The Splanchnotrophidae: Comfy inside a Sea Slug

In previous posts, I've referred to the great significance of the minute crustaceans known as copepods to aquatic ecosystems. At the time, I was referring to free-living members of this group but the copepods also include a wide range of parasitic forms. Some of these parasitic copepods have evolved into forms so derived and bizarre that they are barely recognisable as crustaceans. One example of this is the family Splanchnotrophidae.

Sea slug Janolus fuscus with protruding egg sacs of a splanchnotrophid copepod, probably Ismaila belciki, copyright Michael D. Miller.

Splanchnotrophids are a group of copepods endoparasitic on two orders of shell-less marine gastropods (sea slugs), the Nudibranchia and Sacoglossa. They are characterised by reduced mouthparts and appendages though they retain a distinct pair of claw-like antennae. These antennae seem to be used to hold the copepod in place in their preferred location within the body cavity of their host. Though the exact means of feeding by splanchnotrophids is not certain, their rudimentary mouthparts, combined with a rarity of observations of actual tissue damage in parasitised hosts, indicate that they probably suck nutriment from their host's haemolymph. Females and males live in association within the host, the minute (and slightly more recognisably copepod-y) males holding close to their comparatively gigantic mates. As well as their size, female splanchnotrophids differ from males in the possession of elongate, tubular dorsal outgrowths of the thorax. These are most commonly presumed to function to provide more space for the female's enlarged ovaries, though some have suggested additional functions such as maintaining position within the host, respiration or absorbing nutrients (Anton & Schrödl 2013). The female's tubular egg-sacs extend through an opening in the host's body wall to release eggs into the water column. Usually, these egg-sacs will emerge close to some outgrowth of the host's own body, such as gills or papillae, and may be coiled if relatively long; these measures presumably help protect the egg-sacs from external damage. How the released larvae find and colonise new hosts remains unknown but it is possible the antennules (the smaller second pair of antennae possessed by most crustaceans) are used to locate hosts chemically, with their reduced condition in adults the result of a halt to development once their purpose has been fulfilled.

Female (left) and male Ismaila aliena dissected out from host, from Anton & Schrödl (2013).

Relatively few splanchnotrophids have been recognised to date, maybe about a dozen species divided between five genera. A few other species that had earlier been included in the family on little more grounds than that they were endoparasites of gastropods were excluded by Huys (2001)*. A sixth genus and species Chondrocarpus reticulosus is of uncertain relationships. If correctly associated with the splanchnotrophids, it is of interest in parasitising a different group of sea slugs (the pleurobranchids) and in its massive size (growing to twelve millimetres vs only a few millimetres for females of the other genera), but the only available description is inadequate for its proper characterisation. In some localities, splanchnotrophids have proven to be surprisingly abundant. A once-off survey of potential host species in Oregon found no less than 62% of individuals of one species to be infected (25 other potential host species were completely free of parasites), whereas a longer-term survey off the coast of Chile found an overall infection rate of 13% with some particular host species approaching 100% infection (Schrödl 2002). Host specificity seems to vary within the family: a study by Anton et al. (2018) found that species of the genus Ismaila tended to restrict themselves to a single host species, whereas species of Splanchnotrophus are more catholic and undiscriminating. Nevertheless, a lack of correlation between relationships of splanchnotrophid species and those of their host species suggests that, even in the more discriminating Ismaila, host changes may not have been uncommon.

*As a concise indication of just how sloppy some of the earlier work on 'splanchnotrophids' had been, one misattributed species was re-identified by Huys (2001) as having been based on the detached head of a pelagic amphipod.

The broader relationships of splanchnotrophids within copepods also remain poorly understood. A phylogenetic study by Anton & Schrödl (2013) suggested that Splanchnotrophidae may form a clade with another genus of copepods endoparasitic in gastropods, Briarella, with this clade being in turn derived from ectoparasitic ancestors. However, by the authors' own admission, this study was heavily biased in both taxon and character coverage to the Splanchnotrophidae, and may have been affected by insufficient scrutiny of non-splanchnotrophid taxa. Though derivation of the endoparasitic splanchnotrophids from ectoparasitic ancestors has a definite intuitive appeal, further study is required before we can feel confident about it.

REFERENCES

Anton, R. F., D. Schories, N. G. Wilson, M. Wolf, M. Abad & M. Schrödl. 2018. Host specificity versus plasticity: testing the morphology-based taxonomy of the endoparasitic copepod family Splanchnotrophidae with COI barcoding. Journal of the Marine Biological Association of the United Kingdom 98 (2): 231–243.

Anton, R. F., & M. Schrödl. 2013. The gastropod-crustacean connection: towards the phylogeny and evolution of the parasitic copepod family Splanchnotrophidae. Zoological Journal of the Linnean Society 167: 501–530.

Huys, R. 2001. Splanchnotropid systematics: a case of polyphyly and taxonomic myopia. Journal of Crustacean Biology 21 (1): 106–156.

Schrödl, M. 2002. Heavy infestation by endoparasitic copepod crustaceans (Poecilostomatoida: Splanchnotrophidae) in Chilean opisthobranch gastropods, with aspects of splanchnotrophid evolution. Organisms, Diversity & Evolution 2: 19–26.

The Ant-like Beetles

As I've commented before, the world is home to an overwhelming diversity of small brown beetles, most of them (for me, at least) inordinately difficult to distinguish. One group of tiny beetles that is quite recognisable, though, is the ant-like beetles of the genus Anthicus.

Anthicus cervinus, copyright Robert Webster.

Over a hundred species around the world have been attributed to this genus. Few of them grow more than a few millimetres in length. They are elongate with the elytra more or less rounded and often covered in short hair. The legs are relatively long. The prothorax is globular and generally narrower towards the base. The head is inclined and carried on a narrow neck (Ferté-Sénectère 1848). Many species have the elytra contrastingly patterned with bands or spots. As the vernacular name indicates, the overall appearance is reminiscent of a small ant though I'm not sure if this indicates a protective mimicry or is merely coincidence.

Anthicus antherinus, copyright Udo Schmidt.

The natural history of most Anthicus species is poorly known. The greater number of species are saprophages, found in association with rotting vegetation or scavenging on dead insects. One species, Anthicus floralis, is found worldwide as a storage pest, infesting seed and grain stores. One of the larger North American species, A. heroicus, has larvae that attack masses of dobsonfly eggs on midstream boulders (Davidson & Wood 1969). The larvae feed on the eggs from the inside, using them for shelter as well as nutrition, before emerging from the eggs to pupate.

REFERENCES

Davidson, J. A., & F. E. Wood. 1969. Description and biological notes on the larva of Anthicus heroicus Casey (Coleoptera: Anthicidae). Coleopterists Bulletin 23 (1): 5–8.

Ferté-Sénectère, M. F. de la. 1848. Monographie des Anthicus et genres voisins, coléoptères hétéromères de la tribu des trachélides. Sapia: Paris.

The Camisiids: Cryptic Inhabitants of Soil and Wood

Various views of Camisia biverrucata, copyright Pierre Bornand.

The animal in the above pictures is a typical representative of the Camisiidae, a widely distributed family of oribatid mites. Members of this family can be found in soil, on the trunks of trees, or hidden among mosses and lichens. They are slow-moving animals and are often concealed from potential predators by an encrusting layer of dirt and organic debris. Carrying this encrusting layer may be related to a reduction in the offensive chemical-producing glands that are used by many other oribatids for defense (Raspotnig et al. 2008). In members of the genus Camisia, the openings of these glands are completely covered by dirt, but in the genera Platynothrus and Heminothrus the openings still protrude above the encrustation. The recently described Paracamisia osornensis, which does not carry an encrusting layer, retains a large offensive gland (Olszanowski & Norton 2002).

Close to 100 species have been assigned to this family; though found in most parts of the world, camisiids are most diverse in the Northern Hemisphere. One species in particular, Platynothrus peltifer, is almost global in distribution and the range of habitats in which it has been found includes soil, litter, peat and even aquatic habitats (Norton & Behan-Pelletier 2009) When one is as small and metabolically undemanding as these animals are, there may be surprisingly little difference between being out in the air or immersed in water, and even primarily terrestrial oribatids may survive submersion almost indefinitely. Genetic studies of P. peltifer have identified a high level of within-species divergence and it has been calculated on this basis that this species may have survived almost unchanged in external appearance for some 100 million years (Heethoff et al. 2007).

The ubiquitous Platynothrus peltifer, copyright Centre for Biodiversity Genomics.

The Camisiidae are closely related to another oribatid family, the Crotoniidae, that is found in South America and Australasia. One of the more significant differences between the two families is that whereas the camisiids appear to be entirely parthenogenetic, crotoniids reproduce sexually. Recent analyses, both molecular and morphological, indicate that the 'camisiids' are paraphyletic with regard to the crotoniids, leading Colloff & Cameron (2009) to treat the latter as a subfamily, Crotoniinae, of the former. This re-classification has been accepted by other authors though the law of priority requires that the combined family should be known as the Crotoniidae, not Camisiidae. The nested position of the sexual crotoniines within the asexual 'camisiids', with other related oribatid families also being asexual, has led to the suggestion that the crotoniines have somehow re-evolved sexuality. This would be fascinating if true, seemingly violating the usual principle that complex features can't be re-evolved once lost. Personally, I tend to be sceptical of claims like this (see this old post, for instance). I would like to see evidence beyond simple phylogenetic position to indicate if this is a true re-evolution rather than an historical bias towards loss of sexuality giving a misleading image.

REFERENCES

Colloff, M. J., & S. L. Cameron. 2009. Revision of the oribatid mite genus Austronothrus Hammer (Acari: Oribatida): sexual dimorphism and a re-evaluation of the phylogenetic relationships of the family Crotoniidae. Invertebrate Systematics 23: 87–110.

Heethoff, M., K. Domes, M. Laumann, M. Maraun, R. A. Norton & S. Scheu. 2007. High genetic divergences indicate ancient separation of parthenogenetic lineages of the oribatid mite Platynothrus peltifer (Acari, Oribatida). Journal of Evolutionary Biology 20: 392–402.

Norton, R. A., & V. M. Behan-Pelletier. 2009. Suborder Oribatida. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3^rd ed. pp. 430–564. Texas Tech University Press.

Olszanowski, Z., & R. A. Norton. 2002. Paracamisia osornensis gen. n., sp. n. (Acari: oribatida) from Valdivian forest soil in Chile. Zootaxa 25: 1–15.

Raspotnig, G., E. Stabentheiner, P. Föttinger, M. Schaider, G. Krisper, G. Rechberger & H. J. Leis. 2008. Opisthonotal glands in the Camisiidae (Acari, Oribatida): evidence for a regressive evolutionary trend. Journal of Zoological Systematics and Evolutionary Research 47 (1): 77–87.

Fishing Mice

In a 1950 discussion of the origins of the fauna of South America, the great American palaeontologist G. G. Simpson dismissed the enormous radiation of muroid rodents in that continent as mere "field mice" exhibiting little regional differentiation. George Gaylord Simpson may have been one of the leading thinkers in mid-20th Century evolutionary theory, but in this respect he was just plain wrong. The South American mice and rats include a wide variety of divergent forms, some of them specialised in surprising ways. Consider, for instance, the fishing mice of the Ichthyomyini.

Illustration of Stolzmann's crab-eating mouse Ichthyomys stolzmanni by Joseph Smit.

The Ichthyomyini are a small assemblage of less than twenty species of mice found between Mexico and the north of South America from Peru to French Guiana (Voss 1988). Though some species are known from lower altitudes, the majority are found in alpine habitats in association with fast-flowing mountain streams (albeit in no location are they known to be common). Ichthyomyins seem to show a particular preference for hanging around waterfalls (Barnett 1997) and are not found in association with standing water such as swamps or ponds. They are moderate in size, ranging from ten to twenty centimetres in length excluding the tail. They show a number of adaptations for foraging underwater: the hind feet are partially webbed and have a more or less elongate fringe of stiff hairs that aid in swimming, the tail is furry rather than scaly, the eyes are small, the external ears are reduced in size (in a couple of species they are completely hidden by the fur and in one, Anotomys leander, the external pinnae are missing entirely), and the whiskers are long, strong, and arranged in such a way that they almost look more like the whiskers of a sea lion than of a mouse. The nerves associated with these whiskers are also enlarged and they evidently provide the main means of finding food.

Peruvian fish-eating rat Neusticomys peruviensis, copyright Carlos Boada.

Or, as I should say, finding prey. As far as we know, these mice seem to be entirely carnivorous. Only a couple of examples are known of specimens with plant matter in their stomachs and the significance of those finds remains uncertain. The primary source of food for most species is small invertebrates such as aquatic insects. Where freshwater crabs are available, a number of species show preferences for those. In larger species, the diet may be supplemented to a greater or lesser degree by small vertebrates such as fish or tadpoles. In line with their carnivorous diet, ichthyomyins are also characterised by a shorter, less complicated gut than other mice. Little is known about breeding and nesting habits in ichthyomyins. A specimen of Chibchanomys kept in captivity made tunnels in mossy vegetation (Barnett 1997). The few known specimens of gravid females indicate that litters are small with no more than two foetuses being carried at a time.

Voss (1988) recognised five genera of Ichthyomyini. The largest of these, Neusticomys, includes about half a dozen species that may more closely resemble the ancestral morphology for the group. Their hind feet are narrower than those of other ichthyomyins and the fringe of swimming hairs is shorter (Packer & Lee 2007). Where one species found in Colombia and Ecuador, Neusticomys monticolus, overlaps in range with Anotomys leander, it shows a preference for more sheltered sections of stream banks whereas A. leander is found in more exposed rapids.

Undescribed species of Chibchanomys, copyright Alexander Pari.

In most ichthyomyins, the coat consists of a layer of dense, woolly underfur covered by an overcoat of long guard hairs mixed with glossy, often distally flattened awn hairs. In Anotomys leander, Chibchanomys trichotis, and Neusticomys monticolus, the awn hairs are missing so these species have a dull grayish black appearance overall rather than than the glossy coat of other species. Chibchanomys trichotis retains minute external ear flaps albeit not ones that are visible past the coat; Anotomys leander, as noted above, lacks external ear flaps but does have the positions of the ear openings marked by a prominent white spot. Both these last two species were placed in monotypic genera by Voss (1988), but Barnett (1997) refers to an at-that-point undescribed species of Chibchanomys.

The remaining two genera were recognised by Voss (1988) as including four species apiece. Species of Rheomys, found in the mountains of Central America, have the most extensively webbed hind feet among the ichthyomyins. This is the only genus of fishing mice found in Central America; the other genera are all restricted to South America. The genus Ichthyomys includes the largest species of the group and also the species that feed on the highest proportion of vertebrates. This difference in diet is reflected in their dentition: Ichthyomys species have proportionately larger incisors and smaller molars than other ichthyomyins, with greater emphasis on using the incisors to grasp and slice struggling prey.

Rheomys raptor, from Villalobos-Chaves et al. (2016).

All told, the ichthyomyins are a remarkable radiation. Ecologically, they are close parallels to forms found elsewhere such as water shrews or desmans, but most other semi-aquatic mammals are distinctly larger in size. Even with less than twenty species, the ichthyomyins represent more species than there are of similarly sized semi-aquatic mammals anywhere else in the world. However, as noted above, ichthyomyins are not common anywhere they occur, and factors such as deforestation and climate change could represent a significant threat to their survival. It would be unfortunate if this remarkable radiation was to fade away.

REFERENCES

Barnett, A. A. 1997. The ecology and natural history of a fishing mouse Chibchanomys spec. nov. (Ichthyomyini: Muridae) from the Andes of southern Ecuador. Zeitschrift für Säugetierkunde 62: 43–52.

Packer, J. B., & T. E. Lee Jr. 2007. Neusticomys monticolus. Mammalian Species 805: 1–3.

Voss, R. S. 1988. Systematics and ecology of ichthyomyine rodents (Muroidea): patterns of morphological evolution in a small adaptive radiation. Bulletin of the American Museum of Natural History 188 (2): 259–493.