Belongs within: Florideophycidae.
The Plocamiaceae are a family of red algae including the free-living Plocamium and its adelphoparasite Plocamiocolax.
Algal intrafamilial strife
Published 22 June 2020
Most of you are probably familiar with the old adage that one should keep one’s friends close and one’s enemies closer. From a phylogenetic perspective, the red algal genus Plocamium has certainly achieved the latter.
Plocamiaceae is a cosmopolitan family of marine red algae found mostly in temperate waters. They may grow in a variety of habitats from sheltered to exposed. Phylogenetic analyses have indicated that the family is somewhat distantly related to other red algal families, such that it is currently classified in its own order (Saunders & Kraft 1994). The great majority of the forty-odd known species of Plocamiaceae are currently placed in the genus Plocamium. These are reasonably sized seaweeds with erect or decumbent thalli that can grow about half a metre in length/height. They have flattened, complanately branched axes (that is, the branches are in the same plane as the axis they branch from). Branching is pectinate (comb-like) with each axis producing usually between two and six branchlets. The lower branchlets in a series are usually unbranched but higher ones will produce their own series of side-branchlets. In particular, the last branchlet will generally grow and overtop the axis it arose from to effectively replace it (as a result, the axis of the algal thallus will appear at first glance to have many more side branches than mentioned previously but can be seen on close inspection to have something of a zig-zag appearance representing the successive axes). The comb-like pattern of the branching is particularly evident in terminal branches of the thallus. In section, the axes have a disorganised cortex surrounding the central axial cells. Plocamiaceae have the standard triphasic red algal life cycle with gametophytes and sporophytes similar in outward appearance. Cystocarps appear to be more or less globular and borne along axial margins. Tetrasporangia are borne on the underside of modified branchlets called stichidia in a manner reminiscent of the sporangia of ferns (Gabrielson & Scagel 1989).
Only a few species have been described to date of the other genus of Plocamiaceae, Plocamiocolax. Though its reproductive anatomy demonstrates its relationship to Plocamium, Plocamiocolax is very different in its superficial appearance. It is a parasite, specifically a parasite of its sister genus. As such, they exhibit greatly reduced thalli and coloration. They grow on the host Plocamium as wartlike cushions, up to about five millimetres in diameter. As the cushion grows, it produces short, flattened projections that may be simple or forked. Tetrasporophytes may bear tetrasporangia on greatly reduced stichidia or on partially endophytic, verrucose patches.
Parasitic forms that are closely related to the species they infect are referred to as ‘adelphoparasites’, meaning ‘sister-parasites’. Adelphoparasitism is remarkably common among red algae: of over sixty known genera of parasitic red algae, about 90% are adelphoparasites (Salomaki & Lane 2014). One of the very first posts I ever wrote on this site was about red algal adelphoparasites, way back in…(gosh, really?…doesn’t time fly when you’re marching unceasingly towards oblivion…) It is possible that the regularity of this phenomenon is related to a distinctive feature of red algal development: the ability to open connections between adjacent cells allowing the passage of cytoplasm and organelles. Though the primary function of this process is presumably to facilitate the transfer of cellular products between cells of a single individual, it is not difficult to imagine a scenario where one individual hijacks another. The more closely related the adjacent cells, the greater the chance of an illicit connection succeeding. And succeeding multiple times. Though treated as a distinct genus, ‘Plocamiocolax‘ lineages have apparently arisen within Plocamium multiple times (Goff et al. 1996). In some cases, a Plocamiocolax species proves to be the direct derivative of the Plocamium species they are found infesting. In others, a Plocamiocolax has arisen on one host species but later made the switch to another. Children are supposed to become independent and find their own way in the world, but sometimes the blighters just won’t leave.
Systematics of Plocamiaceae
Characters (Womersley 1994): Thallus (of free-living species) erect, complanately branched, with alternating marginal series of 2–6 compressed lateral branches or ramuli, axes sympodially developed, base stoloniferous (or parasitic and pulvinate); structure uniaxial, with a central filament surrounded by a pseudoparenchymatous cortex. Sexual thalli dioecious; procarpic. Carpogonial branches 3-celled, borne on mid cortical cells, with the supporting cell acting as the auxiliary cell. Carposporophyte outwardly directed, with a basal fusion cell producing several gonimolobes with all except the basal cells forming carposporangia, enclosed in a dense cellular pericarp; cystocarps sessile or pedicellate on the upper ramuli of the series, non-ostiolate. Spermatangia developed from surface cortical cells on upper ramuli or on branched tufts in their axils. Tetrasporangia borne in simple or branched stichidia on the margins of upper ramuli or in branched axillary tufts. Life history triphasic with isomorphic gametophytes and tetrasporophytes.
<==Plocamiaceae [Plocamiales]
|--Plocamiocolax pulvinata Setchell 1923S57
`--PlocamiumHSA03
|--P. abnorme Hooker & Harvey 1845L27
|--P. angustum Agardh 1841L27
|--P. brachiocarpum Kützing 1849L27
|--P. cartilagineumHSA03
|--P. coccineum Lyngbye 1819L27
|--P. confervaceum Bory de Saint-Vincent 1828BS-V28
|--P. costatum Agardh 1841L27
|--P. cruciferum Hooker & Harvey 1855L27
|--P. dilatatum Agardh 1876L27
|--P. dispersum Hooker & Harvey 1855L27
|--P. hamatumNLS03
|--P. leptophyllum Kützing 1849L27
|--P. mertensiiG05
|--P. oregonum Doty 1947S57
|--P. pacificum Kylin 1925S57
|--P. procerum Agardh 1841L27
|--P. rigidum Agardh 1851L27
|--P. telfairiaeL27
|--P. tenue Kylin 1925S57
|--P. violaceum Farlow 1877S57
`--P. vulgare Bory de Saint-Vincent 1828 [incl. Delesseria plocamium γ procera]BS-V28
*Type species of generic name indicated
References
[BS-V28] Bory de Saint-Vincent, J. B. 1828. Voyage Autour du Monde, Exécuté par Ordre du Roi, Sur la Corvette de Sa Majesté, La Coquille, pendant les années 1822, 1823, 1824 et 1825. Botanique. Cryptogamie. Arthus Bertrand: Paris.
Gabrielson, P. W., & R. F. Scagel. 1989. The marine algae of British Columbia, northern Washington, and southeast Alaska: division Rhodophyta (red algae), class Rhodophyceae, order Gigartinales, families Caulacanthaceae and Plocamiaceae. Canadian Journal of Botany 67: 1221–1234.
Goff, L. J., D. A. Moon, P. Nyvall, B. Stache, K. Mangin & G. Zuccarello. 1996. The evolution of parasitism in the red algae: molecular comparisons of adelphoparasites and their hosts. Journal of Phycology 32: 297–312.
[G05] Goldberg, N. A. 2005. Temporal variation in subtidal macroalgal assemblages at Black Island, Recherche Archipelago. Journal of the Royal Society of Western Australia 88 (2): 65–71.
[HSA03] Huisman, J. M., A. R. Sherwood & I. A. Abbott. 2003. Morphology, reproduction, and the 18S rRNA gene sequence of Pihiella liagoraciphila gen. et sp. nov. (Rhodophyta), the so-called ‘monosporangial discs’ associated with members of the Liagoraceae (Rhodophyta), and proposal of the Pihiellales ord. nov. Journal of Phycology 39: 978–987.
[L27] Laing, R. M. 1927. A reference list of New Zealand marine algae. Transactions and Proceedings of the New Zealand Institute 57: 126–185.
[NLS03] Nelson, T. A., D. J. Lee & B. C. Smith. 2003. Are “green tides” harmful algal blooms? Toxic properties of water-soluble extracts from two bloom-forming macroalgae, Ulva fenestrata and Ulvaria obscura (Ulvophyceae). Journal of Phycology 39: 874–879.
Salomaki, E. D., & C. E. Lane. 2014. Are all red algal parasites cut from the same cloth? Acta Societatis Botanicorum Poloniae 83 (4): 369–375.
Saunders, G. W., & G. T. Kraft. 1994. Small-subunit rRNA gene sequences from representatives of selected families of the Gigartinales and Rhodymeniales (Rhodophyta). 1. Evidence for the Plocamiales ord.nov. Canadian Journal of Botany 72: 1250–1263.
[S57] Scagel, R. F. 1957. An annotated list of the marine algae of British Columbia and northern Washington (including keys to genera). National Museum of Canada Bulletin 150: 1–289.