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Bizarre barnacle discoveries from deep South African waters

By Prof. Charles Griffiths, Department of Biological Sciences, UCT and Dr Lara Atkinson, SAEON Egagasini Node
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All barnacles are highly modified crustaceans, bearing little resemblance to their more typical walking or swimming cousins, like shrimps and crabs.

Barnacles have instead taken up a sedentary existence, and have adapted their body form to this lifestyle by becoming encased in a series of protective shell-plates and by modifying their limbs into long, hairy appendages that can be combed though the water to collect floating food particles.

The two main groups within these ‘typical’ barnacles are the familiar acorn barnacles found encrusting rocky shores or ships’ hulls and the stalked barnacles, which are most commonly encountered on floating objects, but in fact reach their greatest diversity in deep-sea habitats.

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Typical stalked (left) and acorn barnacles

Bizarre parasitic barnacles

But there are another bizarre and highly modified group of barnacles that have lost all of their appendages and most of their internal organs and become parasitic on other, larger crustaceans. These are the rhizocephalan (literally ‘root-headed’) barnacles, which would be difficult to recognise as being barnacles at all, except that their larvae have remained exactly like those of their more typical ancestors.

The bodies of rhizocephalans consist of just two structures, a branching, root-like ‘interna’ that extends into the tissues of the host and sucks up nutrients from it, and a sac-like gonad or ‘externa’, visible attached to the outside of the host.

Their life-history is equally unusual. Female larvae produced from the externa swim though the water in search of a new host - usually a crab or prawn - which must be of the same species as that parasitised by the parent. When it finds a host, the female larva attaches to it, bores though the exoskeleton and injects its cell contents into the host’s tissues, where it grows into a branching root-like interna by drawing nourishment from the host’s body fluids.

When the female parasite reaches maturity, it develops a sac-like externa. This externa secretes hormones to attract male barnacle larvae, which must fertilise the female’s eggs. The fertilised externa then produces both larger male and smaller female larvae … and the cycle begins again.

‘Enslaved’ hosts

Infected hosts are usually castrated by the parasite and treat the parasite’s externa as if it were their own eggs, grooming and cleaning it. Sometimes the parasite also prevents the host from moulting, otherwise the externa is shed with the moult and regenerated afterwards from the interna.

Infected hosts thus essentially become ‘enslaved’ by the parasite, the food they eat being sucked up by the parasite’s interna and diverted to the production of parasite larvae!

Rhizocephalans are very poorly studied in South Africa. To date only three species have been described, all known only from their original taxonomic descriptions, which date from the 1950s.

Through collaborations between SAEON and the Department of Agriculture, Forestry and Fisheries (DAFF) and a long-term commitment to monitoring demersal invertebrates, the collection of deep-sea invertebrates for research purposes has been made possible. DAFF demersal staff collected in excess of 1 500 cloaked hermit crabs during the 2016 demersal surveys for researchers from the University of Cape Town (UCT).

New scientific discoveries

Recent work on these deep-sea hermit crabs by research students Amy Wright and Jannes Landschoff has resulted in the discovery of additional rhizocephalan species, as well as a new hermit species. These are all parasites of a single host, the deepwater cloaked hermit crab, Parapagurus bouvieri.

This hermit is itself a fascinating species, as it lives not in discarded gastropod shells, like other hermits, but in ’pseudoshells’ constructed by a commensal zooanthid (a colonial sea-anemone relative). These pseudoshells grow along with the hermits, relieving them from the problem of having to find successively larger shells to occupy as they grow in size. Having these ‘tailor-made’ homes probably also allows cloaked hermits to become far more common than would be the case if they had to rely on empty gastropod shells, as these are relatively rare in the deep-sea.

Examination of some 1 500 cloaked hermit crabs has so far resulted in the discovery of three different rhizocephalans. All of these were rare, which, combined with the challenges of obtaining large numbers of deep-sea hosts, is no doubt why they had not previously been reported.

The most common species, which has a kidney-shaped externa, was recorded about eight times, giving an infection rate of less than 1%. The other two species each produce multiple externa, but of different shapes, one resembling a hand of bananas and the other a group of skittles.

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Two of the newly discovered rhizocephalans

Both of these have so far been recorded on just one individual host. All these species are almost certainly new to science, as most rhizocephalans are host-specific, and none have previously been described from this particular host. Specimens have been sent to a taxonomic specialist in Norway, where they are currently being examined both genetically and morphologically before being described and named.

These fascinating creatures represent just a few of many new taxa that are emerging from invertebrate samples now regularly being collected during the DAFF demersal research surveys. This emphasizes how poorly known the invertebrate fauna of deeper waters off South Africa remains.

Send your images or specimens to the authors

Rhizocephalans are also widespread among other shallow-water crustacean hosts and any readers who may have noticed these strange creatures are encouraged to send images or specimens to the authors, who are keen to build up more information about this fascinating group and which South African hosts they infect.

Additional reading

  • Boschma, H. (1958). Peltogaster contortus, a new rhizocephalan from South Africa. Koninklijke Nederlandse Akademie van Wetenschappen 61: 431-437
  • Griffiths, C.L., Robinson, T.B., Lange, L. and Mead, A. 2010. Marine biodiversity in South Africa - an evaluation of current states of knowledge. PLoS-ONE 5(8) e123008
  • Walker, G. (2001). Introduction to the Rhizocephala (Crustacea: Cirripedia). Journal of Morphology, 249: 1-8 DOI: 10.1002/jmor.1038

 

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