Marine Biology

, Volume 153, Issue 2, pp 213–223 | Cite as

18S rRNA phylogeny of sea spiders with emphasis on the position of Rhynchothoracidae

  • Koichiro Nakamura
  • Yasunori KanoEmail author
  • Nobuo Suzuki
  • Takashi Namatame
  • Akinori Kosaku
Research Article


The phylogenetic relationships among all living families of sea spiders (Arthropoda: Pycnogonida) are investigated using nearly complete 18S rRNA sequences from 57 ingroup species and five chelicerates under the Bayesian and maximum likelihood methods. Monophyly of Colossendeidae, Pycnogonidae, Phoxichilidiidae, Endeidae and Pallenopsidae is consistently supported. However, the genera formerly classified in the family Ammotheidae are split up into two distantly related groups. The genera Ascorhynchus and Eurycyde (here recognized as Ascorhynchidae) are possibly an early offshoot of sea spiders, whereas other ammotheids constitute a robust terminal clade with Pallenopsidae, Phoxichilidiidae and Endeidae. This topology also opposes the prevalent assumption of successive losses and simplification of three kinds of cephalic appendages like in a previous cladistic analysis. At least three independent losses are suggested for palps by the inferred topology, and both chelifores and female ovigers may have been lost twice. Our knowledge of early ontogeny and internal anatomy is more congruent with the present 18S rRNA data. The families Callipallenidae and Nymphonidae with unique “attaching larvae” are grouped together in present molecular trees, suggesting that extended paternal care of offspring evolved only once in Pycnogonida. Confident clustering of Pycnogonidae and Rhynchothoracidae indicates that the number of female genital pores is an evolutionary conservative character.


Bayesian Inference Horseshoe Crab Cladistic Analysis Female Ovigers Bootstrap Probability 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



We are deeply indebted to the following colleagues for their help in obtaining pycnogonid material: Tadashi Akiyama (Okayama University), Kazunori Hasegawa, Katsumi Kubodera and Hiroshi Namikawa (National Science Museum, Tokyo), Isadora Kotuku (Aqua Scape Research Co.), Masaaki Ito (Tokyo), Dhugal Linsay and Hiroshi Miyake (JAMSTEC), Katumi Miyazaki (Kyoto University), Susumu Ohtsuka (Hiroshima University), Suguru Ohta and Takenori Sasaki (University of Tokyo), Hiroshi Oida (Chiba Prefectural Agriculture Research Center), Koji Tojo (Shinshu University), Hajime Ueda (Shizuoka), and Kensuke Yanagi (Natural History Museum and Institute, Chiba). We are also indebted to the crews of R.V. Tansei-Maru and Hakuho-Maru (University of Tokyo), R.V. Natsushima (JAMSTEC) and R.V. Toyoshio-Maru (Hiroshima University). Special thanks are given to Kazuki Kanakatsu and Kunikatsu Hamano (Tokyo University of Agriculture and Technology), Kazuhiro Nakaya (Hokkaido University), Shigeaki Kojima (University of Tokyo), Ryuichiro Machida and Seiji Tokumasu (Tsukuba University), Reiko Nakamura (Sequella Inc.), Hironobu Suzuki (Olympus Co.), and Yoshihisa Suyama (Tohoku University) for technical help and discussion. We also thank Claudia Arango (Queensland Museum), Amy Maxmen (Harvard University) and anonymous reviewers for their invaluable comments on earlier versions of this manuscript. Allan Child gave us suggestions in species identification. This study was partly supported by a research grant from Nikaido Gakuen, Tokyo.


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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Koichiro Nakamura
    • 1
    • 4
  • Yasunori Kano
    • 2
    Email author
  • Nobuo Suzuki
    • 1
  • Takashi Namatame
    • 3
  • Akinori Kosaku
    • 3
  1. 1.Japan Women’s College of Physical EducationTokyoJapan
  2. 2.Department of Biological Production and Environmental ScienceUniversity of MiyazakiMiyazakiJapan
  3. 3.Institute for Medical ScienceDokkyo Medical UniversityTochigiJapan
  4. 4.TokyoJapan

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