Morion

Morion monilicornis, copyright Charles Schurch Lewallen.

Just a quick one today. This is a typical member of Morion, a genus currently recognised as including over forty species of carabid beetles though there may be many more yet to be described. Characteristic features of this genus include a somewhat flattened body form, moniliform antennae (that is, the antennal segments are all short and similar in form, like beads on a string), a more or less cordiform (heart-shaped) pronotum, and a bilobed median tooth on the mentum (a sclerite on the underside of the head that might be thought of as the 'lower lip' of the mouth) (Will 2003). Though currently recognised as pantropical, Will (2003) suggested that its defining features were potentially plesiomorphic relative to some closely related genera. Further studies may identify Morion in its current sense as a paraphyletic grade to those genera, possibly leading to a reclassification.

The flattened body form of Morion and its relatives (the Morionini) reflects their preferred habitat. Like other carabids, Morion species are voracious predators (both as adults and larvae). Morionins are specialised for hunting in dead wood and under back, forcing themselves through enclosed gaps in search of other insects that might have thought themselves secure in their lignified fortresses.

REFERENCE

Will, K. W. 2003. Review and cladistic analysis of the generic-level taxa of Morionini Brullé (Coleoptera: Carabidae). Pan-Pacific Entomologist 79 (3–4): 212–229.

Libellulidae: On the Wing

Dragonflies of the order Odonata are unquestionably one of the more familiar groups of insects to the general public. They are large, visible and eye-catching, and may be quite colourful. Some have even taken to 'twitching' dragonflies in the same manner as bird species, identifying species observed on the wing and keeping a tally of how many they have seen.

And at the top of many people's list: the wandering glider Pantala flavescens, copyright Jeevan Jose, the world's most widespread dragonfly species.

Ecologically, in contrast, dragonflies may be called a relatively conservative group. All begin their lives as aquatic predators before emerging with adulthood as fast-moving aerial predators. All are generalists, feeding on whatever other insects may be unfortunate enough to fall into their grasp. All dragonflies conform to a fairly similar overall bauplan when compared to the diversity of forms that may be found in many other insect orders (for instance, there are no flightless dragonflies). Classification of dragonflies has often focused heavily on features of the wing venation, tracing its lines in their criss-crossing network.

Hind wing of a libellulid with the anal loop highlighted, from here.

The largest of the generally recognised families of dragonflies is the Libellulidae, containing over 1000 of the approximately 6000 known species of Odonata (Pilgrim & von Dohlen 2008). Characteristic features of the Libellulidae include the presence of the 'anal loop', an arrangement of veins in the hind wing forming what has been described as a boot shape. In the case of the genus Libellula, at least, the shape of the anal loop rather reminds me of one of the legs on the Manx flag. Members of the Libellulidae are commonly known as perchers or skimmers in reference to their hunting behaviours; others have similarly composed names such as darters or pondhawks. A number of members of the family have strikingly banded or coloured wings, leading to vernacular labels such as amberwings or pennants. Members of the genus Tramea are commonly known as saddlebags in reference to the dark patches at the base of their hind wings.

Common picturewing Rhyothemis variegata, copyright Tarique Sani.

Members of the Libellulidae have been divided between about a dozen subfamilies, again primarily defined on the basis of wing venation. However, distinctions between the subfamilies have always been vague with many subfamilies recognised by particular combinations of characters rather than characters unique to each subfamily alone. This vagueness has been underlined by recent molecular studies which have identified most subfamilies as polyphyletic. It seems likely that the defining features of these subfamilies are convergences related to similar ecologies. The 'Sympetrinae' include species with a preference for open watery habitats such as ponds and marshes where they spend a lot of time perched on exposed vegetation (Pilgrim & von Dohlen 2008). The 'Tetrathemistinae', with narrow wings with somewhat reduced venation, are found along forest streams (Fleck et al. 2008). The genera Tramea and Pantala, falsely united in the subfamily Trameinae by broadened bases on the hind wings, are specialised for long-distance flights spending extended periods on the wing (Pilgrim & von Dohlen 2008). Indeed, the wandering glider Pantala flavescens is the world's most widespread dragonfly species, being found in warmer regions of the entire globe and seemingly capable of migrations between separate continents.

The slightly freakish-looking larva of Orionothemis felixorioni, from Fleck et al. (2009).

So if we're going to have a stable classification for libellulids, we need to look past their wings. Intriguingly, larval features may prove more useful in this regard than adult characters. Fleck et al. (2008) examined a group of genera previously classified in the Tetrathemistinae but whose larvae were more similar to those found among members of the Libellulinae. A molecular phylogeny showed that, whereas the Tetrathemistinae as a whole were polyphyletic, these genera were indeed associated with the Libellulinae as their larvae indicated. With further research, we find that libellulid classification need not be all in vein.

REFERENCES

Fleck, G., M. Brenk & B. Misof. 2008. Larval and molecular characters help to solve phylogenetic puzzles in the highly diverse dragonfly family Libellulidae (Insecta: Odonata: Anisoptera): the Tetrathemistinae are a polyphyletic group. Organisms, Diversity & Evolution 8: 1–16.

Pilgrim, E. M., & C. D. von Dohlen. 2008. Phylogeny of the Sympetrinae (Odonata: Libellulidae): further evidence of the homoplasious nature of wing venation. Systematic Entomology 33: 159–174.

The Life and Times of Dissodinium

I've referred before to the position of the minute crustaceans known as copepods as one of the major groups of animals making up the marine zooplankton. Copepods form a significant part of the diet for a wide range of other marine animals: fish, molluscs, jellyfish, you name it. They are also targeted by other organisms coming in at a different scale.

Dissodinium pseudolunula: dinospores waiting to be released from the shell of a secondary cyst, copyright Gabriela Hannach.

Dissodinium is a genus of dinoflagellates, another group of organisms that has appeared on this site in the past. Most dinoflagellates are primarily photosynthetic but not Dissodinium: it's a parasite. Specifically, it's a parasite of copepod eggs. Copepods produce relatively large eggs compared to their body size that are full of tasty lipids and other nutrients so it's hardly surprising that they would attract attention. The free-swimming dinospore of Dissodinium initially looks much like a typical dinoflagellate but once they attach to a copepod egg they produce a sucker-like organelle through which they slurp up the egg's contents, swelling to a globular blob. When feeding is finished, this blob detaches from the remains of the egg to begin the process of reproduction.

There are two species of Dissodinium whose asexual life cycles were described by Elbrächter & Drebes (1978). I haven't found any reference to a known sexual reproduction cycle for Dissodinium. In both species, the replete individual forms a spherical primary cyst that floats free within the plankton. The contents of the primary cyst divide within the cyst wall to form the next stage, the secondary cysts. In the most commonly seen species, Dissodinium pseudolunula*, these secondary cysts are distinctively crescent-shaped. Following their release from the original primary cyst wall, the cytoplasm within the secondary cysts further subdivides to form the actively swimming dinospores. These dinospores presumably function as the infective stage for another round of the cycle but it should be noted that Gómez et al. (2009) were unable to induce infection when they incubated newly released dinospores together with copepod eggs. Instead, the dinospores encysted themselves in a hyaline membrane and Gómez et al. suggested that some sort of maturation period may be necessary before infection can take place. The second species of Dissodinium, D. pseudocalani, differs in that the secondary cysts are not crescent-shaped, and divide to release the dinospores while still themselves contained within the original primary cyst wall so the breakdown of the latter releases dinospores directly into the environment. This compression of the life cycle has also sometimes been observed with D. pseudolunula.

*This species has often masqueraded in the past under the name of Dissodinium lunula. The name 'Gymnodinium lunula' was originally used for crescent-shaped cysts by Schütt in 1895. Unfortunately, Schütt's figured examples of this 'species' included representatives of two quite different dinoflagellates, now classified as Dissodinium and another genus Pyrocystis that is not parasitic. The name lunula has become properly attached to the latter species, requiring a different name for the Dissodinium.

Stages in the life cycle of Dissodinium pseudolunula, from Elbrächter & Drebes (1978), running from a freshly released primary cyst at top left to a newly attached parasitic dinospore at bottom right.

Elbrächter & Drebes (1978) included Dissodinium in the Blastodiniales, a morphologically diverse group of parasitic dinoflagellates. The advent of molecular analyses would later demonstrate this grouping to be polyphyletic with parasitic dinoflagellates evolving on numerous occasions from free-living ancestors. Instead, Dissodinium and another parasite of copepod eggs, Chytriodinium, form a clade that is closely related to the major free-living genus Gymnodinium (Gómez et al. 2009). Gómez et al. also found that D. pseudolunula retains some elements of its free-living ancestry: it still retains chlorophyll (chlorophyll is absent in D. pseudocalani and Chytriodinium). Just how functional this chlorophyll remains is an open question: it appears less concentrated within the cell than in a typical photosynthetic dinoflagellate, and Gómez et al. were unable to maintain a culture of D. pseudolunula under conditions that would support a free-living species. Nevertheless, they suggested that a low level of photosynthesis might supplement the dinoflagellate's nutrient requirements while it waited out the aforementioned incubation period before finding itself a host.

REFERENCES

Elbrächter, M., & G. Drebes. 1978. Life cycles, phylogeny and taxonomy of Dissodinium and Pyrocystis (Dinophyta). Helgoländer wiss. Meeresunters. 31: 347–366.

Gómez, F., D. Moreira & P. López-García. 2009. Life cycle and molecular phylogeny of the dinoflagellates Chytriodinium and Dissodinium, ectoparasites of copepod eggs. European Journal of Protistology 45: 260–270.

Protacanthopterygii: A Brief History of a Vague Idea

There are some taxon names whose concepts are rock-solid, that have been universally recognised since their inception almost without variation. There are some taxon names that are coined, potentially linger through one or two subsequent uses, then disappear into the mists of history never to be used again. And then there are some taxon names that are used regularly but whose actual concept shifts wildly over time: names that seem to be used not so much for their own sake as because authors seem to think they need to be in there somewhere. Witness today's subject, the Protacanthopterygii.

Brown salmon Salmo trutta, photographed by Eric Engbretson, about as close to a definitive 'protacanthopterygian' as you're going to get.

The Protacanthopterygii has widely been recognised as a major group of ray-finned fishes since the name was established by Greenwood et al. (1966). Using the modern parlance, Greenwood et al.'s Protacanthopterygii was an explicitly paraphyletic group of euteleost fishes that could be recognised as branching off the lineage leading to the Acanthopterygii and Paracanthopterygii but lacked the full suite of characteristics of the latter group. As such, many of the characters listed by Greenwood et al. as diagnostic of the Protacanthopterygii were expressed in the form of trends: "widespread trend toward the development of premaxillary processes", for instance, or "hyoid and branchiostegal skeleton approaching paracanthopterygian and acanthopterygian form". We also get a number of references to majority rather than universal features: "glossohyal teeth usually prominent", or "few species with opercular spines or serrations". Greenwood et al. included the bulk of their Protacanthopterygii in the order Salmoniformes, but recognised this order in a much broader sense than modern authors. As well as the Salmonidae itself, their Salmoniformes included taxa that would now be placed in the orders Galaxiiformes, Esociformes, Myctophiformes, Aulopiformes and Stomiiformes, among others. Greenwood et al.'s Protacanthopterygii was also supposed to include the orders Cetomimiformes, Gonorynchiformes and Ctenothrissiformes. Their concept of Cetomimiformes is now recognised as polyphyletic and neither Cetomimiformes and Gonorynchiformes include any taxa closely related to Salmonidae; the case of Ctenothrissiformes has been discussed on this site previously.

Northern pike Esox lucius, copyright Jik jik.

In the intervening years, of course, the philosophy of systematics has shifted to prioritising the recognition of monophyletic taxa, requiring the dissolution of the original Protacanthopterygii. Unfortunately, calculating basal euteleost relationships has not proven an easy task. As a result, authors have differed considerably on exactly which fishes should be regarded as 'protacanthopterygians'. About the only constant factor in all circumscriptions of the taxon has been the inclusion of the Salmonidae, the salmons, trouts and the like. Indeed, the most extreme restriction of the Protacanthopterygii would treat it as including this family alone.

Recent molecular studies have agreed on the recognition of a clade uniting the Salmonidae with the Esociformes. The Esociformes is a small order of a bit over a dozen species of freshwater fish found in the Holarctic region, uniting the pikes of the genus Esox with the mudminnows of the Umbridae. Betancur-R et al. (2017) recognised Protacanthopterygii as the name for a clade uniting the Salmonidae, Esociformes, Argentiniformes (a marine order including herring smelts, barreleyes and the like) and Galaxiidae (whitebaits). However, other studies have not supported this clade.

Spotted galaxias Galaxias truttaceus, copyright Nathan Litjens, an Australian member of the whitebait family. Though galaxiids are rather salmon-like in overall appearance, it remains an open question whether this resemblance indicates any sort of direct relationship or just a shared hold-over from some ancestral neoteleost.

Considering the difficulty in defining it, one might question why the concept of a 'Protacanthopterygii' persists at all. Really, there doesn't seem to be much reason for it other than that the Greenwood et al. (1966) classification was long the base standard for teleost classifications, leaving subsequent authors loathe to discard any taxon recognised therein lightly. It might, in theory, be possible to rescue the Protacanthopterygii concept by phylogenetic definition: for instance, as those species more closely related to Salmo than Perca (indeed, I would not be surprised to learn this has already been done). But considering that the uncertain composition of the resulting clade would reduce the practicality of its recognition, I don't think I would be weeping too much if someone would just take the Protacanthopterygii concept out the back and shoot it.

REFERENCES

Betancur-R., R., E. O. Wiley, G. Arratia, A. Acero, N. Bailly, M. Miya, G. Lecointre & G. Ortí. 2017. Phylogenetic classification of bony fishes. BMC Evolutionary Biology 17: 162.

Greenwood, P. H., D. E. Rosen, S. H. Weitzman & G. S. Myers. 1966. Phyletic studies of teleostean fishes, with a provisional classification of living forms. Bulletin of the American Museum of Natural History 131 (4): 339–456.