MadSci Network: Zoology
Query:

Re: Why do both octopi and spiders have eight legs?

Date: Wed Jul 28 05:14:50 1999
Posted By: Trevor Cotton, Grad student, Palaeobiology Research Group, Department of Earth Sciences, University of Bristol
Area of science: Zoology
ID: 932070229.Zo
Message:

Spiders and Octopi are distantly related. Spiders belong to the arthropods, the group of invertebrate animals with jointed legs and an exoskeleton, including spiders and their relatives (scorpions, horshoe crabs, mites), insects, crustaceans (lobsters, prawns, crabs) and centipedes and millipedes. Octopi are cephalopod molluscs (MollusKs in the US), along with squid and cuttlefish. Other molluscs include bivalves (e.g. mussels, clams), gastropods (snails and slugs) and a host of less common groups. Lots more information about Arthropods and Molluscs is available online.

It was once thought that molluscs and arthropods are fairly closely related. Animals (with the exception of very simple animals like sponges and cnidarians) can broadly be divided into two major groups, the deuterostomes and protostomes (these groups are largely based on how the adult body is formed from the embryo, but are supported by many lines of evidence). Deuterostomes include humans, other vertebrates, echinoderms (starfish and sea urchins) and a few minor groups, such as sea-squirts, whereas all the other groups form the protostomes. Traditionally both molluscs and arthropods, along with segmented worms (the annelids, such as earthworms) were considered to form a group of advanced (coelomic) protostomes. Some zoologists (including myself) now think that arthropods are more closely related to some minor worm-like groups (the "aschelminths" e.g. nematodes) then to the molluscs, annelids and other protostomes. This is a matter of current scientific debate (anyone who may want to follow this in the scientific literature, see e.g. Aguinaldo et al. 1997; de Rosa et al., 1999; Nielsen et al., 1996) Either way, octopi are definitely more closely related to animals without "legs" at all than to spiders. Spiders similarly are more closely related to other arthropods with a wide range of numbers of legs than to octopi. The presence of 8 legs in both groups is the result of convergent evolution (as is for example, the presence of wings in bats and birds).

Usually the presence of similar structures in unrelated groups of animals shows that there is a strong functional reason for its evolution. In general the possession of 8 legs allows these animals to move with greater facility in directions other than forwards. Both spiders and octopi can often be seen moving backwards or sideways rather than forwards. In octopi, the legs are also used to manipulate objects and to sense the envrionment and it is also useful for them to be able to do these things in many directions at once. In contrast, insects with six legs have a definite stronger tendency to move and function forwards. The closest relatives of octopi, squid, have the "legs" (the same structures function as tentacles in squid) around the mouth, and are used for feeding in the forwards direction only. The number of tentacles in squids is highly variable.

REFERENCES

  1. Aguinaldo, A.M. et al. (1997) Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387:489-493
  2. de Rosa, R. et al. (1999) Hox genes in brachiopods and priapulids and protostome evolution. Nature 399:772-776
  3. Nielsen, C. et al. (1996) Cladistic analyses of the animal kingdom. Biol. J. Linnean Soc. 57:385-410


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