The brown algae of the Phaeophyceae* are one of the predominant groups of multicellular algae in the modern ocean. Though not the most speciose of the three (that title belongs to the red Macrorhodophytina), they are in many places the most ecologically significant, as this is the group that includes the gigantic kelps that predominate and shape many parts of the marine environment. As significant as kelps are, though, they are not my focus for today. Instead, I'm going to look at a much smaller group of brown algae, the Myrionemataceae.
*The names "Phaeophyceae" or "Phaeophyta" have usually been used to refer to the monophyletic clade of multicellular brown algae. Cavalier-Smith & Chao (2006) recently proposed an expanded "class Phaeophyceae" to include the unicellular Phaeothamniophyceae and the very-basically-multicellular Schizocladia, with the traditional Phaeophyceae referred to as "subclass Melanophycidae". As Cavalier-Smith & Chao did not dispute the monophyly of the traditional Phaeophyceae, this is a simple ranking issue and I suspect it will probably be largely ignored by everyone else.
Myrionemataceae are minute filamentous algae, usually only a couple of millimetres in size (Abbott & Hollenberg, 1993). They generally grow in a disc only one or two cells deep, with numerous filaments growing upwards from the strands of the disc. These filaments may be uniserial or branching; they may or may not carry lateral processes or sporangia. Myrionemataceae most commonly seem to be epiphytic on much larger marine plants and algae.
Myrionemataceae (and small filamentous brown algae in genera) have been a royal pain in the neck for algal taxonomists, and as a result they have been widely shunned. For instance, the main distinguishing characteristic between the Myrionemataceae and the similarly filamentous Ectocarpaceae has been that the former is discoidal and the latter is not (Clayton, 1974). However, the genus Hecatonema (included by some authors in Myrionemataceae and excluded by others) includes some species that always grow in a disc, and others that do not. Some "Myrionemataceae" have been suggested to represent divergent growth forms or dimorphic generations of other brown algae placed in different families (many marine algae have very distinct alternating generations). This seems very probable for some species, but proving such relationships is not easy and Clayton (1972) claimed that at least some cases of myrionematoid algae being grown from cultures of other algae represented contamination instead of relationship. Other Myrionemataceae that have been cultured show no signs of alternating generations, while yet others have only ever been recorded reproducing asexually.
It seems almost certain that the "Myrionemataceae" are a polyphyletic grouping, with their overall lack of distinguishing features obscuring a diversity of origins. Peters (2003) found the type genus, Myrionema, to be itself polyphyletic. The type species of Myrionema, M. strangulans, has a fairly isolated position within the Ectocarpales sensu lato in Draisma et al. (2001), but it is pretty much anyone's guess as to how many of the so far unanalysed myrionematoid species belong with it*. Some species may be established as growth-forms of other taxa and shipped off accordingly, but others may prove more intractable and remain in the "family" - not so much a wastebasket as an inedible core.
*Das et al. (2005), in a paper that had me completely baffled but which I rather suspect was complete twaddle to begin with, cite a later phylogeny which they attribute to Draisma et al. (2002) that positions Myrionema strangulans in the Chordariaceae - unfortunately, Draisma et al. (2002) has absolutely no mention anywhere of Myrionema, chordariaceous or otherwise, so I wouldn't know what reference Das et al. really had in mind.
Abbott, I. A., & G. J. Hollenberg. 1993. Marine Algae of California. Stanford University Press.
Cavalier-Smith, T., & E. E.-Y. Chao. 2006. Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista). Journal of Molecular Evolution 62: 388-420.
Clayton, M. N. 1972. The occurrence of variant forms in cultures of species of Ectocarpus and Giffordia. Br. phycol. J. 7: 101-108.
Clayton, M. N. 1974. Studies on the development, life history and taxonomy of the Ectocarpales (Phaeophyta) in southern Australia. Australian Journal of Botany 22: 743-813.
Das, S., J. Chakrabarti, Z. Ghosh, S. Sahoo & B. Mallick. 2005. A new measure to study phylogenetic relations in the brown algal order Ectocarpales: The “codon impact parameter”. J. Biosci. 30: 699–709.
Draisma, S. G. A., J. L. Olsen, W. T. Stam & W. F. Prud'homme van Reine. 2002. Phylogenetic relationships within the Sphacelariales (Phaeophyceae): rbcL, RUBISCO spacer and morphology. European Journal of Phycology 37: 385-401.
Draisma, S. G. A., W. F. Prud'homme van Reine, W. T. Stanm & J. L. Olsen. 2001. A reassessment of phylogenetic relationships within the Phaeophyceae based on RUBISCO large subunit and ribosomal DNA sequences. Journal of Phycology 37: 586-603.
Peters, A. F. 2003. Molecular identification, distribution and taxonomy of brown algal endophytes, with emphasis on species from Antarctica. In Seventeenth International Seaweed Symposium (A. R. O. Chapman, R. J. Anderson, V. J. Vreeland & I. R. Davison, eds) pp. 293-302. Oxford University Press.
Taşkin, E., M. Öztürk & M. J. Wynne. 2006. First report of Microspongium globosum Reinke (Phaeophyceae, Myrionemataceae) in the Mediterranean Sea. Nova Hedwigia 82 (1-2): 135-142.