Gel Weeds

The red algae of the genus Gigartina are a widespread bunch, most diverse in temperate regions of the southern hemisphere but also found on most coasts of the north. They grow as erect thalli that may come in a variety of forms: foliose or dichotomously branched, cylindrical, compressed or flattened (Hommersand et al. 1993). Like other members of the Gigartinaceae, the family to which they belong, growth is multiaxial (the primary growth axes composed of multiple filaments). The inner cortical and medullary cells are rather loosely arranged and separated by a copious matrix (Hommersand et al. 1999).

Pestle weed Gigartina pistillata, copyright Ignacio Bárbara.

Members of the Gigartinaceae have an isomorphic life history with the alternating haploid gametophyte and diploid tetrasporophyte generations being similar in overall appearance. Historically, Gigartina has included some species that were subsequently found to have a heteromorphic life cycle with very different-looking generations (Guiry & West 1983). Despite the similarities in appearance of the gametophytes to true Gigartina, these species are now thought to belong to a distinct family, the Phyllophoraceae. The specific details of Gigartina reproduction are, as with all red algae, obscenely complicated, but it is on the basis of these details that Gigartina is distinguished from related genera (Hommersand et al. 1993). Gigartina gametophytes may be either monoecious (with male and female gametes formed on a single thallus) or dioecious (with separate male and female individuals). The reproductive structures of the gametophytes are formed near the apex of the thallus on distinct branchlets, pinnules or papillae. Again as is typical for red algae, ova are not released but retained on the gametophyte, and their fertilisation results in the growth of a diploid carposporophyte on the parent gametophyte. The carposporophyte then releases diploid spores (carpospores) that are released to give rise to the tetrasporophyte generation. In Gigartina, the carposporophytes are each surrounded by an envelope of secondary filaments. Filaments of the carposporophyte penetrate between the cells of the envelope and fuse with them to form a placenta composed of heterokaryotic cells (with a mix of haploid and diploid nuclei). Carposporangia are produced in grape-like clusters. In the tetrasporophytes, tetrasporangia develop embedded within the thallus at the boundary between the cortex and the medulla. Tetraspores are released when the tetrasporangium as a whole is released by the breakdown of the containing patch of cortex; the resulting holes can leave the tetrasporophyte thallus with a reticulate appearance.

Mature carposporophyte of Gigartina pistillata, from Hommersand et al. (1993).

Economically, Gigartina species are of most interest to humans as a source of long polysaccharides called carrageenans. Carrageenans are characteristic of the Gigartinaceae; other notable carrageenan producers include the well-known Irish moss Chondrus crispus. Though not digestible by humans (they largely past through the digestive tract unaltered), carrageenans are used in food production to thicken and set liquids in a similar manner to gelatin. According to Wikipedia, the use of Gigartina for food production is known as far back as 600 BC in China. In pre-industrial methods, carrageenan can be obtained by boiling cleaned seaweed and then straining the resulting brew. In modern times, carrageenans are used to provide texture to a wide range of products, including dairy products such as ice cream or yoghurt, processed meats or vegetarian meat substitutes, or cosmetic products such as toothpaste or shampoo. It has even been used in paper production: old-style marbled paper was made by floating ink on a mixture including carrageenan. Truly a versatile little compound!

REFERENCES

Guiry, M. D., & J. A. West. 1983. Life history and hybridization studies on Gigartina stellata and Petrocelis cruenta (Rhodophyta) in the North Atlantic. Journal of Phycology 19: 474–494.

Hommersand, M. H., S. Fredericq, D. W. Freshwater & J. Hughey. 1999. Recent developments in the systematics of the Gigartinaceae (Gigartinales, Rhodophyta) based on rbcL sequence analysis and morphological evidence. Phycological Research 47: 139–151.

Hommersand, M. H., M. D. Guiry, S. Fredericq & G. L. Leister. 1993. New perspectives in the taxonomy of the Gigartinaceae (Gigartinales, Rhodophyta). Hydrobiologia 260–261: 105–120.

Algal Intrafamilial Strife

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.

Plocamium species growing on the coast of South Africa, copyright Derek Keats.

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).

Close-up on terminal brachlets of Plocamium coccineum, copyright Fernan Federici. Some tetrasporangia-bearing stichidia are visible in the lower part of the image.

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.

Plocamiocolax pulvinata growing on Plocamium, copyright Michael Hawkes.

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.

REFERENCES

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.

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.

The Alga of Uncertainty

The red alga Rhodomela confervoides, from Coastal Imageworks.

The tradition in taxonomy that nothing is ever really forgotten (for which there are very good reasons) means that, over the years, we have accumulated a certain amount of excess detritus. Whether referred to as nomina dubia, species inquirendae or just plain unidentifiable, there are a number of names for which the original description or material is not adequate to determine their identity with certainty. Most nomina dubia simply slumber undisturbed, not interfering with standard taxonomic practice; they simply serve to irritate those whose role it is to assemble comprehensive listings.

The red alga Rhodomela preissii was named by Sonder in 1848 for a specimen collected in Western Australia. He diagnosed it as "fronde tereti filiformi siccitate subplicata a basi dichotome ramosa, ramis inferioribus patentibus superioribus brevioribus erectiusculis, ramulis sparsis setaceis simplicibus furcatisve, capsulis subpedicellatis solitariis ramis superioribus adnatis", Latin descriptions being the fashion at the time*. The specimen appears to have never been figured.

*Some of you may be aware that the Botanical Congress recently voted to remove the requirement for Latin diagnoses or descriptions from the Botanical Code of Nomenclature**. Authors are still required to give a diagnosis for new taxa in either Latin or English, so Chinese still doesn't get a look-in.

**Though it was also decided that it would no longer be called the Botanical Code. It's now the "International Code of Nomenclature for Algae, Fungi and Plants". I suppose that we should be just grateful they didn't go with the even more explicit "International Code of Nomenclature for Plants, Fungi, Algae, Oomycetes, Labyrinthuleans, Plasmodiophoromycetes, Mycetozoans, Dinoflagellates, Euglenaceae and Cyanobacteria (and maybe Fossil Bacteria, on alternate Tuesdays)".

Hypnea rosea, photographed by Olivier De Clerck.

Womersley (2003) noted that the type specimen of Rhodomela preissii, held at the Melbourne herbarium, is small and inadequate for the species' identification. True Rhodomela is unknown from Australia, though other members of the Rhodomelaceae occur there. Womersley, however, suggested that R. preissii might be a specimen of Hypnea. If true, this would place 'R.' preissii some distance phylogenetically from Rhodomela, the latter belonging to the order Ceramiales while Hypnea is a member of the Gigartinales. As things stand, though, no-one seems to be faced with a great need to resolve the question.

REFERENCES

Sonder, O. G. 1846-1848. Algae L. Agardh. In: Lehmann, C. Plantae Preissianae sive Enumeratio Plantarum quas in Australasia occidentali et meridionali-occidentali annis 1838-1841 collegit Ludovicus Preiss, Phil. Dr. Acad. Caesar. Leopold, Carol. Natur. Curios. et Reg. Societ. Bot. Ratisbonens, Sodalis, cet. vol. 2 pp. 148-160 (1846), 161-195 (1848). Meissner: Hamburg.

Womersley, H.B.S. 2003. The Marine Benthic Flora of Southern Australia.
Rhodophyta. Part IIID. Ceramiales – Delesseriaceae, Sarcomeniaceae, Rhodomelaceae. Australian Biological Resources Study, Canberra.

Carpospores in Chains (Taxon of the Week: Schizoserideae)

Drachiella spectabilis, one of the few species of Schizoserideae found in the North Atlantic. Note the characteristic iridescence of the fronds. If you look very closely at the photo, you can also see one other characteristic of the family Delesseriaceae to which this species belongs - the fronds are so thin that you can see the features of the rock the alga is growing on through them. Photo by Keith Hiscock.

Despite being the most speciose clade of multicellular marine algae, I must admit I find that the Macrorhodophytina* (multicellular red algae) are not that easy to get a handle on. Most of the significant distinguishing features of various groups of red algae are at the cellular level, and often wrapped up in the eye-wateringly complicated life cycles many macrorhodophytes indulge in. So before I wrote this post, I had to spend a certain amount of time looking up things like just what a "gonimoblast" is. I hope I got it right.

*If you're wondering why I didn't use the name Rhodophyta, that's because Rhodophyta is a larger clade that also includes a few unicellular forms.

The Schizoserideae are a tribe of the red algal family Delesseriaceae containing five genera - Schizoseris, Neuroglossum, Abroteia, Drachiella (Lin et al., 2002) and the recently described Nancythalia (Millar et al., 2002). Delesseriaceae is a large family of red algae with very thin fronds (sometimes only a single cell thick) that may be anything from flat, broad and unbranched to very feathery; however the fronds are not filamentous or polysiphonous (a tubular construstion with a central axial cell surrounded by pericentral cells), distinguishing Delesseriaceae from other families in the order Ceramiales. Ceramiales are in turn distinguished from other red algal orders by the mode of formation of the auxiliary cell. To explain what an auxiliary cell is, I have to tell you that Ceramiales, like many other red algae, alternate between not just two but three distinct generations. As well as having separate multicellular haploid and diploid generations (as also found in many other algae and plants), there is a third stage called the carposporophyte. Mature diploids produce haploid spores that settle and grow into mature male or female haploids. The male haploids release sperm that fertilise the females. However, the resulting zygotes are not released; instead, the diploid nucleus of the zygote abandons the zygote and invades a nearby cell to produce the auxiliary cell (in Ceramiales, the cell that becomes the auxiliary was previously one of the supporting cells for the female gamete). The auxiliary cell then gives rise to a small diploid that remains parasitic on the parent haploid - this is the carposporophyte. The carposporophyte produces diploid spores that grow into new independent diploids.


Mature carposporophyte of Schizoseris condensata, showing the large, branching central fusion cell. Figure from Hommersand & Fredericq (1997).

In members of the Schizoserideae, the female gametangium (the procarp) contains four cells called carpogonia, one of which will get fertilised by the sperm, as well as one or two basal and one or two lateral sterile cells. After the fertilised zygote nucleus has entered the auxiliary cell, the carpogonial cells fuse to form a (wait for it) fusion cell. The auxiliary cell then gives rise to the filaments of the carposporophyte (these are the gonimoblast filaments, in case you were still wondering what that was), which in turn produce the carpospores (diploid spores) in long chains. After forming the carpospore chains, the gonimoblast cells then also fuse with the fusion cell, which ends up being a large, candelabra-shaped supportive structure for the carpospore chains; this candelabra-shaped fusion cell is one of the distinguishing characters for the Schizoserideae* (Hommersand & Fredericq, 1997). Other distinguishing features include the lack of protective covering cells on the procarps, and the arrangement of cell nuclei within the fronds - in growing parts of the fronds, all the nuclei line up in a single plane. As well as the morphological characteristics, the tribe has also been supported by molecular analysis (Lin et al., 2001).

*And just to show how much I do for you - those four sentences were the result of probably about an hour of me reading and re-reading the original paper trying to work out just what the heck was going on.


Sections through fronds of Schizoseris condensata, showing the linear arrangement of nuclei. In the bottom section, the alignment is breaking apart in the older part of the frond to the right. Figures from Hommersand & Fredericq (1997).

The main centre of distribution for the Schizoserideae is in the Southern Hemisphere; Abroteia and Nancythalia are both (as far as is known) monotypic and endemic to New Zealand (Millar et al., 2002). The genus Drachiella is the exception, with three of its four species found in the northern Atlantic and the fourth species described only recently from Taiwan and the Philippines (Lin et al., 2002).

REFERENCES

Hommersand, M. H., & S. Fredericq. 1997. Characterization of Schizoseris condensata, Schizoserideae trib. nov. (Delesseriaceae, Rhodophyta). Journal of Phycology 33 (3): 475-490.

Lin, S.-M., S. Fredericq & M. H. Hommersand. 2001. Systematics of the Delesseriaceae (Ceramiales, Rhodophyta) based on large subunit rDNA and rbcL sequences, including the Phycodryoideae, subfam. nov. Journal of Phycology 37: 881-899.

Lin, S.-M., J. E. Lewis & S. Fredericq. 2002. Drachiella liaoii sp. nov., a new member of the Schizoserideae (Delesseriaceae, Rhodophyta) from Taiwan and the Philippines. European Journal of Phycology 37: 93-102.

Millar, A. J. K., & W. A. Nelson. 2002. Nancythalia humilis gen. et sp. nov. and Abroteia suborbiculare (Delesseriaceae, Rhodophyta) from New Zealand. Phycologia 41 (3): 245-253.

Coral - It's Not Just an Animal Thing

The encrusting coralline alga Hydrolithon onkodes. Photo by J. Orempüller.

Coral reefs form the world's most renowned tropical marine habitat, and are some of the most inspiring structures on the planet's surface. An incredible wealth of biodiversity hides among the reef structure, and reefs are a justly celebrated subject of many popular books. Most of such books will (hopefully) dedicate a reasonable amount of space to the primary constructors of the reef, the corals, describing how these colonial anemone-related cnidarians lay down skeletons of carbonate that, as the layer of coral grows upwards, accumulate to form the reef structure. For today's Taxon of the Week post, I'm going to showcase another significant group of organisms in the formation of coral reefs that tends not to get quite so much press - the coralline red algae of the order Corallinales.

The coralline algae are one of the most distinct groups of the red algae. They receive their name because, like coral, they lay down skeletons of calcium carbonate that both support the organism and dissuade potential grazers. Coralline algae are a very significant component of coral reefs - in at least some areas, coralline algae actually make up a higher percentage of the surface cover of the coral reef than the coral itself. Overgrowth by coralline algae can bind the reef structure, making it more resilient to erosion and wave action. In areas of coral reef restoration, coral larvae settle preferentially on substrate that is already overgrown by coralline algae (Precht, 2006). Coralline algae are also able to withstand lower temperatures and higher-energy environments than corals, meaning that algae-dominated reefs also have a much wider distribution than true coral reefs. Coralline algae also include some of the deepest-living photosynthetic organisms in the world, with some species growing at depths of 300 m, in light levels only 0.0005% of surface level that would be imperceptible to humans.


The articulate coralline alga Corallina officinalis. Photo from habitas.org.uk.

Morphologically, coralline algae are often divided into two groups, geniculate or articulate and non-geniculate or crustose coralline algae. Geniculate coralline algae grow in branching upright thalli formed of a series of segments divided by joints called genicula. Crustose coralline algae generally do just what the label says they do - they usually grow as flat crusts, though sometimes they may grow upright branches which never grow as high as geniculate algae and are never segmented. Some crustose coralline algae actually grow unattached to the substrate and form often rounded, rock-like structures called rhodoliths that in some places litter the sea floor in large rhodolith beds. Rhodoliths grow extremely slowly - Frantz et al. (2000) calculated a growth rate of approximately 0.6 mm per year using carbon-14 mass spectrometry of rhodolith sections, suggesting that the largest rhodoliths with diameters of over ten centimetres could be 100 years old or more. Such slow growth is characteristic of coralline algae as a whole - experimental observations on reef sections where grazers were excluded showed that in their absence the coralline algae quickly became overgrown by fouling macroalgae.

Unfortunately, and despite their ecological significance, systematic studies of coralline algae are relatively few and far between, largely because of the difficulties of working with them taxonomically. Many morphologically informative characters require examination under a compound microscope, a particularly challenging prospect for crustose forms which are, after all, essentially a thin layer of living tissue over a core of solid rock. Despite its superficial convenience, smaller scale morphological features and molecular phylogeny indicate that the division between geniculate and crustose forms is not phylogenetically informative (Harvey et al., 2003). Instead, Harvey et al. (2003) divided the living Corallinales into three families, the Sporolithaceae, Corallinaceae and Hapalidiaceae, based on molecular data as well as the arrangement and morphology of the sporangia. Corallinales have the stupidly complicated life-cycles of the average floridean red alga (though not so stupidly complicated as some), but the important detail is that generations alternate between morphologically identical haploid and diploid generations (Hoek et al., 1995). Corallinales have also not indulged in parasitism to the extent of other red algal groups, though two members of Hapalidiaceae, Choreonema thuretii and Austrolithon intumescens, are endophytic parasites of the Corallinaceae species Jania tenella (Broadwater et al., 2002).


Rhodolith bed off the coast of Alaska. Photo by Brenda Konar.

Not surprisingly, coralline algae have a pretty detailed fossil record compared to other algal groups, but, unfortunately, I suspect that a certain degree of scepticism is called for here. I've alluded before to the difficulty of interpreting fossil "algae" in relation to modern taxa - because the type of fine-scale microscopic features used in establishing the relationships of living algal taxa are usually unknown for fossil taxa, the risk that supposed overall resemblances may be the results of convergence rather than true relationship runs pretty high. Fairly unequivocal crown-group coralline algae of all three living families date back to the Cretaceous (Aguirre et al., 2000), but taxa dating right back to near the beginning of the Palaeozoic have also been identified as Corallinales or close relatives. Indeed the Silurian Gracticulaceae have been regarded as being only doubtfully distinct from the living Sporolithaceae (Harvey et al., 2003) - though accepting them as such would demand one heck of a ghost lineage between the Silurian and the Cretaceous. The Palaeozoic "Solenoporaceae" also turn up repeatedly in connection with discussions of Corallinales, but the integrity of this group has been regarded as debatable in recent years. Most significantly, the actual type species of the Solenoporaceae, the Ordovician Solenopora spongioides, was recently demonstrated by Riding (2004) to be not an alga at all, but actually a sponge!

REFERENCES

Aguirre, J., R. Riding & J. C. Braga. 2000. Diversity of coralline red algae: origination and extinction patterns from the early Cretaceous to the Pleistocene. Paleobiology 26 (4): 651-667.

Broadwater, S. T., A. S. Harvey, E. A. Lapointe & W. J. Woelkerling. 2002. Conceptacle structure of the parasitic coralline red alga Choreonema thuretii (Corallinales) and its taxonomic implications. Journal of Phycology 38 (6): 1157-1168.

Frantz, B. R., M. Kashgarian, K. H. Coale & M. S. Foster. 2000. Growth rate and potential climate record from a rhodolith using ¹⁴C accelerator mass spectrometry. Limnol. Oceanogr. 45 (8): 1773-1777.

Harvey, A. S., S. T. Broadwater, W. J. Woelkerling & P. J. Mitrovski. 2003. Choreonema (Corallinales): 18S rDNA phylogeny and resurrection of the Hapalidiaceae for the subfamilies Choreonematoideae, Austrolithoideae, and Melobesioideae. Journal of Phycology 39 (5): 988-998.

Hoek, C., D. G. Mann & H. M. Jahns. 1995. Algae: An Introduction to Phycology. Cambridge University Press.

Precht, W. F. 2006. Coral Reef Restoration Handbook. CRC Press.

Riding, R. 2004.
Solenopora is a chaetetid sponge, not an alga. Palaeontology 47 (1): 117-122.

A Parasite in the Family

In the previous post, I wrote about an epiphytic red alga, and mentioned in passing the interesting phenomenon of adelphoparasitism, where a parasite is very closely related phylogenetically to its host. Since then, I've been wondering how such a situation arose, and specifically whether there was a connection between red algal adelphoparasitism and the complexities of red algal life cycles (see here for a summary).

Red algae fall into three to seven classes - Rhodellophyceae (which Yoon et al., 2006, divide into five, but as I haven't yet seen the paper I'll let it slide), Bangiophyceae and Florideophyceae. Rhodellophyceae are unicellular, and I confess I don't know the details of their life cycles. Bangiophyceae (which include Porphyra, the nori used in making sushi) alternate between distinct haploid and diploid generations. Florideophyceae include the vast majority of red algae, and verge on the completely insane in life style complexity. The basic florideophycean life cycle (which, as shown in the previous post, not all members of the class go through) involves no less than three alternating generations (I linked to the diagram here in the previous post, but I'll do it again because it's a good'un). Starting with the diploid tetrasporophyte, the tetrasporophyte releases haploid spores that settle and grow into gametophytes. Male gametophytes release spermatia (aflagellate sperm) that are captured by the female gametophytes and fertilise the carpogonia. The carpogonium (and this is the interesting part for this post) then grows into a carposporophyte, which remains attached to the parent gametophyte, releasing diploid spores that grow into new tetrasporophytes. So in effect, parasitism is already part of the florideophycean life cycle. Is it somehow possible that this parasitism is behind the rise of adelphoparasitism?

It's worth noting here that similar patterns to "adelphoparasitism" are not unique to red algae. They have also been recorded among social Hymenoptera as well as mistletoes. Red algal parasites have traditionally been divided between adelphoparasites (which are closely related to their hosts) and alloparasites (not so closely related). The two classes are also supposedly distinguished by the mode of parasitism. In both, after the parasite rhizoid invades the host it adheres to and fuses with the host cells, injecting parasite nuclei and mitochondria. In adelphoparasites, the parasite nuclei then multiply within the host cell, hijacking it and causing the formation of growths which release spores of the parasite species (Goff et al., 1997), In alloparasites, the parasite nuclei do not divide in the host cytoplasm, though they do alter its physiology to facilitate the transfer of nutrients from host to parasite, and (I assume) the parasite reproductive bodies grow from the parasite rhizoid itself. Goff et al. (1997) demonstrated that one 'genus' of adelphoparasites had actually arisen polyphyletically from the host 'genus'. Zuccarello et al. (2004) demonstrated the same thing for a 'family' of alloparasites. The latter authors therefore suggested that the terms 'adelphoparasite' and 'alloparasite' were not useful. However, this does still leave the question of the different cytoplasmic interactions (Zuccarello et al. implied that this might be due to the taxa studied belonging to different orders).

Goff et al. (1997) give two possible scenarios for the origin of parasitic red algae. In one, the parasites are ancestrally epiphytic, later becoming endophytic and eventually parasitic. In the second, the parasites derive directly from spores that lose the ability to survive independently of the parent. The existence of the carposporophyte, in my opinion, gives a lot of support to this option. One possibility is that adelphoparasites arose by the second method while alloparasites arose by the first.

Goff et al. also examined the main complaint towards the second origin - even if some mutant parasitic individual does arise, what is to stop it backcrossing to the parent population? How does the parasite become established as a new species? At present, there is no really satisfying answer to this question. Goff et al. do point out that parasitic taxa have life cycles taking a fraction of the time of the host species. At any given time, only a small percentage of the individuals in a population of algae are reproductive - perhaps the difference in timing of life cycles simply meant that the chance of backcrossing between parasite and non-parasite was too low to prevent speciation?

REFERENCES

Goff, L. J., J. Ashen & D. Moon. 1997. The evolution of parasites from their hosts: a case study in the parasitic red algae. Evolution 51 (4): 1068-1078.

Zuccarello, G. C., D. Moon & L. J. Goff. 2004. A phylogenetic study of parasitic genera placed in the family Choreocolacaceae (Rhodophyta). Journal of Phycology 40: 937-945.

Little Discs of Doom

Okay, total hyperbole in the title, but I wanted to get your attention. Today I'll be looking at Pihiella liagoraciphila, a very distinctive member of the red algae that was only described a few years ago (Huisman et al., 2003).

Pihiella is an endo/epiphyte found on members of the red algal family Liagoraceae, but not a parasite as far as I can tell (red algae are notable for the range of associations between different taxa, most interestingly the occurrence of what is call 'adelphoparasitism', where parasitic species are closely related to their hosts - I'll have to write on that some day). It has a quite simple disc-shaped or subspherical morphology with rhizoids to attach it to the host and long hairs and trichogynes (hair-like appendages of the female carpogonia that catch the male gametes). Mature discs are very small, up to 400 μm in diametre and 150 μm thick, though the hairs can be up to 800 μm long. Specimens were first observed as long ago as 1858, but were interpreted as buds of the host plant. Authors thereafter disagreed as to whether the so-called 'monosporangial discs' were asexual reproductive organs of the host or an independent organism. All authors agreed that the discs were asexually reproductive.

Sexually reproductive organs on the discs weren't recorded until 2003, when Huisman et al. established that the discs were indeed a separate organism from the host. Pihiella seems to lack the obscenely complicated triphasic life cycles of other red algae (see here for an example). As already mentioned, the carpogonia (sexual organs) possess a long hair-like trichogyne, and Huisman et al. did observe examples with spermatia (the aflagellate male sex cells) attached. Nevertheless, Huisman et al. were unable to conclude whether the mature sporangia observed were asexually produced monosporangia, sexually produced zygotosporangia, or both (I feel the last option seems most likely, but what do I know?). No carposporophytes or tetrasporangia were observed (see the link above to find out what these are).

The morphology of Pihiella was too distinct from any other red alga to be phylogenetically informative, but Huisman et al. were able to assess the phylogeny molecularly. Pihiella turned out to be quite isolated from other red algae, enough that Huisman et al. established a new monotypic order for it. Interestingly, the trees recovered Pihiella as sister taxon to Ahnfeltia, another phylogenetically isolated taxon, with a high level of support. Morphologically, Ahnfeltia is very distinct from Pihiella, being a large cartilaginous plant with a triphasic life cycle found in cool waters (the host family of Pihiella, Liagoraceae, is a mostly warm-water group). Though Ahnfeltia and Pihiella are each other's closest relatives, the relationship is not close. Liagoraceae, in contrast, was in a quite distant part of the tree.

REFERENCES

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.