The Grapsidae: From Sea to Shore

Sally Lightfoot, Grapsus grapsus, photographed by Victor Burolla. The vernacular name refers to their walking on the points of their legs.

In a post from back in 2008, I wrote about the group of crabs known as the Grapsoidea. As described in that post, the classification of the Grapsoidea has been shuffled in recent years, and the subjects of today's post, the Grapsidae, would have previously been classed as the Grapsinae within a larger Grapsidae. The more restricted Grapsidae has been supported by numerous recent analyses, both morphological (Karasawa & Kato 2001) and molecular (Schubart et al. 2000). Morphologically, grapsids are united by having an expanded anterolateral corner to the merus* of the third maxilliped, oblique ridges on the lateral surfaces of the meri of the pereiopods, and (in many species) oblique ridges on the dorsum of the carapace (Karasawa & Kato 2001). Studies of the larvae of grapsids have also identified distinctive characters by which grapsid larvae can be distinguished from those of other grapsoids (Cuesta & Schubart 1999).

*The merus is the first elongate segment of crustacean appendages, corresponding to the femur of other arthropods. The maxillipeds are feeding appendages; the pereiopods are the walking legs.

The Columbus crab Planes major, photographed by Denis Riek. This species comes in a wide range of colours, from brown to blue to almost white; the page linked to shows a number of examples.

Most grapsids are intertidal shore-dwellers, but there are some exceptions. Species of the genus Planes, known as Columbus crabs, are small oceanic forms. They live on objects floating in the open water: seaweed, driftwood and other debris, or even other animals such as by-the-wind sailors or turtles (Spivak & Bas 1999). Columbus crabs differ from other grapsids in having flattened pereiopod meri for swimming, and two of the three species lack oblique ridges on the carapace. The aforementioned phylogenetic analyses also agree in placing Planes as the sister group to other grapsids analysed.

Geograpsus grayi, from here.

Also distinctive are species of the genus Geograpsus, which are one of a number of crab groups to have developed a terrestrial lifestyle, found on islands of the Indo-Pacific and Atlantic. In the Indo-Pacific G. crinipes, it has been shown that dense bunches of setae between the second and third walking legs are long enough to contact the ground when the animal sits back on its haunches (McLay & Ryan 1990). Water on the surface of the ground is drawn up through the setae by capillary action and conducted into the gill chamber, keeping the gills damp and functioning. Terrestrial Geograpsus retain marine larvae as do many other terrestrial crabs; the larval development has been studied for the eastern Pacific G. lividus which goes through nine larval stages (eight zoeae and the megalopa) over the period of two months (Cuesta et al. 2011). This happens to be the longest developmental pathway of any known crab: the previous confirmed maximum was eight larval stages.

REFERENCES

Cuesta, J. A., G. Guerao, C. D. Schubart & K. Anger. 2011. Morphology and growth of the larval stages of Geograpsus lividus (Crustacea, Brachyura), with the descriptions of new larval characters for the Grapsidae and an undescribed setation pattern in extended developments. Acta Zoologica 92 (3): 225-240.

Cuesta, J. A., & C. D. Schubart. 1999. First zoeal stages of Geograpsus lividus and Goniopsis pulchra from Panama confirm consistent larval characters for the subfamily Grapsinae (Crustacea: Brachyura: Grapsidae). Ophelia 51 (3): 163-176.

Karasawa, H., & H. Kato. 2001. The systematic status of the genus Miosesarma Karasawa, 1989 with a phylogenetic analysis within the family Grapsidae and a review of fossil records (Crustacea: Decapoda: Brachyura). Paleontological Research 5 (4): 259-275.

McLay, C. L., & P. A. Ryan. 1990. The terrestrial crabs Sesarma (Sesarmops) impressum and Geograpsus crinipes (Brachyura, Grapsidae, Sesarminae) recorded from the Fiji Is. Journal of the Royal Society of New Zealand 20 (1): 107-118.

Schubart, C. D., J. A. Cuesta, R. Diesel & D. L. Felder. 2000. Molecular phylogeny, taxonomy, and evolution of nonmarine lineages within the American grapsoid crabs (Crustacea: Brachyura). Molecular Phylogenetics and Evolution 15 (2): 179-190.

Spivak, E. D., & C. C. Bas. 1999. First finding of the pelagic crab Planes marinus (Decapoda: Grapsidae) in the southwestern Atlantic. Journal of Crustacean Biology 19 (1): 72-76.

Biantidae: The Importance of Titillators

An unidentified member of Biantinae, photographed in Singapore by WJ.

Long-time readers of this site will be familiar with my rants about the influence of early 20th-century arachnologist Carl-Friedrich Roewer on Opiliones taxonomy (just enter 'Roewer' into the search box near top right on this page). One of Roewer's larger errors (but an understandable one in context) involved the species he included in the family Phalangodidae. As Roewer had it, this was an almost cosmopolitan family, with representatives on all continents. However, as research has progressed, it has become clear that Roewer's Phalangodidae was a polyphyletic assemblage of a number of different lineages of relatively generic-looking Laniatores (short-legged harvestmen). Firm distinction of some of these lineages (now recognised as separate families) often requires examination of the male genitalia, something Roewer never did.

Specimen of Lacurbs, from A. B. Kury.

The Biantidae are one of these families of ex-phalangodids. They are a pantropical family, with representatives in South America, Africa and Asia. Biantids have eyes well-separated on the carapace instead of on a common central eye-mound, and they bear a strong external resemblance to another family of Laniatores that I've covered before, the South American Stygnidae. Distinguishing biantids from stygnids depends on two features: the presence of a process of the tarsi of the third and fourth legs in Stygnidae, and the presence of a ventral sclerotised plate on the penis of the Stygnidae versus no plate and dorsal processes called titillators (named for an obvious possible function) on the penis of Biantidae. However, despite the strong external similarity, stygnids and biantids are not closely related: a recent molecular phylogenetic analysis of Laniatores places them in separate superfamilies, with stygnids in the Gonyleptoidea and biantids in the Samooidea (Sharma & Giribet 2011).

Representative biantid penes, from Kury & Pérez González (2007). Left: Caribbiantes (Stenostygninae), dorsolateral view; right: Biantes sherpa (Biantinae), lateral and dorsal views. Ti = titillators; Co = conductors.

Biantidae are divided between four subfamilies (Kury & Pérez González 2007). The African genera Lacurbs and Zairebiantes are more divergent than the other two subfamilies: Lacurbs has the scutum (dorsal shield) of the opisthosoma (the rear part of the body) widest in the middle and narrowing towards the front and back, while other biantids are more or less straight-sided, and has the tibiae and metatarsi of the hind legs armed and swollen. Zairebiantes has its eyes placed closer together and further forward than other biantids, and Pinto-da-Rocha (1995) suggested that its classification as a biantid may require re-evaluation. The other two subfamilies, the South American Stenostygninae and the African and Asian Biantinae, contain the great majority of biantids and share the presence of dense scopulae (pads of hairs) on the third and fourth tarsi. Apart from their distribution, the latter two subfamilies are distinguished by genital morphology: in Stenostygninae, the titillators are rigid and sit forward to cover the capsula interna of the penis, while in Biantinae they are soft and fold back and out so that they don't cover the capsula interna.

REFERENCES

Kury, A. B., & Pérez González, A. 2007. Biantidae Thorell, 1889. In Harvestmen: The Biology of Opiliones (R. Pinto-da-Rocha, G. Machado & G. Giribet, eds) pp. 176-179. Harvard University Press: Cambridge (Massachusetts).

Pinto-da-Rocha, R. 1995. Redescription of Stenostygnus pusio Simon and synonymy of Caribbiantinae with Stenostygninae (Opiliones: Laniatores, Biantidae). Journal of Arachnology 23 (3): 194-198.

Sharma, P. P., & G. Giribet. 2011. The evolutionary and biogeographic history of the armoured harvestmen—Laniatores phylogeny based on ten molecular markers, with the description of two new families of Opiliones (Arachnida). Invertebrate Systematics 25: 106-142.

A Bizarre New Shark

Live goblin shark Mitsukurina owstoni, from here.

It's a bit unusual for me to be posting anything on a Sunday, but I've just received notice of something so incredibly cool that I couldn't wait to tell you all about it. A new paper has just come out describing a truly remarkable new species of shark:

Takahashi, N., & N. Yuasa. 2012. First recorded use of weaponised light by an elasmobranch. National Daiei Journal 7: 17-87.

The new species, Neomitsukurina nodai, is most closely related to the unusual goblin shark Mitsukurina owstoni, and the resemblance between the two is clearly visible in the head region:

Photo of the new shark species from Takahashi & Yuasa.

Nevertheless, it possesses several remarkable differences. First there is the distinctive fin array, somewhat more extensive than that found in most shark species. The denticles in the skin are much reduced, giving the body an almost rubbery appearance. Furthermore, in a remarkable case of life imitating art, Neomitsukurina differs in its jaw morphology. The vast majority of depictions of goblin sharks show it with protruding jaws but, as can be seen in the photo at the top of the post, this is not the usual appearance of this species: the jaws are generally only protruded when the shark is picking up food. In Neomitsukurina, however, the jaws are seemingly permanently protruded, and the upper jaw has been modified into a sharpened beak. The most interesting distinction of all, however, is the presence of a massively enlarged photophore on the underside of the rostrum, above the jaws:

Close-up of the head of Neomitsukurina nodai, from Takahashi & Yuasa.

The photophore contains a unique lens structure that focuses the light it produces. So strongly focused is the light, in fact, that it can be used in prey capture by the shark. Through a mechanism not yet fully understood, but possibly a shock reaction to its brightness, the light causes potential prey animals to become stunned, after which they can be easily picked off. Preliminary observations of Neomitsukurina suggest that it may be willing to take on quite large prey: even turtles have not proven immune to stunning, though the shark did not always immediately ingest stunned prey animals. Neomitsukurina has also been observed gliding above the surface of the water through the use of its enlarged pectoral fins.

It might be wondered how such a distinctive and mobile predator eluded discovery until the present, but Neomitsukurina's strict nocturnality might have something to do with it. It is also worth noting that sightings of what may, in hindsight, have been Neomitsukurina have been described in the past (a particularly famous sighting occurred in 1971, near the island of Niemonjima), but attempts to follow up such records have so far only collected other animals such as sea bass.

Sending Forget-me-nots

The Chatham Islands forget-me-not Myosotidium hortensia, from here.

I haven't been able to prepare a full post lately as we're currently in the field conducting our next survey round for the day job. In the meantime, I'll just content myself with a brief introducion to the Cynoglosseae. This is a tribe in the plant family Boraginaceae, redefined by Långström & Chase (2002) on the basis of molecular phylogeny to effectively correspond to the clade of Boraginaceae with heterocolpate pollen, as well as an undivided style with a single stigma (another tribe of Boraginaceae, the Boragineae, was covered in an earlier post). In the heterocolpate pollen of Cynoglosseae, the three apertures found in the pollen of other Boraginaceae alternate with an equal number of 'pseudoapertures'. The pseudoapertures represent gaps in the outer exine coat of the pollen grain like the apertures, but lack certain other features of the latter such as a concentration of cytoplasmic vesicles, as well as being longer and narrower (Hargrove & Simpson 2003).

Flower of camelbush Trichodesma zeyanicum, photographed by Ethel Aardvark.

Perhaps the most familiar members of this usage of Cynoglosseae are the forget-me-nots of the genus Myosotis, with other members including the hound's-tongue Cynoglossum officinale and, here in Australia, the camelbush Trichodesma zeylanicum. Offhand, camelbushes are generally one of the more prominent flowering plants here on Barrow Island, my current location, though they're one a bit of a low right now. There has been a bit of rain, and camelbush doesn't like to get its feet wet.

The traditional associations of forget-me-nots, of course, are right there in their name. There are a number of stories supposedly explaining how these flowers came to be associated with the memory of loved ones (surely the most ridiculous being the one that apparently has a knight drowning under the weight of a bouquet of the things) but the true reasons are probably lost to history. My own suspicion is that it is perhaps ultimately because forget-me-nots are relatively unassuming as flowers go, making them an ideal symbol of beauty that should not be overlooked for the sake of more flashy but perhaps less reliable competitors.

Hound's-tongue Cynoglossum officinale, from here. Native to Europe, this plant has become established in many parts of North America.

Hound's-tongue, on the other hand, seems to get its name from the resemblance of its leaves to its namesake. This plant doesn't seem to have quite the same hold on human affection as the forget-me-not, and the reason for this may be indicated by some of its other vernacular names: 'monk's nit' or 'beggar's lice', in reference to its sticky seeds that adhere to clothing (perhaps 'gypsy flower' derives from the same source?) and, even more damning, 'rats and mice', referring to its unmistakeable smell.

REFERENCES

Hargrove, L., & M. G. Simpson. 2003. Ultrastructure of heterocolpate pollen in Cryptantha (Boraginaceae). International Journal of Plant Sciences 164 (1): 137-151.

Långström, E., & M. W. Chase. 2002. Tribes of Boraginoideae (Boraginaceae) and placement of Antiphytum, Echiochilon, Ogastemma and Sericostoma: a phylogenetic analysis based on atpB plastid DNA sequence data. Plant Systematics and Evolution 234: 137-153.

The Nostocaceae: Tangled Filaments

Macroscopic growth of the cyanobacterium Nostoc commune, from here.

The cyanobacteria, commonly referred to as the 'blue-green algae', were one of the first groups of bacteria to be recognised as distinct. As our knowledge of bacteria has improved over the years, the distinctiveness of cyanobacteria continues to be supported: they are the only bacteria to contain chlorophyll, capturing energy from the light through the oxidation of water. Unfortunately, this confidence in their separation has not necessarily been carried over at lower levels. Many cyanobacterial 'families' and 'genera' have not been supported by more recent molecular analyses. Nevertheless, one clade that has been well supported is the nitrogen-fixing cyanobacteria, Hormogoneae.

Morphology-based classifications of cyanobacteria started, firstly, with the question of whether a species existed as independent cells, or whether they formed thread-like chains. The Hormogoneae are all chain-forming species, with the chain contained within a but they are also distinguished by the formation of heterocysts, large morphologically distinct cells within the chain that specialise in nitrogen fixation. Heterocysts are so specialised that they are unable to photosynthesise for themselves and are dependent on their neighbour cells for nutrition. So, with the presence of differentiated, interdependent cells, Hormogoneae can be regarded not simply as colonies of cells but as true, multicellular bacteria. A number of species of Hormogoneae (particularly in the genus Nostoc) form symbiotic associations with plants that take advantage of their nitrogen-fixing properties. One of the best-known examples is the association of the aquatic floating fern Azolla with the cyanobacterium 'Anabaena' azollae, and Azolla is used as a source of nitrogen in rice paddies. In another post, I have described the association between a Nostoc species and the plant genus Gunnera.

Trichomes of Anabaena, from here. The yellowish cells are heterocysts.

Within the Hormogoneae, classifications have traditionally distinguished between the orders Nostocales and Stigonematales. Stigonematales have branching trichomes, while those of Nostocales are unbranched. The Nostocales have been divided between the Nostocaceae, Rivulariaceae and Scytonemataceae: Rivulariaceae have trichomes that show a distinct base-to-apex polarity, while those of Scytonemataceae show a feature called 'false branching'*. Nostocaceae are defined by the lack of these features: however, it should not be surprising that molecular studies have not supported a group defined solely by the absence of characters, and both Rivulariaceae and Scytonemataceae (and possibly Stigonematales as well) are probably derived (possibly polyphyletically) from 'nostocacean' ancestors. However, actual relationships within the Hormogoneae remain poorly resolved, and no formal reclassification has been proposed (one of the authoritative texts on bacterial classification, Bergey's Manual of Systematic Bacteriology, replaces the cyanobacterial 'orders' with numbered subsections [Nostocales, for instance, is treated as Cyanobacteria subsection IV]—Castenholz 2001).

*True branching as in Stigonematales occurs when cells within a trichome divide at right angles to the direction of the trichome. In 'false branching', the trichome breaks within the containing sheath and then grows out of the sheath, but the division of the individual cells remains linear.

Trichomes of Cylindrospermum licheniforme, from André Advocat. The heterocysts are the round terminal cells, while the large elongate cells behind them are akinetes.

Phylogenetic analyses have also failed to confirm many of the genera recognised within the Nostocaceae (treated by Bergey's Manual as 'form-genera' only), distinguished by features such as whether trichomes are generally straight or coiled, and the positions within the trichome of heterocysts and other specialised cells called akinetes, thick-walled cells that function as resistent spores. The traditional genus Nostoc also differs from other Nostocaceae by the formation of hormogonia, motile trichomes with smaller cells and without differentiated heterocysts. Hormogonia form the dispersal stage of the Nostoc life cycle; it is as hormogonia, for instance, that symbiotic Nostoc are transmitted to new hosts. Mature Nostoc trichomes are embedded in a gelatinous matrix, and in some species this matrix may form a globular ball containing large numbers of radially arranged trichomes. Though usually microscopic, these globular clusters can get very large, sometimes more than twenty centimetres in diameter. Species attributed to other genera of Nostocaceae do not generally produce differentiated hormogonia (though some do, such as the aforementioned Anabaena azollae). Trichomes of these species may remain motile throughout the life cycle, or they may be permanently immotile (the latter state is characteristic of planktonic species).

REFERENCE

Castenholz, R. W. 2001. Phylum BX. Cyanobacteria. Oxygenic photosynthetic bacteria. In: Boone, D. R., & R. W. Castenholz (eds) Bergey's Manual of Systematic Bacteriology 2nd ed., vol. 1, pp. 473-599. Springer.

Beaver Fever

Eurasian beaver Castor fiber, from here.

Beavers are one of those animals that are familiar even to people who do not live in parts of the world where you can find beavers. The two living species of beaver are semi-aquatic rodents with one species each native to Eurasia (Castor fiber) and North America (C. canadensis) (though the North American beaver has been introduced to several parts of Europe). Differences between the two are slight: the Eurasian beaver is generally larger (up to 35 kg) and has a somewhat longer skull and a less rounded tail. Beavers are best known, of course, for their construction of elaborate subaquatic nests and dams*. Dams are generally about fifteen to seventy metres across, but have been recorded over 600 metres across (Rybczynski 2008). Beavers may also dig burrows connected to their dams, and construct canals over one hundred metres long (Rybczynski 2008).

*The original text here has been edited following Howard's comment below.

Phylogenetically, beavers are somewhat remote from other rodents, and represent the last survivors of a once more diverse lineage. First known from the late Eocene, the members of the beaver family Castoridae are divided in the most recent treatments between five subfamilies (Korth 2001, 2004). The plesiomorphic subfamilies Agnotocastorinae and Anchitheriomyinae are not well known, and the Agnotocastorinae in particular may be non-monophyletic (Rybczynski 2007). The remaining subfamilies fall into two distinct lineages: one containing the Palaeocastorinae (Oligocene-Miocene), the other the Castoridinae (Oligocene-Pleistocene) and Castorinae (Oligocene-present). Of these two lineages, only the latter are known to have been semiaquatic: the Palaeocastorinae are strictly terrestrial.

Preserved Daimonelix burrow in the American Museum of Natural History, with specimen of Palaeocastor fossor in the presumed nesting chamber, photographed by Inazakira.

The palaeocastorines, a strictly North American lineage, were specialised burrowers. Their incisors, which have rounded faces in modern beavers, became flattened and adapted for digging. Their burrows were distinctive helicoidal structures, described as trace fossils under the name of Daimonelix ('devil's spiral'), that could reach over 2.5 metres in depth and twenty centimetres in diameter. These burrows were constructed in 'towns' with multiple burrows in close proximity. Though each burrow was independent, without connections between adjacent burrows, such close positioning suggests that palaeocastorines may have had a well-developed social structure (Hugueney & Escuillié 1996). However, though the palaeocastorines were much more diverse at their apogee than the castoroidine-castorine lineage, they became extinct after a relatively short period.

Reconstructed skeleton of Castoroides ohioensis alongside that of (I presume) a modern beaver in Earlham College, from here.

The Castorinae and Castoroidinae may never have achieved the diversity at any one point in time of the palaeocastorines, they were more successful over the long haul: the more diverse of the two subfamilies, the Castoroidinae, only became extinct fairly recently. Castoroidines are commonly referred to as the 'giant beavers', and while not all castoroidines were giant (many, if not most, were smaller than modern beavers), the largest of them certainly were: the North American Castoroides reached an estimated size of about 100 kg, and would have been as large as a small bear. Whether the giant Castoroides produced similarly gigantic dams, however, is uncertain. Evidence of wood-chopping behaviour like that known for modern beavers (in the form of preserved wood bearing identifiable tooth marks, in association with beaver remains) is only well supported for one fossil species, the castoroidine Dipoides (suggested evidence for wood-chopping in Castoroides is more equivocal) (Rybczynski 2008). Phylogenetic bracketing between Dipoides and modern beavers would suggest that wood-chopping arose at the base of the castoroidine-castorine clade; alternatively, the absence of direct evidence of such behaviour may suggest convergence between these two species. Also, Dipoides was a less efficient wood-cutter than modern Castor, cutting with the rounded edges of its incisors while Castor uses the flattened ends, and if it used chopped wood to construct nests then they would have probably been correspondingly more simple (beavers also use chopped wood for food, eating the leaves and bark, so wood-chopping does not automatically indicate dam-building). There are other indications that fossil beavers may not have been as specialised aquatically as the modern species: the early castorine Steneofiber, for instance, did not possess the flattened tail of Castor (a flattened tail has been indicated for Castoroides but Castor and Castoroides probably developed such tails independently) (Hugueney & Escuillié 1996).

Reconstruction of Trogontherium cuvieri, from Fostowicz-Frelik (2008).

Perhaps the primary enigma among fossil beavers is the European Pleistocene Trogontherium. Although also referred to as a 'giant beaver', and often implied to be a European parallel to Castoroides, Trogontherium was a quite different animal. Fostowicz-Frelik (2008) argued that leg proportions and other features indicate that Trogontherium was a more terrestrial, cursorial animal than other beavers (in particular, its narrowed rather than flattened toe bones suggest that it lacked the webbed feet of modern beavers). The phylogenetic analysis of beavers by Rybczynski (2007) placed Trogontherium as closely related to Castoroides, but certain plesiomorphies in its tooth morphology lead Rybczynski to suggest that this position was probably an artifact of convergences due to large size, and that Trogontherium should perhaps be in a much more basal position.

REFERENCES

Fostowicz-Frelik, Ł. 2008. First record of Trogontherium cuvieri (Mammalia, Rodentia) from the middle Pleistocene of Poland and review of the species. Geodiversitas 30 (4): 765-778.

Hugueney, M., & F. Escuillié. 1996. Fossil evidence for the origin of behavioral strategies in early Miocene Castoridae, and their role in the evolution of the family. Paleobiology 22 (4): 507-513.

Korth, W. W. 2001. Comments on the systematics and classification of the beavers (Rodentia, Castoridae). Journal of Mammalian Evolution 8 (4): 279-296.

Korth, W. W. 2004. Beavers (Rodentia, Castoridae) from the Runningwater Formation (Early Miocene, early Hemingfordian) of western Nebraska. Annals of Carnegie Museum 73 (2): 1-11.

Rybczynski, N. 2007. Castorid phylogenetics: implications for the evolution of swimming and tree-exploitation in beavers. Journal of Mammalian Evolution 14: 1-35.

Rybczynski, N. 2008. Woodcutting behavior in beavers (Castoridae, Rodentia): estimating ecological performance in a modern and a fossil taxon. Paleobiology 34 (3): 389-402.

Name the Bug # 58

It's been a while since we last had one of these, but I thought I'd put this up in preparation for tomorrow's post:

What kind of animal was this, photographed in a Polish forest? Because this is one of the easier ones, tell me the exact species, please. Attribution, as always, to follow.