Cirripedes (barnacles) are known for their sessility – it’s their defining characteristic. They count as one of the first model organisms of evolutionary biology, having been comparatively studied by Darwin for over 8 years (one would think they’re his favourite animals – although this letter says otherwise!). The 4 resulting monographs (two for Recent, two for fossil) are still some of the best examples of systematic biology done right (cf. Newman, 1987), and his experience working on them was extremely influential in setting his ideas on adaptation straight; among other things, it led to him stressing the importance of embryology for identification of possible homologies, laying the foundation for Haeckel’s expansion of that idea.
While they may be seen as a mindfuck taxon, it was actually recognised fairly early on, by surgeon J. Vaughan Thomson in 1830, that they were crustaceans, since they have a naupliar larva. This hasn’t been doubted since.
Systematically, cirripedes are a member of the Thecostraca, along with the Facetotecta and the Ascothoracida. Autapomorphic for the thecostracans is a second larval stage, the cypridoid, which comes between the nauplius and the adult. However, keep in mind that we currently have no clue where the Thecostraca fit in the crustacean system (Regier et al., 2005)
Cirripedes have several autapomorphies identifying them as a monophylum. These are taken and translated from the seminal Ax (1999). Cirripedes have three stages to their life cycle: the naupliar larva is planktonic and serves for dispersal, turning into a cypris larva that looks for a suitable substrate (with the help of compund eyes!) and attaches itself to it before metamorphosing into the adult (Høeg & Møller, 2006); each of these stages has its own set of autapomorphies.
Cuticular frontolateral horns, with glands at the tips. These are unique and allow immediate identification of a cirripede larva. Darwin (1854) wrongly suggested that they are homologous to other crustaceans’ primary antennae; this is false. They’re labelled as “fh” in the above SEMs, taken from the gorgeous Semmler et al. (2009) paper.
- Univalved dorsal carapace.
- First antenna modified to an attachment organ, with cement glands.
- Thoracomeres 6 and 7 fused. A penis attachment structure is found on the 6th thoracical segment, even though it should be on the 7th. Therefore, there was a fusion of the 6th and 7th segments. They’re followed by 2 abdominal segments and the telson with a furca.
- No feeding apparatus.
- Two segments visible from the outside: the capitulum (“head”) and peduncle (the attachment shaft), instead of seeing the actual body of the creature. Keep in mind that the capitulum includes part of the thorax, not just the head.
- Carapace fused ventrally into a sac. Only a slit remains for the cirri (feeding) and the penis.
- Calcareous plates. The standard is to have five plates: a dorsal carina, paired scuta and paired terga, but this can be modified.
- Thoracopods 1-6 modified to cirri, for filter-feeding.
- Thoracopod 7 modified into penis. It’s long and freely moveable.
- Reduced abdomen, most often to the point of complete absence.
Basically, to recognise the cirripedes by look is easy: if it’s sessile, enclosed in a house-like carapace, with feathery appendages sticking out of the top (cirri), it’s a cirripede. The lack of an abdomen is also unique, but not visible without dissection. The name-giving cirri develop from the cypris’s thoracopods and are recognisable as multisegmented hairy appendages. The setae are there to filter whatever is flowing in the current (organic particles, plankton).
Neuroanatomically, cirripedes are pretty interesting. The naupliar larva is standard and doesn’t have major differences from other crustaceans: there’s a tripartite brain, with a central protocerebrum, a deuterocerebrum controlling the primary antennae and a tritocerebrum for the antennules. The only divergence has to do with the innervation of the autapomorphic frontolateral horns, which are sensory structures – what they sense is still anyone’s guess though.
The cypris larva’s brain is pretty complex, befitting an animal with complex behaviour. Being the stage at which the cirriped chooses its final living space, it has to have multiple sensory systems to detect suitable sites. Harrison & Sandeman (1999) showed that they have the neural substrate needed to achieve this.
The interesting part comes with the metamorphosis to the adult, where the brain gets reduced (Gwilliam & Cole, 1979), a change undoubteably related to the sessile way of life (brain simplification is a trend observed in all suspension-feeders).
Cirripedes colonise hard substrates. These may be immobile (sea floor, rocks), or mobile – they’re known to foul ship hulls, or they can colonise the skin of wales (Coronula diadema) or the shells of marine turtles (Chelonibia testudinaria); back in the Cretaceous, some lived on ammonites (Ifrim et al., 2011). It doesn’t matter, as long as they can anchor themselves. Except for the parasitic Rhizocephala, all cirripedes are suspension-feeders.
We split the cirripedes into three groups: the Acrothroacica, the Thoracica and the Rhizocephala.
The Rhizocephala are the parasitics, mostly on decapods, but can also occur on stomatopods, peracarids and even other cirripedes. As is always the case with parasitism, these forms are even more derived than the already highly-derived cirripedian body plan (only the pentastomids rival them as the weirdest crustaceans). The adult is nothing more than a bag of reproductive organs anchored into the host by a system of roots – all organs, body structures, etc. are reduced. However, the presence of a naupliar larva confirms they are crustaceans, and the frontolateral horns on this larva confirms they are cirripedes.
The most well-known rhizocephalan is Sacculina carcini, a crab parasite. When the naupliar larva finds its host crab, it attaches on a joint and metamorphoses into the cypris, drilling through the crab’s cuticle and into the haemocoel. Here, it metamorphoses again into the vermigon, which is what the adult is called. From this point on, any resemblance to a cirripede disappears: the vermigon is a wormish thing that can swim, has no appendages, segments or organs (it only has four cell types!); it basically consists of only a thin epidermis and a ball of ovaries. The vermigon, swims to a specific site where it will stick out of the host crab to reproduce (Høeg, 1995).
The poorly-known Acrothoracica are unique in that they are plate-less, instead burrowing into living corals or inhabited gastropod shells. They are also suspension-feeders, and do not count as ectoparasites.
However, the bulk of the cirripedes are the regular Thoracica; if you’ve ever seen a cirripede (in tidal pools, on whales or ships, or as fossils are the most accessible ways to see them), it was most likely a thoracican.
If you’re lucky, you might catch them mating, which is an interesting act in itself. At reproduction time, the males stick out their penis, made really really long by turgor pressure, out over the substrate and move it around until they find a female. To help them out, the penis is equipped with many sensory structures and setae (see Klepal et al. (1972) for any details you want on their penis). Then they ejaculate. The human equivalent is to run blindly around and masturbate on the first woman you touch. (Don’t try this at home, you might hit your mother and you’ll end up a bunch of diseased inbreds like the British Royal Family. Do it in public just to be safe.)
Here’s a video of this. If you look closely, you’ll see that at several points, one of the cirri is longer than the rest. It goes beyond the crown, searching randomly and eventually going into other barnacles. That’s the penis, and every time that happens, it ejaculates.
And really, once that’s said, there’s nothing I can say to make cirripedes more interesting.
Ax P. 1999. Das System der Metazoa II.
Darwin C. 1854. A Monograph on the subclass Cirripedia, with figures of all the species.
Gwilliam GF & Cole ES. 1979. The morphology of the central nervous system of the barnacle, Semibalanus cariosus (Pallas). Journal of Morphology 159, 297-310.
Harrison PJH & Sandeman DC. 1999. Morphology of the Nervous System of the Barnacle Cypris Larva (Balanus amphitrite Darwin) Revealed by Light and Electron Microscopy. The Biological Bulletin 197, 144-158.
Høeg JT. 1995. The biology and life cycle of the Rhizocephala (Cirripedia). Journal of the Marine Biological Association of the United Kingdom 75, 517-550.
Høeg JT & Møller OS. 2006. When similar beginnings lead to different ends: Constraints and diversity in cirripede larval development. Invertebrate Reproduction & Development 49, 125-142.
Ifrim C, Vega FJ & Stinnesbeck W. 2011. Epizoic Stramentid Cirripedes on Ammonites from Late Cretaceous Platy Limestones in Mexico. Journal of Paleontology 85, 524-536.
Klepal W, Barnes H & Munn EA. 1972. The morphology and histology of the cirripede penis. Journal of Experimental Marine Biology and Ecology 10, 243-265.
Newman WA. 1987. Evolution of Cirripedes and their major groups. In: Southward AJ (ed.). Barnacle Biology.
Regier JC, Schultz JW & Kambic RE. 2005. Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic. Proc. R. Soc. B 272, 395-401.
Semmler H, Høeg JT, Scholtz G & Wanninger A. 2009. Three-dimensional reconstruction of the naupliar musculature and a scanning electron microscopy atlas of nauplius development of Balanus improvisus (Crustacea: Cirripedia: Thoracica). Arthropod Structure & Development 38, 135-145.
Thomson JV. 1830. On the cirripedes or barnacles. Zoological Research 1, 69-85.
Research Blogging necessities :)
Høeg, J. (1995). The biology and life cycle of the Rhizocephala (Cirripedia) Journal of the Marine Biological Association of the United Kingdom, 75 (03) DOI: 10.1017/S0025315400038996