Fleshy Farewells

Three recent arrivals on the carnival rounds:

For things wet and squashy, see Carnival of the Blue at The Saipan Blog.

For things green and hayfevery, see Berry Go Round at Not Exactly Rocket Science.

For things hard and sedimentary, see The Boneyard at When Pigs Fly Returns.

And a reminder that Linnaeus' Legacy posts need to be in by the day after tomorrow!

When Ferns Don't Look Like Ferns

I suspect that I would hardly need to explain to anyone what a fern looks like - their cool, green, graceful appearance makes them a favourite of holders of foliage fetishes everywhere. What you possibly may not be aware with is that the classic fern is actually only part of the story. Odds are that the parent of the fern you next see growing in a pot or in a damp grove looked nothing like that fern, and if you took the spores of that fern and grew them, you may not recognise the product. Welcome to the world of alternating generations.



Alternation of generations is actually something that all land plants indulge in. A diploid sporophyte asexually produces haploid spores that grow into haploid gametophytes whose haploid gametes fuse to form the zygotes that grows into new sporophytes, as shown in the diagram above by Jeffrey Finkelstein. In seed plants, the gametophyte has been severely reduced and does not grow outside its parent - the female gametophyte remains contained within the parent flower or cone as the ovule, while the male gametophyte is only a few cells in size and forms the pollen grain. In ferns, the gametophyte grows as a separate (albeit really small - perhaps only about a centimetre across) individual with an undifferentiated thallus. Each gametophyte produces both male and female gametes at different places on the thallus, and male gametes require a layer of moisture across the surface to swim across to the female gametes and fertilise them. Cross-fertilisation occurs when multiple gametophytes grow in close proximity and joined by a common covering of moisture. The sporophyte then grows directly out of the parent gametophyte.

In the majority of fern species, the gametophyte is a small heart-shaped structure like in the diagram above. The meristem, the growing part of the plant, is restricted to the recessed point of the heart. In three fern families, though, the gametophyte is ribbon-like or filamentous with multiple marginal meristems and grows indeterminately. While gametophytes of other fern families tend to be short-lived affairs, the gametophytes of Hymenophyllaceae, Vittariaceae and Grammitidaceae can be much longer-lived. Dassler and Farrar (1997) recorded an individual gametophyte of the Hymenophyllaceae species Callistopteris baueriana still growing seven years after germination. What is more, some inderminately-growing gametophytes are able to reproduce asexually as well as sexually through the production of gemmae, side-buds that can detach and grow into new individuals (anyone who has owned a hen-and-chickens fern or a mother-of-millions plants may have seen gemmae growing along the edge of their leaves). For a very few species, this capacity for sexual reproduction has allowed them to bypass the sporophyte phase of the life-cycle entirely.


The gametophyte-only fern species Vittaria appalachiana. Photo by Bob Klips.

Currently, independent gametophytes (i.e. those that are able to establish populations without forming sporophytes) are known from a single species of Grammitidaceae, two Vittariaceae and nine Hymenophyllaceae (Lindsay, 2003). Most of these species also produce sporophytes over part of the distribution, but the gametophytes are able to survive in areas that are seemingly not conducive to sporophyte production. Vittaria graminifolia, for instance, is known in Louisiana only as gametophytes, with the nearest sporophytes of the species over a thousand kilometres away in Mexico (Lindsay, 2003). As yet, only three species are known that seemingly never produce sporophytes - Vittaria appalachiana, Hymenophyllum tayloriae and Trichomanes intricatum (Raine et al., 1991; Farrar, 1992). Nevertheless, there are good reasons to suspect that the diversity of unrecognised independent gametophytes out there might be much higher. Fern gametophytes have been studied much less than sporophytes - not only are they small and difficult to find, but they have generally been regarded as decidedly low on taxonomically useful characters. Vittaria appalachiana, the first-known gametophyte-only species, was actually discovered sixty years before it was confirmed to be identifiably distinct from sporophyte-producing species of Vittaria. It does not escape notice that all three known gametophyte-only species come from the eastern United States, even though the families involved are found in tropical and subtropical habitats throughout the world. Things become particularly suspicious when you realise that a single person, Donald Farrar of Iowa State University, has been privy to the description of all three. More than likely, the apparent absence of gametophyte-only species from other parts of the world does not suggest that there is something unusual about the eastern United States, but simply that no-one has really looked anywhere else.

Like the relationship between asexually- and sexually-reproducing fungi, the taxonomic and ecological implications of the independent gametophyte may be significant. Rumsey et al. (1999) demonstrated that the Killarney fern (Trichomanes speciosum), previously regarded as extremely rare in the British Isles based on the distribution of the sporophyte, was actually fairly widespread and common as the gametophyte. The wide distribution of the eastern North American Trichomanes intricatum, including areas previously subject to glaciation and despite the apparent low dispersal potential of gametophytes reproducing by gemmae only, led Farrar (1992) to suggest that the "extinction" of the sporophyte form may have happened only recently. Has this species really forever lost the ability to produce sporophytes, or might a change of climate lead to the unfurling of a long-forgotten frond deep within the forests of New England?

REFERENCES

Dassler, C. L., & D. R. Farrar. 1997. Significance of form in fern gametophytes: Clonal, gemmiferous gametophytes of Callistopteris baueriana (Hymenophyllaceae). International Journal of Plant Sciences 158 (5): 622-639.

Farrar, D. R. 1992. Trichomanes intricatum: the independent Trichomanes gametophyte in the eastern United States. American Fern Journal 82 (2): 68-74.

Lindsay, S. 2003. Considerations for a revision of the fern family Vittariaceae for Flora Malesiana. Telopea 10 (1): 99-112.

Raine, C. A., D. R. Farrar & E. Sheffield. 1991. A new Hymenophyllum species in the Appalachians represented by independent gametophyte colonies. American Fern Journal 81 (4): 109-118.

Rumsey, F. J., J. C. Vogel, S. J. Russell, J. A. Barrett & M. Gibby. 1999. Population structure and conservation biology of the endangered fern Trichomanes speciosum Willd. (Hymenophyllaceae) at its northern distributional limit. Biological Journal of the Linnean Society 66 (3): 333-344.

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.

The Strangest of Spiders

In the comments to an earlier post, I promised to write a post sometime on micro-spiders. As alluded to in that post, some of the smallest spiders are mind-bogglingly tiny - the smallest known male spider, Patu digua, reaches all of 0.37 mm in length as an adult, but at least one other species known as yet only from females could potentially have a male even smaller. If one of these spiders crawled into your ear while you were sleeping, it could probably slip into your Eustachian tubes and tap on the back of your eyeballs. But even more remarkable than their small size is the bizarre morphologies on show among the micro-spiders. And no group of micro-spiders is more bizarre than the Archaeidae.

Archaeids are a bit bigger than Patu, but still pretty small - the largest examples reach about six millimetres. The name "Archaeidae", of course, means "old", and archaeids received their name because they were first described in 1854 from fossils in Baltic amber from northern Europe. In Europe, the archaeids are long gone (they may have disappeared along with the amber forests), but nearly thirty years after their initial description living examples were found in Madagascar. They are also known from Australia, while a specimen from Cretaceous Burmese amber has been placed in a living genus from South Africa and Madagascar (Penney, 2003). A species has also been described from the Jurassic of Kazakhstan, but it is uncertain whether this species is an actual archaeid or belongs to another micro-spider family such as Pararchaeidae.

Many micro-spiders show relatively long chelicerae (the fangs and their base) relative to body size, but in Archaeidae this is taken to the extreme, as can well be seen in the photo by Jeremy Miller at the top of this post. Because the trochanter (base) of the chelicerae is a rigid structure, lengthening them in spiders requires that the carapace as a whole be raised, otherwise the fangs would not be able to get anywhere near the mouth. Archaeids have developed a long "neck" supporting the eyes and chelicerae. The distinct shape of the cephalothorax together with the long chelicerae gives them an unmistakeable profile, and one common name used for the group is "pelican spiders". Despite their small size, archaeids are active hunters and voracious exclusive predators of other spiders (another common name is "assassin spiders"). It has been suggested that the lengthened chelicerae are directly related to their araneophagous diet, allowing them to strike their prey without getting too close, but as I already noted archaeids are not the only small spiders with lengthened chelicerae (though they are still the most dramatic), and I'd be interested to know if there is a correlation between small size and long chelicerae.



I'd also like to share this diagram from Wood et al. (2007) showing a molecular-derived phylogeny of the endemic Madagascan genus Eriauchenius. As can be seen, there is a fair amount of variation in the thickness of the "neck" (the darkness of the bars reflects the mean carapace height/length ratio for whichever group they subtend), and it had been suggested that those species with a particularly slender neck formed a derived clade. Wood et al. (2007) found that this does not appear to be the case, with at least two extreme narrow-neck groups - E. workmani in one and E. gracilicollis and E. lavatenda in the other - at quite divergent points in the tree. I also looks to me like at least one group - E. tsingyensis and its allies - may have gone the other way. To paraphrase a Rocky Horror Picture Show audience member - that spider has no neck.

REFERENCES

Penney, D. 2003. Afrarchaea grimaldii, a new species of Archaeidae (Araneae) in Cretaceous Burmese amber. Journal of Arachnology 31 (1): 122-130.

Wood, H. M., C. E. Griswold & G. S. Spicer. 2007. Phylogenetic relationships within an endemic group of Malagasy ‘assassin spiders’ (Araneae, Archaeidae): ancestral character reconstruction, convergent evolution and biogeography. Molecular Phylogenetics and Evolution 45 (2): 612-619.

Linnaeus' Legacy: Legs Eleven

The next edition of Linnaeus' Legacy, the taxonomy and systematics blog carnival, will be coming up shortly at The Other 95%. Last month's edition at A DC Birding Blog matched its host by holding lots of birds, so my challenge to you all would be to try to make this month follow suit and bring out the marine invertebrates. Submissions can be directed to Eric Heupel via eric.heupel at gmail.com, or use the BlogCarnival submission form.

More Juvenilia

Chris M at The Echinoblog has put up a list of odd things that sit in his laboratory, and notes that people look askance at the pile of toilet paper (a vital tool in drying specimens). Well, I can top that:



K-Y jelly is actually fantastic stuff for preparing temporary slide mounts. It's transparent, it holds things in place reasonably well (though it does heat up and start flowing a bit if you leave it under the light for too long) and it's water-soluble, so you can just take a specimen preserved in alcohol out of its vial and put it straight onto the slide then return it straight to the vial when finished without needing to wash or prepare it in any way. Putting the specimen in alcohol on a concave slide is still preferable, because alcohol is more optically clear than K-Y, but alcohol won't hold the specimen at an angle in any way if you need to look at the specimen in a particular position. Seeing as how this is invertebrate systematics we're talking about, the thing I most commonly need to look at on a slide are reproductive organs. Which leads to an actual exchange that took place:

Colleague: "Why do you have K-Y jelly in your office?"

Me: "I use it for mounting... I mean, I put genitalia in it... Crap."

Offhand, I have also now discovered that if you do an image search for K-Y jelly, it pays to have the SafeSearch option turned on.

Ye Gods I'm Immature

Sometimes when naming a species, it pays to be careful...

In 1954, Roewer described a new species of harvestman named Metagagrella mysoreana (so named, I assume, because it came from Mysore). Metagagrella has since been synonymised with the older genus name Psathyropus, but most of the appropriate new combinations have not yet appeared in print. I was just entering in names for the Psathyropus section of the Palpatores nomenclator, which requires me to form said new combinations. However, because Psathyropus is a masculine name, I had to correct species name genders.

Yep.

Psathyropus mysoreanus.

The fact that I giggled when I realised shows just how much of a child I am.