Field of Science

Sea Spiders

With arthropods being such a massively diverse sector of the global biota (and even that feels like an understatement; describing arthropods as 'very diverse' seems a bit like describing the Andromeda Galaxy as 'very far away'), it is only to be expected that it contains some very weird corners. And definitely among the weirder of those corners are the Pycnogonida, commonly known as the 'sea spiders'.

Anoplodactylus evansi, copyright Mick Harris & Claudia Arango.

Pycnogonids are a group of marine arthropods found around the world (not actual spiders, of course, though honest-to-goodness marine spiders are a thing that does exist). Their relationships to other arthropods have long been in dispute but the majority view is that they are distant relatives of the terrestrial arachnids. Pycnogonids are not uncommon in both coastal and deep-sea habitats but tend to go unnoticed: they feed on rock-encrusting colonial animals such as hydrozoans and are often coloured to disguise themselves against their prey. If one ever does see a sea spider, the first thing to stand out about them is how they are made of legs. The central body is often remarkably small compared to its limbs, to the extent that the dubbing of pycnogonids as 'no-bodies' by an early 20th Century author has become something of a cliché. Certain major organs, such as the gonads and parts of the digestive system, have even been diverted into the legs to make up for the lack of space in the body. Most pycnogonids possess four pairs of walking legs though there are species with more. At the front of the body on the underside of the head is a large proboscis that is used for sucking the juices out of prey, flanked by pairs of pincer-bearing chelifores and/or palps used for tearing it open. Near the first pair of walking legs there is often a pair of slender leg-like appendages known as the ovigers, used for carrying bundles of eggs until they hatch. The greater part of the body behind the head is taken up by the leg-bearing thorax; the legless abdomen is reduced to the merest nub like the docked tail of a dog.

Close-up on preserved male Anoplodactylus lentus, from Florida Museum of Natural History.

One of the largest recognised genera of pycnogonids is Anoplodactylus, with over 130 species worldwide and many continuing to be described (Lucena et al. 2015). This genus can be distinguished by the possession of chelifores with functional chelae (pincers) but palps are absent or reduced to buds. Both the chelifores and the proboscis are relatively short (Child 1998). Ovigers are five- or six-segmented and present in males only (male care of eggs is the standard pattern among pycnogonids). Species vary from 0.6 to 6 millimetres in body length. The majority of species of Anoplodactylus are found in shallow waters in temperate and tropical regions with a smaller number of species found in polar and deep waters. Alvarez & Ojeda (2018) record finding a single specimen of the species A. batangensis among vegetation on the surface of an anchialine pool in the Yucatan Peninsula of Mexico. Though the surface of these pools is more or less fresh water, deeper sections are saline owing to subterranean connections to the sea. The collection of a pycnogonid near the surface of this pool suggests an ability to adjust to very low salinity though one questions whether it would be able to survive indefinitely.

Larvae of Anoplodactylus are very small compared to those of other pycnogonids and have what has been termed an 'encysting' development (Burris 2011). As bizarre as the appearance of adult pycnogonids is, their larvae are arguably even weirder, being essentially nothing more than a head bearing chelifores, proboscis, and two pairs of undifferentiated appendages. The remaining segments of the body are added over the course of development. In Anoplodactylus, the larvae develop as parasites, forming a cyst in the gastrocoel (the stomach cavity) of cnidarians (having presumably been placed there somehow by their fathers, though I haven't found if we know how). They become free-living upon reaching the first juvenile stage, emerging from their host to pursue their predatory lives.


Alvarez, F., & M. Ojeda. 2018. First record of a sea spider (Pycnogonida) from an anchialine habitat. Latin American Journal of Aquatic Research 46 (1): 219–224.

Burris, Z. P. 2011. Larval morphologies and potential developmental modes of eight sea spider species (Arthropoda: Pycnogonida) from the southern Oregon coast. Journal of the Marine Biological Association of the United Kingdom 91 (4): 845–855.

Child, C. A. 1998. The Marine Fauna of New Zealand: Pycnogonida (Sea Spiders). National Institute of Water and Atmospheric Research (NIWA).

Lucena, R. A., J. F. de Araújo & M. L. Christoffersen. 2015. A new species of Anoplodactylus (Pycnogonida: Phoxichilidiidae) from Brazil, with a case of gynandromorphism in Anoplodactylus eroticus Stock, 1968. Zootaxa 4000 (4): 428–444.

Of Hawks and Marble

The acanthomorph fishes (a major clade of fishes mostly characterised by the presence of spines at the front of the dorsal fin) have long been recognised as a particularly thorny problem for higher-level systematics. Morphological relationships between many of the large number of families recognised in this clade have been almost impossible to unravel, and it is only in recent years that molecular analyses have been able to start making sense of the rapid divergences. Nevertheless, there are some subgroups of the acanthomorphs that have been recognised for a long time and which recent analyses have continued to support. One such group is the cirrhitoids.

Spottedtail morwong Goniistius zonatus, copyright Joi Ito.

Variously referred to in recent sources as the Cirrhitoidea, the Cirrhitoidei, or the Cirrhitiformes, the cirrhitoids include about eighty known species usually divided between five families. These are the hawkfishes of the Cirrhitidae, the trumpeters and morwongs of the Latridae, the Cheilodactylus fingerfins, the Chironemus kelpfishes and the Aplodactylus marblefishes (the morwongs were historically placed with the fingerfins in the Cheilodactylidae but have recently been transferred based on molecular data—Ludt et al. 2019). The largest cirrhitoid is the dusky morwong Dactylophora nigricans of western and southern Australia, growing to 1.2 metres in length, but most species are only a fraction of this size. Some of the largest species are of note to fisheries. Cirrhitoids are generally inhabitants of reefs, mostly feeding on benthic invertebrates such as crustaceans. They have long been recognised as a coherent group owing to their distinctive fin structure. The lower rays of the pectoral fins are not branched, and in a number of species they are thickened and protrude past the fin membrane (observant readers of this post may have already noticed a theme in many of the genus names given to cirrhitoids, relating to this feature). The pelvic fins are set well behind the pectoral fins. Other notable features of the clade include a relatively high number of vertebrae, a relatively low number of rays in the caudal fin, and the presence in juveniles of a fatty sac running along the fish's underside (Greenwood 1995).

Coral hawkfish Cirrhitichthys oxycephalus, copyright Aquaimages.

Both morphological and molecular studies have agreed that the hawkfishes of the Cirrhitidae represent the sister clade to the remaining cirrhitoids. Hawkfishes are brightly coloured inhabitants of the tropics, usually well under a foot in length. They are distinguished by bundles of trailing filaments emerging from the ends of the spines on the dorsal fin. Perhaps the most familiar member of the group is the longnose hawkfish Oxycirrhites typus, a regular in marine aquaria. However, this is also perhaps the most atypical member of the family as other species do not have the elongate snout. Hawkfishes commonly perch atop corals on the uppermost part of the reef, protected by the coral's sting and able to maintain a clear view of their surrounds. Wikipedia suggests that this behaviour is the inspiration for the name of 'hawkfish', but I'm not sure I buy this. I mean, it sounds plausible, but it also sounds like the sort of thing you would have to be diving below the reef to see. Vernacular names for fish tend to more often refer to things you might observe while hauling them onto a boat.

Marblefish Aplodactylus arctidens, copyright Peter Southwood.

The remaining cirrhitoids are all found in cooler waters, mostly in the Southern Hemisphere. Two species of Latridae, the redlip morwong Goniistius zebra and the spottedtail morwing G. zonatus, are found in the northern Pacific off the coast of eastern Asia (the kind of distribution shown by the genus Goniistius, where species are found in northern and southern temperate waters but not in the intervening tropics, is known as 'anti-tropical' and it's an interesting question how such a distribution would come to be). They are mostly found among rocky reefs, with the kelpfishes Chironemus and marblefishes Aplodactylus being particularly associated with patches of seaweed. The marblefishes feed on algae (particularly reds) as well as on some invertebrates and are characterised by a transverse mouth that is little or not protractible (Regan 1911). As noted above, the family Latridae has been inflated recently by the inclusion of most of the species previously included in the Cheilodactylidae. Cheilodactylus itself is now restricted to two species found around southern Africa. They differ from the remaining species in the latrids by the absence of a gas bladder as well as by elements of the skeleton. Many of the latrids are favourites of anglers, being well regarded as eating fish. By contrast, the herbivorous marblefishes are maligned as very poor fare and avoided. There's something to be said for eating your greens.


Greenwood, P. H. 1995. A revised familial classification for certain cirrhitoid genera (Teleostei, Percoidei Cirrhitoidea), with comments on the group's monophyly and taxonomic ranking. Bulletin of the Natural History Museum of London (Zoology) 61 (1): 1–10.

Ludt, W. B., C. P. Burridge & P. Chakrabarty. 2019. A taxonomic revision of Cheilodactylidae and Latridae (Centrarchiformes: Cirrhitoidei) using morphological and genomic characters. Zootaxa 4585 (1): 121–141.

Nelson, J. S., T. C. Grande & M. V. H. Wilson. 2016. Fishes of the World 5th ed. Wiley.

Regan, C. T. 1911. On the cirrhitiform percoids. Journal of Natural History, series 8, 7: 259–262.

The New Centaury

In an earlier post, I described the South American flowering herbs known as the Coutoubeinae. In this post, I'm going to take a step back and look at a clade of which the coutoubeines form a part, the Chironieae.

Seaside centaury Centaurium littorale, copyright Anne Burgess.

The Chironieae are one of the major tribes of the flowering plant family Gentianaceae, including about 160 known species. Representatives are found in most parts of the world, though as part of the native flora in Australasia they do not extend past the north of Australia (some exotic species have been introduced further south). The Chironieae seem to primarily be supported as a clade on the basis of molecular data (Struwe et al. 2002). All members are herbs, from annuals to short-lived perennials. Most have an erect growing habit; members of the Caribbean genus Bisgoeppertia are annual climbers and some species of the Mexican genus Geniostemon are creeping perennials. There may or may not be a basal rosette of leaves, and a number of genera have winged stems. Flowers are solitary or borne in cymose or racemose inflorescences. These flowers are most commonly salver-shaped (that is, shaped like a flat dish) or tubular, and usually have four or five petals (some species may have up to twelve). The calyx is usually comprised of fused sepals and is unwinged and tubular. The fruit is usually a septicidal capsule (splitting along the septa between carpels), more rarely a berry.

Yellow centaury Cicendia filiformis, copyright Hajotthu.

Members of the Chironieae are divided between three subtribes that are mostly distinct both morphologically and biogeographically. As described in the previous post, the Neotropical Coutoubeinae are characterised by releasing their pollen in tetrads whereas the other subtribes shed individual pollen grains. The Canscorinae are mostly found in the Old World tropics and have white or cream-coloured flowers (less commonly yellow, pink or purple) with the calyx tube longer than the calyx lobes. The Chironiinae mostly includes found in northern temperate regions, as well as the southern African genera Chironia and probably the South American Zygostigma. Their flowers come in a range of colours—pink, yellow, purple or blue, but less commonly white or cream-coloured—and may have calyx lobes longer than the tube. Many chironiine flowers also have anthers that become spirally twisted after releasing pollen whereas those of Canscorinae are always straight. Molecular data usually support the monophyly of the three subtribes and the majority view seems to be that the temperate Chironiinae represent the sister group of a tropical clade of Canscorinae and Coutoubeinae.

Cultivated Eustoma, copyright Rameshng.

Perhaps the best known members of the Chironieae are the centauries of the genus Centaurium. Historically, about fifty species across the Holarctic have been included in this genus. However, phylogenetic studies have demonstrated that this broad sense of the genus is polyphyletic and thus it has been cut down to a group of about twenty species found in Europe and western Asia. The name 'centaury' refers to the use of common centaury Centaurium erythraea as a medicinal herb, after the legendary centaur healer Chiron. Other Old World species are now placed in the genus Schenkia whereas North American species form the genera Gyrandra and Zeltnera. The yellow centauries of Cicendia are small, filiform annuals native to Europe and the Americas that have been introduced to Australia. The rose gentians Sabatia of North America bear pinkish-purple flowers, often in lax cymes. There are also the prairie gentians of the genus Eustoma. Native to southern North America, these plants bear large, showy flowers that have become popular in cultivation. Commercially, they are labelled as lisianthus. This is not to confused with Lisianthius, a distinct genus of Gentianaceae, or Lisyanthus, a name that has been used in the past for members of yet another gentianaceous genus. Both of these belong to completely different tribes in the family, and may be subjects for another day.


Struwe, L., J. W. Kadereit, J. Klackenberg, S. Nilsson, M. Thiv, K. B. von Hagen & V. A. Albert. 2002. Systematics, character evolution, and biogeography of Gentianaceae, including a new tribal and subtribal classification. In: Struwe, L., & V. A. Albert (eds) Gentianaceae: Systematics and Natural History pp. 21–309. Cambridge University Press: Cambridge.