Air-breathing Limpets

For many people, the most familiar members of the gastropods are the terrestrial snails. Gastropods started their evolution as marine animals, breathing through gills, but members of one lineage would instead evolve their own version of a lung, a large hollow in the mantle cavity opening through a hole alongside the head called the pneumostome. Possession of this lung cavity would enable slugs and snails to thrive in the terrestrial environment but the structure had originally evolved in a marine context, and even today one may find marine lung-bearers occupying habitats along the coast. One such group is the siphon limpets or 'false limpets'* of the Siphonarioidea.

*You know, normally I don't overly concern myself with vernacular names. They are not regulated and not obliged to follow reason. But even so, the name 'false limpet' makes me grit my teeth. The name is presumably inspired by the fact that siphonariids are not direct relatives of the 'true' limpets of the Patellogastropoda. But the limpet morphotype, where the typical spiral gastropod shell is reduced to a simple cap, has evolved on multiple occasions. As well as the siphonariids and patellogastropods, there are the keyhole limpets of the Fissurellidae, the freshwater limpets of the Ancylini, and many others, all consistently referred to as 'limpets'. The name refers to a morphology, not to a clade, and by that measure the siphonariids are no more 'false' than any other limpets.

Flat siphon limpets Siphonaria atra, copyright Ria Tan.

Living siphonarioids are placed within a single family, the Siphonariidae, whose members with their cap-shaped, often radially ribbed shells are found in littoral environments in temperate and tropical regions of the world. A second family, the Acroreiidae, is recognised from the Cretaceous and early Tertiary; the inclusion of these smooth, thin shells in the Siphonarioidea is somewhat tentative (classification of limpets in the fossil record is always a challenge because their simple shell form renders them light on distinguishing characters). Siphonariids are readily distinguished from other living limpets by the presence of a groove on the underside of the right side of the shell marking the position of the pneumostome. In dorsal view, this groove is often indicated by an asymmetry in the outline of the shell with one side produced. The pneumostome is also associated with a broad gap in the ring of muscle holding the shell in place; the ring is more complete in other limpets. Seemingly as a result of this lower extent of muscle, siphonariids cling to their home rocks with less tenacity than other limpets and are mostly restricted to more sheltered locations (Simone & Seabra 2017). On the other hand, they do have a more flexible foot than their competitors, allowing them to potentially move more quickly. Like other limpets, siphonariids are grazers, scraping microalgae as they crawl about. Siphonariids have a weaker radula than patellogastropods and so scrape somewhat less forcefully; when members of the two clades occupy the same habitats, patellogastropods are generally the more abundant. The majority of siphonariids (where known) have planktonic larvae but some species are known to be direct developers.

Siphonaria lessonii, copyright Mikelzubi.

Obviously, the marine but lung-possessing siphonariids are potentially of great interest in understanding how the gastropod lung evolved. Many earlier researchers thought that the siphonariids may have evolved from terrestrial ancestors who had returned to the seashore but this is no longer thought likely to be the case. In most lunged gastropods, gas exchange is effected in the mantle cavity via dense blood vessels in the cavity wall but in siphonariids a gill structure is present within the lung (this lung-gill combination makes the siphonariids particularly well suited for moving freely both above and below the water surface). The gill of siphonariids is quite similar to that of the sacoglossans, a group of herbivorous sea-slugs. Though it was long presumed that the lung-bearing gastropods belonged to a single clade, more recent molecular phylogenies have confused the issue (Kocot et al. 2013). The sacoglossans are likely to be close to the ancestry of lunged gastropods as a whole, but it is possible that siphonariids are more closely related to the sacoglossans than the other lung-bearers. It remains an open question whether the siphonariid combination of lung and gill represents an intermediate stage towards the vascular lung of the terrestrial forms, or whether siphonariids and other lung-bearers each evolved their pneumostome from close but distinct ancestors.

REFERENCES

Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

Simone, L. R. L., & M. I. G. L. Seabra. 2017. Shell and body structure of the plesiomorphic pulmonate marine limpet Siphonaria pectinata (Linnaeus, 1758) from Portugal (Gastropoda: Heterobranchia: Siphonariidae). Folia Malacologia 25 (3): 147–164.

The Colletinae: Going to Ground

In a recent post, I considered one of the families of short-tongued bees, the Halictidae. In this post, I'll turn my attention to members of one of the other short-tongued bee families, the Colletidae. Specifically, I'm looking at members of the subfamily Colletinae.

Mating ball of male ivy bees Colletes hederae, copyright Charles J. Sharp.

Members of the Colletidae differ from other bee families in that their glossa, the 'tongue' at the end of the proboscis, is apically bilobed or bifurcate. They are also distinctive in lining their nests with a plasticky, cellophane-like material. It has been thought that this material was made from dry saliva but the bulk of it is now known to come from a large gland in the abdomen that opens near the base of the sting (Almeida 2008). A nesting female will swallow droplets of the glandular secretion from her partially protruded sting then regurgitate it as she licks the wall of the nest cell. This waterproof lining both protects the cell from outside elements while preventing the loss of moisture from within. Many colletids, including colletines, leave the cell food provisions in a semi-liquid state; other bees whose nests are less watertight will dry and compact the provisions, presumably because the bulk of them would otherwise be lost before the larva hatched. In colletines, the egg is attached to the cell lining when laid, suspended above the provisions for the hatching larva to swandive into upon emergence. All colletids are solitary nesters with species nesting either in burrows in the ground or in hollows in vegetation; the majority of colletines are ground nesters*.

*One species, Colletes daviesianus, has apparently taken in Germany to boring its nests in the sandstone and mortar used in building construction.

Female Colletes daviesianus, copyright Donald Hobern.

In his 2007 edition of The Bees of the World, Charles Michener recognised five subfamilies within the Colletidae. The Colletinae were distinguished from three of these subfamilies by their retention of a covering of dense hair over the body (from the last subfamily, the Diphaglossinae, they differ in features of the glossa and wing venation). The hind leg of the female bears a well-developed scopa (dense arrangement of hairs for the carrying of pollen) on the femur and tibia with a corbicula (bare patch within the scopa where a ball of compacted pollen may be carried) on the underside of the femur. Hairiness is an ancestral characteristic for bees and phylogenetic studies have established that the Colletinae as recognised by Michener is a paraphyletic grouping (e.g. Almeida & Danforth 2009). As a result, it has been further subdivided with the name 'Colletinae' now restricted to what Michener had recognised as the tribe Colletini. As such, the Colletinae now includes just two genera of moderate-sized bees (seven to sixteen millimetres in length). The larger of these, Colletes, is found in temperate and tropical regions around the world except for the Indo-Australian region where it is notably absent. The other genus, Maurecolletes, is restricted to South America. One of the most distinctive features of Colletinae in the strict sense compared to other ex-colletines is the lack of the basitibial and pygidial plates, flattened and hardened plates possessed by other hairy colletids at the base of the hind tibia and at the end of the abdomen.

The absence of these plates is intriguing in light of the ground-nesting habits that seem to be the norm for Colletes (the nesting habits of Maurecolletes seem to be unknown). In other ground-nesting bees, the basitibial and pygidial plates are used to press the soil of the nest walls and opening into place. One would think this would mitigate against their loss. An explanation may be provided by the fact that some South American Colletes nest in the hollows of dead, pithy plant stems instead of in the ground, a characteristic shared with members of the less hairy colletid subfamilies belonging to the sister group of the colletines (Almeida & Danforth 2009). Ground-nesting Colletes species also bear noteworthy resemblances to stem-nesting colletids. Nest cells are closed with a layer of the cellophane-like wall membrane rather than the earthen plugs used by other ground-nesting bees. In many species, cells are lined up in a burrow divided by transverse partitions rather than placed in their own individual side branches. The possibility has been suggested that stem-nesting arose within the common ancestors of modern colletines and less hairy colletids. Ground-nesting in Colletes would then represent a secondary reversion by these species to the previous habit. When they did so, they retained the adaptations and habits that had originally been associated with their time in the twigs.

REFERENCES

Almeida, E. A. B. 2008. Colletidae nesting biology (Hymenoptera: Apoidea). Apidologie 39: 16–29.

Almeida, E. A. B., & B. N. Danforth. 2009. Phylogeny of colletid bees (Hymenoptera: Colletidae) inferred from four nuclear genes. Molecular Phylogenetics and Evolution 50: 290–309.

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.

Colpochila: The Chafing of a Mega-genus

Just a few weeks ago, I discussed the melolonthines, a hyperdiverse group of beetles including the chafers that have historically presented something of a taxonomic challenge. In the comments on that post, Adam Yates brought up one aspect of the difficulties presented by this group that I hadn't gotten around to discussing. This is the presence among melolonthines of a number of what may be called 'mega-genera', large genera containing literally hundreds of species that defy attempts to break them down into more manageable units. So on that note, it's only appropriate that I move on to an example of one of these mega-genera, Colpochila.

Colpochila obesa, from Insects of Tasmania.

Colpochila is an Australian genus of melolonthines belonging to a group currently recognised as the tribe Liparetrini (Britton 1986) though readers of the earlier post may recall that relationships between Australian melolonthines and taxa elsewhere in the world remains something of an open question. Liparetrins are, on the whole, a fairly generalised group: characters of the group include a lack of metallic coloration, a labrum which sits underneath and is not fused to the clypeus, simple claws, and relatively broad hind tibiae that end in a pair of widely separated spurs that are placed one above and one below so that the tarsus when moved from side to side can move between the spurs. The two largest genera in the tribe, by a significant margin, are Colpochila and Liparetrus. Somewhere in the region of 130 species are currently recognised in Colpochila whereas Liparetrus is even more diverse. However, both genera were referred to by Britton (1986) as 'polythetic': that is, both represent assemblages of species that, while clearly connected to each other overall, are difficult to characterise from a diagnostic perspective. Species of the genus possess enough features in common that we can readily recognise them as related but it is difficult to drill down on any individual feature or set of features that is shared between all species without exception. Similarly, while I can say from experience that it is generally easy to tell at a glance whether a given species is a Colpochila or a Liparetrus, it is a lot harder to actually define what separates the two genera. The most obvious distinction is size: Colpochila species are relatively large chafers, over a centimetre in length, whereas Liparetrus are smaller. Other features that each separate most Colpochila species from most (though not all) Liparetrus are circular eyes (most Liparetrus have eyes with flattened edges in back so the eye is closer to semi-circular), antennae with more segments in the terminal club, longer elytra that leave less of the end of the abdomen exposed, and hind coxae without the translucent margins found in many Liparetrus.

The lifestyles of Colpochila species are still not very well known. As with other melolonthines, most of the life is spent underground with mature adults only emerging very briefly to breed. The active adults fly at night and may be attracted to lights; it seems unclear whether they feed at maturity. This genus is mostly found in drier habitats such as open woodland, grasslands or semi-desert (mind you, this is Australia we're talking about; drier habitats are 90% of what's going). Of the known species, over half are found in Western Australia.

A second Colpochila species, from Friends of Queens Park Bushland.

So why are Colpochila and other melolonthine mega-genera so diverse? It should be noted that straight geographical and/or ecological divergence does not appear to be the reason: not only is it possible to find multiple species of a single genus in one location but one may even collect very similar species together. It might be that the diversity of the mega-genera is artefactual, a reflection of the failure of taxonomists to properly identify relationships: any study that wanted to explain their diversity would have to study their phylogenetic relationships with related smaller genera to confirm their evolutionary coherence and/or age of divergence. However, if the current generic classification of melolonthines reflects a real evolutionary pattern, a potential explanation was proposed by Britton (1986). Adult melolonthines do not emerge immediately upon maturing but remain dormant underground awaiting a suitable environmental signal such as rainfall. However, rainfall in the arid zone at any one time is often uneven. Dormant beetles at one spot may feel the urge to emerge while others nearby may be left to wait for the next shower. The first wave will have died off before the second wave emerges, and their offspring will not yet be mature. As a result, sub-populations in a single region may become temporally staggered allowing the possibility of divergence via genetic drift. Eventually, their emergence times may drift back into sync but by then they may no longer be able to breed successfully. Could this be the reason why so many species may be found in a single location or may other factors be more significant?

REFERENCE

Britton, E. B. 1986. A revision of the Australian chafers (Coleoptera: Scarabaeidae: Melolonthinae) vol. 4. Tribe Liparetrini: genus Colpochila. Australian Journal of Zoology, Supplementary Series 118: 1–135.

Austrotritia: Jack-in-the-Box Mites

We just keep coming back to the oribatids, don't we?

In an earlier post, I introduced you to Oribotritia, one of the genera of box mites. These, you may recall, are the armoured mites that have evolved the ability to curl the front of the body under themselves and tuck back their legs to form a solid case (in the Oribotritiidae, that mechanical defense is supplemented by the production of a defensive chemical, chrysomelidial, from glands in the cuticle—Shimizu et al. 2012). In the earlier post, I also gave you a quick overview of the families of what are known as the 'true' box mites. Today's post is for another component of the family Oribotritiidae, the genus Austrotritia.

Austrotritia lebronneci, copyright R. Penttinen.

Austrotritia accounts for nearly twenty species of box mite, the great majority of which are found in Australasia and southern and eastern Asia (Liu et al. 2009). Outliers are A. engelbrechti in South Africa, A. herenessica in the Canary Islands and, most unexpected of all, A. finlandica in Finland. Austrotritia differs from all other oribotritiids except the small Bornean genus Terratritia in lacking any division between the genital and aggenital plates on the underside of the body. The distinction between Austrotritia and Terratritia perhaps requires reassessment: Niedbała (2000) distinguished them by the presence of five-segmented palps and a single pair of exobothridial setae in Austrotritia versus three-segmented palps and two pairs of exobothridial setae in Terratritia (the bothridia are the structures bearing large sensory setae on the prodorsum of the mite; exobothridial setae are thus setae sitting alongside the bothridia). However, Liu & Zhang (2014) redescribed the widespread species Austrotritia lebronneci as having three-segmented palps but only a single pair of exobothridial setae. Note that classification of oribatids has mostly been conducted from a diagnostic rather than a phylogenetic perspective; it would not surprise me if Terratritia turned out to be a derived subgroup of Austrotritia.

Schematic of jump performance by Indotritia cf. heterotrichia from Wauthy et al. (1998); the solid line represents observed jumps, the dashed lines modelled jumps. Line drawings represent (a) body posture when beginning jump, (b) rotation during jump, and (c) enclosed posture after jumping.

As well as the aforementioned defenses standard for box mites, Austrotritia and the related genus Indotritia stand out from other oribotritiid genera in that at least some species have the ability to jump. The mechanics of jumping were described for a species of Indotritia by Wauthy et al. (1998) who recorded the mites jumping nearly a centimetre in height over a distance of just under an inch (for perspective, the mite itself is about half a millimetre in length). Jumping was preceded by compressing the notogaster while raising the ventral plates under the opisthosoma, together with lowering the prosoma and bringing the legs together under the body. Small hooks at the end of femur of the first pair of legs were used to catch ridges on the side of the prodorsum in order to hold the body compression. The force for the jump was presumably supplied by the release of the hydraulic compression of the body fluids when the legs disengaged from the prodorsum, propelling the mite backwards while the body rolled forwards: essentially, the mite would star-jump away. The mite would curl up after jumping to lie in an enclosed state.

Whether all Austrotritia species are jumpers is not entirely certain. The femoral hooks that seem to play a significant role in jumping have not been described in all species. However, it is not clear if this lack of observation represents an actual absence or whether this minute feature has simply been overlooked. I also wonder whether the aforementioned fusion of the ventral plates in Austrotritia is related to their jumping abilities (Indotritia species also have the genital and aggenital plates fused anteriorly though they retain a degree of separation at the rear of the plates; non-jumping Oribotritia have the plates entirely separated). As always, there's still a lot we could potentially find out.

REFERENCES

Liu, D., J. Chen & G. Qiao. 2009. Review of Austrotritia (Acari: Oribatida: Oribotritiidae), with descriptions of two new species from China. Zootaxa 2144: 54–64.

Liu, D., & Z.-Q. Zhang. 2014. Redescription of Austrotritia lebronneci (Oribotritiidae) and descriptions of two new species of Euphthiracaridae (Acari, Oribatida) from Australian region. International Journal of Acarology 40 (1): 43–51.

Niedbała, W. 2000. The ptyctimous mites fauna of the Oriental and Australian regions and their centre of origin (Acari: Oribatida). Polskie Towarzystwo Taksonomiczne: Wrocław (Poland).

Shimizu, N., R. Yakumaru, T. Sakata, S. Shimano & Y. Kuwahara. 2012. The absolute configuration of chrysomelidial: a widely distributed defensive component among oribotritiid mites (Acari: Oribatida). Journal of Chemical Ecology 38: 29–35.

The Cervini: Deer of Temperate Eurasia (and Beyond)

A couple of years back, I presented you with a post giving a quick overview of the classification of deer. For this post, I'm going to look a bit closer at a particular subgroup of deer: the species of the tribe Cervini.

Wapiti Cervus canadensis, photographed by Mongo.

For most people outside the Americas, a member of the Cervini will probably represent the first image that comes to mind when picturing a deer. The same goes for many Americans, for that matter, though in that part of the world they face a bit more competition. Cervins are the most diverse group of deer in temperate Eurasia, with representatives also being found in northernmost Africa, North America, India and southeast Asia (as well as introduced species in Australasia). The Monarch of the Glen was a cervin: specifically, a red deer Cervus elaphus. Bambi, in his Disney film incarnation, was also a cervin, a wapiti C. canadensis (in his original literary form, probably less familiar to modern audiences with little interest in Austrian novels about all the miserable ways that animals can die, he was a roe deer Capreolus capreolus and so not a cervin) (Edit: Scratch that, he's a apparently a non-cervin white-tailed deer, see comment below). The group has long been recognised by features of the skull and leg bones, and also is well supported by molecular data (Heckeberg 2020). Males produce antlers with multiple branches (at least in typical individuals) with the branches or tines usually directed forwards from the main shaft of the antler (the Père David's deer Elaphurus davidianus differs from other cervins in having the tines directed rearwards). The first of these branches, the brow tine, usually originates close to the base of the antler. In a number of Asian species, such as the chital Axis axis and sambar Cervus unicolor, there is usually on one more branch on the antler so each antler ends with three points. Species with such antlers are generally found in dense forests and their simpler antlers may represent an adaptation to these habitats (Heckeberg 2020). Other cervin species may have more extensively branched antlers with a tendency for antler complexity to correlate with overall body size; the largest living cervins, the red deer and wapiti, also have the most branched antlers. Larger extinct species had even more extravagant headgear with the apex of insanity being perhaps the bush-antlered deer Eucladoceros dicranios of the lower Pleistocene of Europe: each antler of this species might carry a dozen points.

Skull of Eucladoceros dicranios, photographed by Aldo Cavini Benedetti.

To describe the classification of cervins as having recently been in a state of flux is something of an understatement. A conservative presentation of the group may refer to thirteen or fourteen living species in four genera (e.g. Macdonald 1984). More recent authors, however, might refer to up to ten genera and nearly forty species. In a way, this difference is not really as dramatic as it may seem: multiple subspecies have long been recognised for most cervin species and some authors have argued for the recognition of many of these 'subspecies' as distinct species. Classification at generic level has mostly been affected by recognition that the genus Cervus as previously recognised is not monophyletic. Most recent authors agree on the recognition of at least four genera of Cervini (Cervus, Dama, Axis and Rucervus) with two further genera (Rusa and Elaphurus) also commonly recognised.

Persian fallow deer Dama mesopotamica, copyright Rufus46.

The genus Dama is usually recognised as including two species, the fallow deer D. dama and Persian fallow deer D. mesopotamica. These species are readily distinguished from other cervins by the form of their antlers which are distally palmate. Palmate antlers are also characteristic of the extinct giant Irish elk Megaloceros giganteus and many recent authors have regarded the two as closely related. The white spots that many deer species possess when young are commonly retained by fallow deer into adulthood though the coat will often become darker and the spots disappear during winter. Melanistic and leucistic individuals of fallow deer are also common. Defining the native range of the fallow deer is a bit of a tricky question. This inhabitant of open woodlands is currently widespread in Europe but was probably restricted to a region of the eastern Mediterranean during the last ice age. Its current range in northern Europe may in large part be the result of human transportation. The fallow deer has also been widely introduced elsewhere: herds may now be found in numerous locations in Africa, Australasia, North and South America. The Persian fallow deer, in contrast, is now endangered, its range restricted to a small number of localities in Iran. Indeed, it was once thought to be extinct prior to the rediscovery of a population of about two dozen individuals in the mid-1950s; the current population is perhaps only a few hundred.

Thorold's deer Cervus albirostris, copyright Heather Paul.

The genus Cervus in its current, more restricted sense includes the red deer and wapiti as well as the sika C. nippon* of eastern Asia. Sika are generally smaller than the other two species and, like fallow deer, usually retain the juvenile spots into adulthood. Excluding occasional small accessory branches, the antlers of sika also possess no more than four tines (Heckeberg 2020) in contrast to the commonly further branched antlers of red deer and wapiti. Four-tined antlers are also characteristic of the Thorold's or white-lipped deer C. albirostris, an inhabitant of the Tibetan Plateau that has sometimes been treated recently as the only representative of a separate genus Przewalskium. White-lipped deers have broad, cow-like hooves for navigating the steep, rocky slopes of their homeland. More commonly accepted classification-wise is the separation of two species found in southern Asia, the rusa C. rusa and sambar C. unicolor, as the genus Rusa. Both these species have three-tined antlers and their fawns lack spots.

*Commonly referred to as the sika deer. 'Sika' (or, as it's more commonly transliterated these days, 'shika') is Japanese for deer, so the common vernacular name of Cervus nippon is, indeed, 'deer deer'. The same issue arises for the rusa deer in Malay.

Chitals Axis axis, copyright Charles J. Sharp.

Axis is a genus of four species of smaller forest-dwelling deer found in southern Asia. Antlers are generally three-tined with the upper beams curving inwards towards each other. The chital remains spotted at maturity whereas the other species loose their spots. These species include the hog deer A. porcinus, named for its low, short-legged build, and two closely related insular species. Recent years have seen some authors separate the hog deers as a separate genus Hyelaphus, restricting Axis to the chital, owing to molecular phylogenies casting doubt on the genus' monophyly. However, it seems that these studies may have been mislead by a contaminated sample for the hog deer (Gilbert et al. 2006) and other studies have retained a monophyletic Axis. The thamin Rucervus eldii and barasingha R. duvauceli are also found in southern Asia where they tend to be associated with marshy habitats. Their antlers curve outwards then inwards to form a bow-shaped curve; those of the thamin are three-tined whereas the barasingha possesses further tines, sometimes up to ten on each antler. Again, some studies have questioned the monophyly of Rucervus and suggested the thamin be moved to a separate genus Panolia.

Père David's deers Elaphurus davidianus, copyright Peter O'Connor.

Finally, there is Père David's deer, arguably the weirdest of all the cervins, most often placed in its own genus Elaphurus but sometimes included in Cervus. By the time this species became known to European naturalists, it was already extinct in the wild, surviving only as a herd kept in a hunting garden near Peking belonging to the emperor of China. This herd was exterminated during the Boxer Rebellion but specimens that had been transported to Europe saved the species from total extinction. It is now widely kept in captive herds and has also been returned to the wild in a couple of locations in China. Père David's deer has a number of features that make it stand out from other deer: as well as the aforementioned backwards antlers, it has wide, splayed hooves and a remarkably long tail. But in other regards, Père David's deer is not anywhere as weird as it should be. In particular, its karyotype is very similar to that of the red deer: close enough, in fact, that not only are the two species capable of hybridising in captivity but the resulting hybrids are fully fertile (such matings are unlikely in the wild owing to the two species normally having different breeding seasons). Heckeberg (2020) found that Père David's deer was associated with Cervus species in analyses of nuclear genes and cranial characters but with Rucervus species in analyses of mitochondrial genes and dentition; other authors had previously found similar results. It has been suggested that these schizoid tendencies with regard to phylogenetic analysis might indicate a hybrid origin for Père David's deer from ancestors related to the wapiti on one side and the thamin on the other. Such a hybridisation event would have happened some time ago—fossils related to Père David's deer seem to date back at least to the late Pliocene—allowing enough time to pass for the new population to develop its own idiosyncracies not acquired directly from either parent.

Sticking Your Sulphur on the Outside

The ability to obtain energy from sunlight through photosynthesis is a feature of a range of bacterial lineages, using a number of different processes and growing under a variety of different conditions. Perhaps the best known such group is the oxygen-producing blue-green algae or Cyanobacteria, but there are other photosynthetic bacteria that do not release oxygen. One such group is the purple sulphur bacteria of the Ectothiorhodospiraceae.

Individual Ectothiorhodospira mobilis, from Trüper (1968).

Purple sulphur bacteria are members of the hyperdiverse array of bacteria known as the Proteobacteria, specifically of a subgroup known as the Gammaproteobacteria (other notable Gammaproteobacteria include such luminaries as the various Pseudomonas species, the plant-attacking Xanthomonas, and perhaps the single most intensely studied bacterial species of all, Escherichia coli). They are found growing under anoxic conditions, using light energy to assimilate carbon via the oxidation of sulphides to organic sulphur, which is in turn further oxidated to sulphate. Purple sulphur bacteria can be divided between two families, the Chromatiaceae and Ectothiorhodospiraceae, that may be distinguished by how they deposit the sulphur globules produced during photosynthesis. In Chromatiaceae, the sulphur globules are retained within the cell membrane but in Ectothiorhodospiraceae, they are deposited externally (one species of Ectothiorhodospiraceae, Thiorhodospira sibirica, does deposit sulphur both internally and externally, but the internal globules are restricted to the peripheral region of the cell within the periplasmic space). Photosynthetic pigments are bacteriochlorophyll a or b, together with carotenoids; these pigments are attached to intracellular membranes appearing as lamellar stacks. Ectothiorhodospiraceae species are found in marine and other saline habitats, often in environments with a more or less alkaline pH. Species of the genus Halorhodospira, which have been found in salt and soda lakes, require exceedingly saline conditions, being unable to grow at total salt concentrations below 10%.

Cross section of Ectothiorhodospira mobilis showing photosynthetic membrane stack, from Trüper (1968).

Though the genera Halorhodospira and Thiorhodospira are strictly anaerobic and invariably photosynthetic, some species of the genus Ectothiorhodospira are able to obtain energy heterotrophically from organic compounds, and may grow under microaerobic conditions in the dark. The range of organics they can utilise in this way is limited to relatively simple compounds: they generally cannot break down carbohydrates, for instance, but they can grow on organic acids such as acetates. Molecular analysis has also indicated the inclusion in the Ectothiorhodospiraceae of a number of non-photosynthetic bacteria. The species Arhodomonas aquaeolei was originally isolated from brine from a subterranean oil reservoir; it breaks down organic compounds using oxygen or nitrate but has the same limitations to simple molecules as Ectothiorhodospira. It seems easy to imagine it evolving from an Ectothiorhodospira-like ancestor but losing its photosynthetic capabilities due to its subterranean habitat, like an animal in a cave losing its eyesight. The genera Nitrococcus and Thioalkalivibrio are lithoautotrophs (that is, they synthesis organic compounds like photosynthetic forms but use the energy from chemical reactions using minerals rather than from light) and generally aerobic (at least one strain of the species Thioalkalivibrio denitrificans is facultatively anaerobic). Thioalkalivibrio species oxidise sulphur, sulphides and other sulphur-containing compounds whereas Nitrococcus convert nitrate to nitrite. Despite their lack of photosynthetic abilities, Nitrococcus still carry indications of their photosynthetic ancestry in the presence of tubular intracellular membranes, the repurposed derivatives of the original stacks.

REFERENCE

Brenner, D. J., N. R. Krieg & J. R. Staley (eds) 2005. Bergey’s Manual of Systematic Bacteriology 2^nd ed. vol. 2. The Proteobacteria pt B. The Gammaproteobacteria. Springer.