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Class: Pycnogonida (sea spiders)

Life >  Kingdom: Metazoa (animals) > Phylum: Arthropoda > Subphylum: Cheliceriformes

Pycnogonids bear a superficial resemblance to terrestrial spiders (Araneae) and are found in the sea, hence their common name 'sea spiders'. They are strange-looking animals because they have long legs in relation to the body. Over 1000 species in nearly 100 genera have been named worldwide and evidently there are many more species still to be described. Sixty-four species are known from the seas round South Africa (Adie et al. 2005). They are found from intertidal regions down to a depth of nearly 7000 metres and our found in oceans worldwide. They range in size from a leg span of only 2 mm through to deep sea species in the genus Colossendeis that have leg spans of up to 60 cm. 

Diagnosis

The defining characters of the pycnogonids are as follows:

  • unique pre-oral proboscis (no proboscis in other cheliceriformes).
  • ovigers, which are specialised leg-like appendages between the pedipalps and first pair of walking legs (absent in females of some species). They can be used in a number of ways but in males are used for brooding batches of eggs that are obtained from the female (see Life Cycle below).
  • opisthosoma reduced or absent (reproductive systems extend into the legs so the legs help in storing organs that would normally take up space in the opisthosoma).  
  • walking legs nine-segmented (up to 7-segmented in other cheliceriformes [check]). 
  • multiple pairs of gonopores, located on some or all legs (1 pair in other cheliceriformes).

Food

Most species are predators of hydroids, nudibranchs, polychaetes and other small invertebrates. These species feed by using three cuticular teeth at the end of the proboscis to pierce their prey and they then suck out the contents of their prey through the proboscis. In addition to the predators, there are some species that feed on algae and others that scavenge.

Life cycle

  • Legs of females are usually swollen in the femur to provide space for storing unfertilised eggs. 
  • Legs with eggs have gonopores at their base through which the eggs are laid.
  • The male fertilizes the eggs as they are laid by the female - he either hangs beneath her or is over her back. 
  • The male gathers up the eggs and glues them to his ovigers using a sticky solution from special femoral glands.
  • After the eggs have been looked after by the male for some time, they typically hatch out as unique protonymphon larvae but in some species they hatch out as juveniles that are miniature versions of the adult.
  • The protonymphon larvae live symbiotically with cnidarians, molluscs or echinoderms. This relationship appears to be parasitic in some cases and commensal in others. 
  • The larva passes through a series of moults in which segments and appendages are added on to produce a juvenile. 
  • The juvenile becomes free-living (?) 

References

  • Adie, H., Wethered, R., Herbert, D. & Lewis, F. 2005. The state of the nature of South African biosystematics study. Final Report. Institute of Natural Resources, Scottsville, Pietermaritzburg Investigational Report No. 254.  

  • Arnaud, F & Child, C.A. 1988. The South African Museum's Meiring Naude cruises. Part 17. Pycnogonida. Annals of the South African Museum 98: 121-187.

  • Brusca, R.C. & Brusca, G.J. 2003. Invertebrates - 2nd Edition. Sinauer Associates, Sunderland, Massachusetts.

  • Stock, J.H. 1962. Second list of Pycnogonida of the University of Cape Town ecological survey. Transactions of the Royal Society of South Africa 36: 273-286.

  • Stock, J.H. 1963. South African deep-sea Pycnogonida, with descriptions of five new species. Annals of the South African Museum 46: 321-340.

Text by Hamish Robertson 


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