Field of Science

The Cordia Clade

The tropics are home to a wide diversity of plant species, many of them belonging to groups less familiar in cooler regions of the world. Prominent among these are members of the family Cordiaceae, a group of about 350 known species of mostly trees and shrubs. The Cordiaceae (alternatively treated as the subfamily Cordioideae of the family Boraginaceae) are a well distinguished clade both molecularly and morphologically. Most members of the clade have flowers with the stigma divided between four lobes, fruits with an undivided endocarp, and plicate cotyledons (Miller & Gottschling 2007).

Beach cordia Cordia subcordata, copyright Tauʻolunga.

Historically, most members of the clade have been assigned to a single genus, Cordia. This arrangement was revised by Miller & Gottschling (2007) who recognised the separate genus Varronia for about 100 species of multi-stemmed shrubs native to the New World. The remaining 250 or so species, most of them single-trunked trees, remained in the pantropical Cordia. The two genera also generally differ in their leaves (most Varronia have leaves with serrate margins whereas Cordia have entire margins) and inflorescences (most Cordia have broad cymose inflorescences whereas Varronia have smaller, more compact inflorescences). Few species of Cordiaceae are not assigned to either Cordia or Varronia. Three previously recognised small genera, Auxemma, Patagonula and Saccellium, are now synonymised with Cordia. The small African genus Hoplestigma and the prostrate annual herb Coldenia procumbens are placed in Cordiaceae primarily on the basis of molecular data (Miller & Gottschling 2007; Weigend et al. 2014).

Black sage Varronia curassavica, copyright Mauricio Mercadante.

A number of Cordia species are grown for their wood, with South American species providing timbers known as bocote, freijo (C. alliodora), and ziricote (C. dodecandra). These are only moderately strong woods but strikingly patterned and are more often used for aesthetic rather than structural purposes (such as cabinet veneers and musical instruments). Cordia alliodora has become an invasive in regions where it has been planted outside its native range such as Africa and Vanuatu. Various species are also grown for their edible fruits, such as the Assyrian plum C. myxa and the fragrant manjack C. dichotoma. These fruits are decidedly gooey when ripe and are often given names reflecting this fact such as glue berries, clammy cherries or, here in Australia, snotty gobbles (though this name is more widely used for fruits of the unrelated genus Persoonia). Pulp from unripe fruits of C. myxa can supposedly also be used as a type of glue. Your office reports may not be informative but they will at least be tasty!


Miller, J. S., & M. Gottschling. 2007. Generic classification in the Cordiaceae (Boraginales): resurrection of the genus Varronia P. Br. Taxon 56 (1): 163–169.

Weigend, M., F. Luebert, M. Gottschling, T. L. P. Couvreur, H. H. Hilger & J. S. Miller. 2014. From capsules to nutlets—phylogenetic relationships in the Boraginales. Cladistics 30: 508–518.


Most of the various 'seaweeds' found around the world can be assigned to one of three major groups, each named for their most characteristic pigments: green algae, red algae and brown algae. Of these, green algae are the closest relatives of land plants, and red algae are the most taxonomically diverse. But for many people, the most familiar of the three will be brown algae. Owing to their often relatively large size and predilection for growing in visible locations, brown algae are likely to be the first examples to come to mind when one thinks of seaweed. For this post, I'm examining a particular subgroup of the brown algae, the family Dictyotaceae.

Forkweed Dictyota dichotoma, copyright Ria Tan.

Representatives of the Dictyotaceae can be found around the world but are more diverse in warmer tropical and subtropical waters. They seem to be particularly diverse in the Australasian region. Dictyotaceae are moderately sized seaweeds with flattened thalli that may grow as branching ribbons or radiating fans. One fan-shaped species of Dictyotaceae, Padina pavonica, has earned itself the vernacular name of 'peacock's tail'(this species is also notable for being one of the few calcified brown algae). These thalli grow apically from meristematic cells. Dictyotaceae have an isomorphic life cycle with the alternating sexually and asexually reproducing generations being similar in overall appearance. Sporangia in asexual individuals grow as superficial nodules scattered over the surface of the thallus; the resulting spores usually differ from those of other brown algae in lacking flagella. The less abundant sexual individuals are mostly divided between separate males and females (Bittner et al. 2008).

Peacock's tail Padina pavonica, copyright Diego Delso.

Dictyotaceae are distinct enough from other brown algae to have consistently been treated as their own order (indeed, their sporangia are unique enough that some very early authors did not even regard them as brown algae). Two species found around Australasia, Dictyotopsis propagulifera and Scoresbyella profunda, have previously been considered distinct enough to warrant their own separate families within this order Dictyotales. Dictyotopsis propagulifera has a monostromatic thallus (that is, the thallus is only one layer of cells thick). Scoresbyella profunda has an apical growing cell that divides lengthwise to the thallus instead of transversely as in other Dictyotales. However, molecular data have indicated that these two genera are nested within Dictyotaceae and so only the single family is currently recognised. Dictyotaceae has also been divided in the past between tribes Dictyoteae and Zonarieae based on the nature of the apical growing cells (Dictyoteae have a single meristematic cell whereas Zonarieae have a cluster or row of cells) and some authors have even treated them as distinct families. Again, however, molecular data have not corroborated this division (Bittner et al. 2008).

Lobophora variegata, copyright John Turnbull.

For most species of Dictyotaceae, their greatest significance to humans probably comes from the role they play in providing habitats to fish and other marine animals. As with other algae, Dictyotaceae produce a range of secondary metabolites that serve functions such as protecting them from grazers, and some of these may prove to have economic applications. Some species of Dictyotaceae, on the other hand, have become significant invasive species. A dramatic recent example has been provided by the northern Pacific species Rugulopteryx okamurae which was probably first imported to the Mediterranean as a contaminant on farmed oysters (García-Gómez et al. 2020). This species was recorded on the southern coast of France in 2002 and was later recorded on the coast of Ceuta in 2015. Within a year of the latter record, its presence in Ceuta had reached absolute plague proportions. Most of the illuminated rocky sea bottom was covered by R okamurae, up to about 90% coverage at optimal depths about ten to twenty metres. Over 5000 tons of washed-up seaweed was removed from the beaches of Ceuta in 2016. Needless to say, native seaweeds, and other sessile marine organisms such as corals, would have been severely impacted by this spread.

Rugulopteryx okamurae in Morocco, from El Aamri et al. (2018).

What caused this dramatic invasion? It would have certainly been a factor that defensive metabolites produced by Rugulopteryx okamurae had a negative impact on competitors. But perhaps even more significant a factor was climate change. Rising sea temperatures in the Straits of Gibraltar would have made things uncomfortable for native marine life used to cooler conditions. Meanwhile, the subtropical immigrant would have found things increasingly to its liking. With its competition hobbled and nothing to hold it back, R. okamurae was set to take over.


Bittner, L., C. E. Payri, A. Couloux, C. Cruaud, B. de Reviers & F. Rousseau. 2008. Molecular phylogeny of the Dictyotales and their position within the Phaeophyceae, based on nuclear, plastid and mitochondrial DNA sequence data. Molecular Phylogenetics and Evolution 49: 211–226.

García-Gómez, J. C., J. Sempere-Valverde, A. R. González, M. Martínez-Chacón, L. Olaya-Ponzone, E. Sánchez-Moyano, E. Ostalé-Valriberas & C. Megina. 2020. From exotic to invasive in record time: the extreme impact of Rugulopteryx okamurae (Dictyotales, Ochrophyta) in the strait of Gibraltar. Science of the Total Environment 704: 135408.

Snails of Crystal

In many parts of the world, searching under pots or among other garden rubbish may turn up minute snails with translucent shells. Among the various families which might be found in this way are representatives of the family Pristilomatidae, commonly known as crystal snails.

Common crystal snail Vitrea crystallina, copyright O. Gargominy.

All members of the Pristilomatidae* are tiny: the minute gem snail Hawaiia minuscula, one species which has become widespread, is a giant within the family at close to three millimetres in diameter. The shells have a low spire, growing in more or less a disc shape, and are generally smooth or ornamented with very fine radial lines. In life, they are transparent or a cloudy white, explaining their vernacular name. Internal organs are often visible through the shell. The Pristilomatidae are part of the broader group of mostly tiny snails known as the Gastrodontoidea (which I've covered on this site earlier, albeit in a rather inept fashion). Even among this array, however, they are notably small. Within the gastrodontoids, the pristilomatids are primarily distinguished by the structure of the male genitalia, in which the vas deferens in attached to the proximal end of the penial tunica (a sheath of muscle tissue around the penis; Hausdorf 1998). However, there is a bit of an open question about how well supported they are as a group. Their distinguishing features could all be side effects of their reduced size.

*In older texts, you may find this family referred to as the Vitreidae, after one of the larger genera included. However, the name Pristilomatidae has priority.

Minute gem snail Hawaiia minuscula, copyright Chris Mallory.

Within their native range, crystal snails may mostly be found in western North America and the western Palaearctic. Several species, however, have become further distributed in association with humans. As such, they are mostly found in damp, disturbed habitats, such as gardens, nurseries and parks. They will be found in secluded locations such as under flower pots or buried among moss or leaf litter. Some species prefer to fully bury themselves within the soil. Some other members of the gastrodontoids are known to be predatory, feeding on small arthropods or other snails and their eggs, but I haven't been able to find any direct reference to such habits among pristilomatids. It seems more likely that they prefer to feed on decaying fragments of vegetation. They do not seem to be regarded as presenting a challenge to the gardener; rather, they may provide their own small amount of assistance in keeping things tidy.


Hausdorf, B. 1998. Phylogeny of the Limacoidea sensu lato (Gastropoda: Stylommatophora). Journal of Molluscan Studies 64 (1): 35-66.

Hausdorf, B. 2000. Biogeography of the Limacoidea sensu lato (Gastropoda: Stylommatophora): vicariance events and long-distance dispersal. Journal of Biogeography 27: 379–390.

The Stizus Sand Wasps

Some years ago, I presented a post on the sand wasps of the tribe Bembicini. Bembicini are just part of the broader range of sand wasps that have been variously classified as the Bembicidae, Bembicinae or Nyssoninae (Bohart & Menke 1976; Sann et al. 2018). Another diverse subgroup of the bembicids is the genus Stizus, of which more than 120 species are found in Eurasia, Africa and North America (but not in Australia or South America). Stizus species are relatively large wasps, getting up to 3.5 cm in length. Like Bembicini, they are often brightly coloured, black banded with yellow and/or red. They are otherwise fairly generalised in appearance: the labrum is exserted but is not remarkably long like that of bembicins, and the ocelli are not reduced (Bohart & Menke 1976).

Stizus pulcherrimus, copyright Phonon B.

The nesting behaviour of Stizus species was reviewed by Evans & O'Neill (2007). All known Stizus nests are constructed in soil and sand, sometimes in relatively damp locations such as salt marshes or near water bodies. Burrows of nests may be a foot or more deep and contain multiple cells; acessory burrows are common. Though females construct their burrows strictly single-handedly, they will often nest in clusters with other females. Polidori et al. (2008) found that this clustering behaviour in the European Stizus continuus was due to females being actively attracted to nests of other females, rather than just a side effect of limited nest sites. The most commonly used prey are various Orthoptera (grasshoppers or katydids); a handful of species instead prey on mantids. Prey are paralysed by repeated stinging before being flown back to the nest carried under the female. After the prey insect has been placed in a nest cell, the female lays an egg on its thorax. In most cases, cells are fully stocked with prey before laying, but females of S. continuus have been observed carrying fresh prey back to nests in which larvae have already hatched and begun eating.

Stizus perrisi female constructing nest, copyright David Genoud.

Mating between males and females generally occurs as the newly matured females emerge from the parent nest. Males often emerge before females and begin patrolling the nesting area, searching for females and chasing away other males. In some cases, they may begin actively digging for females emerging from burrows, and a newly emerged female may find herself surrounded by a pack of competing males. In their eagerness, males may become rather hasty: males of the Japanese S. pulcherrimus have been observed attempting to force themselves on females of the related genus Bembix!


Bohart, R. M., & A. S. Menke. 1976. Sphecid Wasps of the World. University of California Press: Berkeley.

Evans, H. E., & K. M. O'Neill. 2007. The Sand Wasps: Natural History and Behavior. Harvard University Press.

Polidori, C., P. Mendiola, J. D. Asís, J. Tormos, J. Selfa & F. Andrietti. 2008. Female-female attraction influences nest establishment in the digger wasp Stizus continuus (Hymenoptera: Crabronidae). Animal Behaviour 75: 1651–1661.

Sann, M., O. Niehuis, R. S. Peters, C. Mayer, A. Kozlov, L. Podsiadlowski, S. Bank, K. Meusemann, B. Misof, C. Bleidorn & M. Ohl. 2018. Phylogenomic analysis of Apoidea sheds new light on the sister group of bees. BMC Evolutionary Biology 18: 71.

Hydroglyphus pusillus, the Tiny Tiger

Hydroglyphus pusillus, copyright Udo Schmidt.

Let's take another visit to the world of diving beetles. Above is Hydroglyphus pusillus, one of the few representatives in northern Europe of a genus that otherwise includes close to ninety species spread through the Old World, primarily in the tropics. Hydroglyphus species are tiny diving beetles, only about two or three millimetres in length, with an elongate oval body shape. Characteristic features of the genus include basal striae on the pronotum and elytra, sutural striae on the elytra, and no transverse stria on the top of the head (Watts 1978, as Guignotus, a subsequently synonymised name). Species are often marked with distinctive colour patterns of streaks and blotches.

Hydroglyphus pusillus attacking larva of mosquito Culex pipiens, from Bellini et al. (2000).

Despite their small size, Hydroglyphus species are (like other diving beetles) voracious predators of other aquatic insects. Bellini et al. (2000) investigated the possible role of H. pusillus in controlling mosquito larvae in flooded rice fields in Italy. The larvae of H. pusillus mostly kept to the bottom sediment (so might be expected to be hunting prey other than mosquitoes) but adults were the most abundant diving beetle in the water column at the surveyed locations. One might expect that H. pusillus would not be effective predators of mosquito larvae that greatly outsized them. One would be wrong: not only are they indeed capable of taking down mosquitoes, Bellini et al. went so far as to describe their effects as "a real slaughter". A diving beetle latching onto a mosquito larva would soon find itself joined by others seemingly scenting haemolymph in the water. Between them, this mob of beetles could destroy a larva in a matter of seconds. Tiny, but terrifying.


Bellini, R., F. Pederzani, R. Pilani, R. Veronesi & S. Maini. 2000. Hydroglyphus pusillus (Fabricius) (Coleoptera Dytiscidae): its role as a mosquito larvae predator in rice fields. Boll. Ist. Ent. "G. Grandi" Univ. Bologna 54: 155–163.

Watts, C. H. S. 1978. A revision of the Australian Dytiscidae. Australian Journal of Zoology, Supplementary Series 57: 1-166.