires some degree of reduction in the size of the cirral fan, and hence reduces feeding efficiency. It has long been suggested that at least some pyrgomatids may compensate for the reduced feeding ability through some degree of parasitism from the host coral, either through tissue feeding or absorption of dissolved nutrients. While many species do show a trend towards weakening of or the development of pores in the basal shell or membrane separating the barnacle from its host (and this basis is completely lost in Hoekia), direct evidence for parasitism in genera other than Hoekia is slight. No evidence of nutrient transfer was found in a study of Newmania milleporum, but Anderson (1992) points out that Newmania is one of the more actively-feeding species, without a reduced basis, so is not one of the most likely candidates for parasitism anyway.
On the basis of morphology, Anderson (1992) suggested a phylogeny for the pyrgomatid subfamily Pyrgomatinae that placed the derived genera in three groups arising independently from the basal Cantellius, which was itself derived from Armatobalanus or an Armatobalanus-like ancestor. However, this phylogeny was not supported by the more recent molecular study by Simon-Blecher et al. (2007). In their phylogeny, Armatobalanus is actually nested within the pyrgomatids (and one “pyrgomatid” genus, Wanella, seems to actually be a convergent member of the Balanidae). If the phylogeny of Simon-Blecher et al. (2007) is correct, then there appears to have been a fair degree of homoplasy in the fusion of the wall plates from the ancestral six retained in Armatobalanus. The most interesting possibility suggested to me by the molecular phylogeny, however, is that the mechanistic method of coral exclusion of Armatobalanus, rather than being ancestral, may actually be derived relative to the chemical inhibition method. Cantellius, the genus Anderson (1992) suggested retained relictual features of the mechanistic method, is sister in Simon-Blecher et al.‘s (2007) tree to Armatobalanus, adding more credility to the idea that we should reverse our ideas of ancestral vs. derived.
Systematics of Balanomorpha
<==Balanomorpha |--Chionelasmatidae [Chionelasmatoidea]MD01 | |--Chionelasmus darwiniBWW93 | `--EochionelasmusBWW93 |--PachylasmaBWW93 [Pachylasmatidae, PachylasmatoideaMD01] | `--P. veteranum Buckeridge 1983BWW93 |--TetraclitoideaMD01 | |--BathylasmaJB12 [BathylasmatidaeMD01] | | `--B. rangatira Buckeridge 1983BWW93 | `--TetraclitidaeC03 | |--TetraclitaC03 | |--Tesseropora rosea (Krauss 1848)HJ08 | |--Eopopella eosimplex Buckeridge 1983BWW93 | `--ElminiusC03 | |--E. modestusDL77 | `--E. plicatus Gray 1843C03 |--CoronuloideaMD01 | |--PlatylepasC79 [PlatylepadidaeMD01] | |--ChelonibiaC79 [ChelonibiidaeMD01] | | |--C. patulaC79 | | `--C. testudinariaA99 | `--CoronulidaeMD01 | |--Emersonius cybosyrinx Ross 1967BWW93 | `--CoronulaW81 | |--C. balaenarisW81 | |--C. diademaW81 | `--C. reginaeW81 |--BalanoideaMD01 | |--BalanidaeMD01 | `--+--PyrgomatidaeA92 | `--ArchaeobalanidaeA92 | |--SolidobalanusBWW93 | `--Armatobalanus Hoek 1913 [Archaeobalaninae]A92 | |--A. allium (Darwin 1854) [=Balanus allium]A92 | |--A. arcuatus (Hoek 1913) [=Balanus (Armatobalanus) arcuatus; incl. Cantellius tredecimus]A92 | |--A. cepa (Darwin 1854) [=Balanus cepa]A92 | |--A. circe (Kolosvary 1947) [=Balanus circe]A92 | |--A. filigranus (Broxh 1916) [=Balanus filigranus]A92 | |--A. funiculorum (Annandale 1906) [=Balanus funiculorum]A92 | |--A. nefrens (Zullo 1963) [=Balanus (Armatobalanus) nefrens]A92 | |--A. oryza (Broch 1931) [=Balanus (Armatobalanus) oryza]A92 | |--A. palaoensis (Hiro 1937) [=Balanus (Armatobalanus) palaoensis]A92 | |--A. quadrivittatus (Darwin 1854) [=Balanus quadrivittatus]A92 | |--A. quinquevittatus (Hoek 1913) [=Balanus (Armatobalanus) quinquevittatus]A92 | `--A. terebratus (Darwin 1854) [=Balanus terebratus]A92 `--ChthamaloideaMD01 |--CatophragmidaeMD01 | |--Catophragmus polymerusH04 | `--Pachydiadema cretaceumBWW93 `--ChthamalidaeY03 |--Hexelasma hirsutumJB12, PP64 |--Chinochthamalus Foster 1980Y03 | `--*C. scutelliformis (Darwin 1854) [=Chamaesipho scutelliformis]Y03 `--ChthamalusY03 |--C. anisopomaR96 |--C. antennatusH04 |--C. challengeriY03 |--C. dalliY03 |--C. depressusPP64 |--C. fissusY03 |--C. fragilisRS10 |--C. malayensisY03 |--C. montagui Southward 1976C03 `--C. stellatus Poli 1971C03
*Type species of generic name indicated
References
[A92] Anderson, D. T. 1992. Structure, function and phylogeny of coral-inhabiting barnacles (Cirripedia, Balanoidea). Zoological Journal of the Linnean Society 106: 277–339.
[A99] Ax, P. 1999. Das System der Metazoa II. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fisher Verlag: Stuttgart (transl. 2000. Multicellular Animals: The phylogenetic system of the Metazoa vol. 2. Springer).
[BWW93] Briggs, D. E. G., M. J. Weedon & M. A. Whyte. 1993. Arthropoda (Crustacea excluding Ostracoda). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 321–342. Chapman & Hall: London.
[C03] Chan, B. K. K. 2003. Studies on Tetraclita squamosa and Tetraclita japonica (Cirripedia: Thoracica) II: larval morphology and development. Journal of Crustacean Biology 23 (3): 522–547.
[C79] Crisp, D. J. 1979. Dispersal and re-aggregation in sessile marine invertebrates, particularly barnacles. In: Larwood, G., & B. R. Rosen (eds) Biology and Systematics of Colonial Organisms pp. 319–327. Academic Press: London.
[DL77] Dinamani, P., & P. A. Lenz. 1977. Some aspects of spatfall of the New Zealand rock oyster during 1974. Veliger 20 (1): 17–26.
[H04] Haeckel, E. 1899–1904. Kunstformen der Natur. Bibliographisches Institut: Leipzig und Wien.
[HJ08] Huisman, J. M., D. S. Jones, F. E. Wells & T. Burton. 2008. Introduced marine biota in Western Australian waters. Records of the Western Australian Museum 25 (1): 1–44.
[JB12] Johnson, M. E., & B. G. Baarli. 2012. Development of intertidal biotas through Phanerozoic time. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 63–128. Springer.
[MD01] Martin, J. W., & G. E. Davis. 2001. An updated classification of the Recent Crustacea. Natural History Museum Los Angeles County, Science Series 39: 1–124.
[PP64] Peres, J. M., & J. Picard. 1964. Nouveau manuel de bionomie benthique de la mer Mediterranee. Recueil des Travaux de la Station Marine d’Endoume, Bulletin 31 (27): 5–137.
[RS10] Regier, J. C., J. W. Shultz, A. Zwick, A. Hussey, B. Ball, R. Wetzer, J. W. Martin & C. W. Cunningham. 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature 463: 1079–1083.
[R96] Roff, D. A. 1996. The evolution of threshold traits in animals. Quarterly Review of Biology 71 (1): 3–35.
Simon-Blecher, N., D. Huchon & Y. Achituv. 2007. Phylogeny of coral-inhabiting barnacles (Cirripedia; Thoracica; Pyrgomatidae) based on 12S, 16S and 18S rDNA analysis. Molecular Phylogenetics and Evolution 44 (3): 1333–1341.
[W81] Watson, L. 1981. Sea Guide to Whales of the World. Hutchinson: London.
[Y03] Yan, Y. 2003. Larval development of the barnacle Chinochthmalus scutelliformis (Cirripedia: Chthmalidae) reared in the laboratory. Journal of Crustacean Biology 23 (3): 513–521.