Letter | Published:

Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment

Nature volume 437, pages 11441148 (20 October 2005) | Download Citation



Independent specialization of arthropod body segments has led to more than a century of debate on the homology of morphologically diverse segments1,2, each defined by a lateral appendage and a ganglion of the central nervous system. The plesiomorphic composition of the arthropod head remains enigmatic because variation in segments and corresponding appendages is extreme. Within extant arthropod classes (Chelicerata, Myriapoda, Crustacea and Hexapoda—including the insects), correspondences between the appendage-bearing second (deutocerebral) and third (tritocerebral) cephalic neuromeres have been recently resolved on the basis of immunohistochemistry1 and Hox gene expression patterns3,4. However, no appendage targets the first ganglion, the protocerebrum, and the corresponding segmental identity of this anterior region remains unclear5. Reconstructions of stem-group arthropods indicate that the anteriormost region originally might have borne an ocular apparatus and a frontal appendage innervated by the protocerebrum6. However, no study of the central nervous system in extant arthropods has been able to corroborate this idea directly, although recent analyses of cephalic gene expression patterns in insects suggest a segmental status for the protocerebral region7,8,9,10. Here we investigate the developmental neuroanatomy of a putative basal arthropod11, the pycnogonid sea spider, with immunohistochemical techniques. We show that the first pair of appendages, the chelifores, are innervated at an anterior position on the protocerebrum. This is the first true appendage shown to be innervated by the protocerebrum, and thus pycnogonid chelifores are not positionally homologous to appendages of extant arthropods but might, in fact, be homologous to the ‘great appendages’ of certain Cambrian stem-group arthropods.

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  1. 1.

    & Development of the nervous system in the ‘head’ of Limulus polyphemus (Chelicerata: Xiphosura): morphological evidence for a correspondence between the segments of the chelicerae and of the (first) antennae of Mandibulata. Dev. Genes Evol. 213, 9–17 (2003)

  2. 2.

    On the relation of the arthropod head to the annelid prostomium. Q. J. Microsc. Sci. 247, 248–268 (1897)

  3. 3.

    & Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment. Proc. Natl Acad. Sci. USA 95, 10671–10675 (1998)

  4. 4.

    , , & A conserved mode of head segmentation in arthropods revealed by the expression pattern of Hox genes in a spider. Proc. Natl Acad. Sci. USA 95, 10665–10675 (1998)

  5. 5.

    in Evolutionary Developmental Biology of Crustacea (ed. Scholtz, G.) 135–167 (A. A. Balkema, Berlin, 2004)

  6. 6.

    A paleontological solution to the arthropod head problem. Nature 417, 271–275 (2002)

  7. 7.

    , & Analysis of neural elements in head-mutant Drosophila embryos suggests a segmental origin of the optic lobes. Wilhelm Roux Arch. Dev. Biol. 205, 31–44 (1995)

  8. 8.

    & Early steps in building the insect brain: neuroblast formation and segmental patterning in the developing brain of different insect species. Arthropod Struct. Dev. 32, 103–123 (2003)

  9. 9.

    & Structure of the insect head as revealed by the EN protein pattern in developing embryos. Development 122, 3419–3432 (1996)

  10. 10.

    & A single cell analysis of engrailed expression in the early embryonic brain of the grasshopper Schistocerca gregaria: ontogeny and identity of the secondary headspot cells. Arthropod Struct. Dev. 30, 207–218 (2002)

  11. 11.

    , & Arthropod phylogeny based on eight molecular loci and morphology. Nature 413, 157–161 (2001)

  12. 12.

    & Invertebrates 2nd edn (Sinauer, Sunderland, Massachusetts, 2003)

  13. 13.

    , & Head development in the onychophoran Euperipatoides kanagrensis with particular reference to the central nervous system. J. Morphol. 255, 1–23 (2003)

  14. 14.

    Bullock, T. H. & Horridge, G. A. (eds) Structure and Function in the Nervous Systems of Invertebrates (W. H. Freeman & Company, San Francisco, 1965)

  15. 15.

    & Pycnogonid affinities: a review. J. Zool. Syst. Evol. Res. 43, 8–21 (2005)

  16. 16.

    , & Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic. Proc. R. Soc. Lond. B 272, 395–401 (2005)

  17. 17.

    & A larval sea spider (Arthropoda: Pycnogonida) from the Upper Cambrian ‘Orsten’ of Sweden, and the phylogenetic position of pycnogonids. Palaeontology 45, 421–446 (2002)

  18. 18.

    , , & A Silurian sea spider. Nature 431, 978–980 (2004)

  19. 19.

    & Onychophoran cephalic nerves and their bearing on our understanding of head segmentation and stem-group evolution of Arthropoda. Arthropod Struct. Dev. 29, 197–209 (2000)

  20. 20.

    On the shape of the foregut lumen in sea spiders (Arthropoda: Pycnogonida). J. Mar. Biol. Assoc. UK 82, 1037–1038 (2002)

  21. 21.

    Larval types and a summary of postembryonic development within the pycnogonids. Invertebr. Reprod. Dev. 43, 193–222 (2003)

  22. 22.

    & Larval development and morphogenesis of the sea spider Pycnogonum litorale (Ström, 1972) and the tagmosis of the body of Pantopoda. Arthropod Struct. Dev. 32, 349–383 (2003)

  23. 23.

    Pycnogonids (Hutchinson & Co. Ltd., London, 1973)

  24. 24.

    , & Atlas of serotonin-containing neurons in the optic lobes and brain of the crayfish, Cherax destructor. J. Comp. Neurol. 269, 465–478 (1988)

  25. 25.

    , & Commissure formation in the embryonic insect brain. Arthropod Struct. Dev. 32, 61–77 (2003)

  26. 26.

    On the evolutionary significance of Pycnogonida. Smithson. Misc. Coll. 106, 1–53 (1947)

  27. 27.

    Beitrage zur Morphologie und Embryologie des vordern Korperabschnitts (Cephalosoma) der Pantopoda Gerstaecker, 1863. I. Entstehung und Struktur des Zentralnervensystems. Z. Zool. Syst. EvolForsch. 18, 27–61 (1980)

  28. 28.

    in Zologiska Bidrag Från Uppsala Vol. 6, 41–181 (Univ. Uppsala, Uppsala, 1918)

  29. 29.

    A contribution to the embryology and phylogeny of the pycnogonids. Stud. Biol. Lab. Johns Hopkins Univ. 5, 1–76 (1891)

  30. 30.

    & Upper Cambrian stem-lineage crustceans and their bearing upon the monophyly of Crustacea and the position of Agnostus. Lethaia 23, 409–427 (1990)

  31. 31.

    , & Development of embryonic cells containing serotonin, catecholamines, and FMRFamide-related peptides in Aplysia californica. Biol. Bull. 199, 305–315 (2000)

  32. 32.

    , in Traité de Zoologie: Anatomie-Systematique Biologie (ed. Grasse, P. P.) 906–941 (Masson et Cie, Paris, 1949)

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We thank W. Morrisey for diagram preparation; E. C. Seaver for assistance with immunohistochemistry; and J. Hanken, G. Das, G. Edgecombe and A. Hejnol for advice and discussion. We thank the Developmental Studies Hybridoma Bank for the anti-tubulin and Elav antibodies. This material is based on work supported by the National Science Foundation AToL program to G.G. and M.Q.M.

Author information


  1. Department of Organismic & Evolutionary Biology and Museum of Comparative Zoology, Harvard University, 26 Oxford Street,

    • Amy Maxmen
  2. Department of Organismic & Evolutionary Biology and Museum of Comparative Zoology, Harvard University, 16 Divinity Avenue, Cambridge, Massachusetts 02138, USA

    • Gonzalo Giribet
  3. Kewalo Marine Lab, Pacific Biosciences Research Center, University of Hawaii, 41 Ahui Street, Honolulu, Hawaii 96813, USA

    • William E. Browne
    •  & Mark Q. Martindale


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Competing interests

Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Corresponding author

Correspondence to Amy Maxmen.

Supplementary information


  1. 1.

    Supplementary Video 1

    The nervous system of an Anoplodactylus protonymphon larva (anti-tubulin (red) and anti-serotonin 5HT (green)), corresponding to text Figure 3b. Anterior chelifores are directed towards the upper right corner. The pair of bifurcating ocular nerves exit the protocerebral commissure dorsally. The rotating projection has been obtained by converting a z-stack of cLSM images into a projection using Zeiss LSM 510 software.

  2. 2.

    Supplementary Video 2b

    The innervated esophagus has been converted to greyscale. The spatial relationships of neuropil ring components are visible by scrolling through a z-stack of cLSM images captured incrementally between the ventral to dorsal surface of the protonymphon

  3. 3.

    Supplementary Video 2c

    The spatial relationships of neuropil ring components are visible via rotating projection of the z-stack created using Zeiss LSM software.

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  1. 1.

    Supplementary Figure 2

    These images show an Anoplodactylus larva labelled with the neuronal marker, Elav (red). Anterior chelifores are directed up. The ocular nerves exiting the dorsal surface of the protocerebrum have been converted from red to yellow. A. Scale bar: 25 µm. In the upper row, ocular nerves are artificially coloured, the bottom row is raw data.

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