Distribution of the brittle star Ophiocoma erinaceus Echinodermata, Ophiuroidea on a mixed coral rub

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Distribution of the brittle star Ophiocoma
erinaceus (Echinodermata, Ophiuroidea) on a mixed
coral rubble reef flat

• Lucretia Neff
• Biology 493
• 30 November 2001

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Introduction

• Different ophiuroids species found at different
  depths (Shin and Koh 1993)
• Population size affects distribution (McClintock
  et al 1993)
• Astrobranchion constrictum found mostly on coral
  colonies (Stewart and Mladenov 1997)
• Asteronyx loveni competes interspecifically for
  host (Fujita and Ohta 1988)

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Introduction cont

• Water temperature plays a role in distribution
  (Dahm 1993, Muus 1981, Shin and Koh 1993)
• Ophiocoma erinaceus is positively associated with
  Ophicoma pica (Chartock 1983)

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Introduction cont

• Fretwells distribution model (as quoted in
  Brewer, 1994)
• Organisms select the most suitable habitat
• If habitat becomes crowded, suitability decreases
• Organism will select a less favorable habitat

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Introduction, cont

• Observations suggested that brittle stars
  partitioned available substrate according to
  rubble size
• Smaller brittle stars under smaller rocks
• Larger brittle stars under larger rocks
• As the brittle stars grew, they migrated to
  larger rubble

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Purpose

• Determine if there was a positive correlation
  between the number and size of O. erinaceus and
  the size of their coral rubble habitat
• Establish whether O.erinaceus were distributed
  randomly or selectively

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Method

• Hauula Beach Park
• 26 m2 coral rubble site
• Coral rubble size groups
• increments of 10 cm circumference, ranging from
  40 to 160 cm

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Methods cont

• Measured coral rubble by circumference at base
• Counted O. erinaceus under each piece of rubble
• Measured diameter of central disk of each O.
  erinaceus

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Size variation in O. erinaceus
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Number and size of rubble
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Number of O. erinaceus per 1000 sq cm of rubble
cover
 
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association of O. erinaceus compared to area
covered
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Average number of O. erinaceus per rubble piece
compared to rubble size
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Number of O. erinaceus compared to available
cover
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Average O. erinaceus size under each rubble
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Conclusions

• Number of O. erinaceus occupying each piece of
  rubble increased as size of rubble increased
• Size of individual O. erinaceus did not increase
  with size of coral rubble
• High frequency of brittle stars of all sizes
  selected coral rubble with circumference greater
  than 79 cm

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Conclusions, cont

• Possibilities
• Active selection of favorable substrate of
  settling larva
• Increased chance of survival
• Passive settling of larva over a wide area
• Decreased survival rates on unfavorable substrate

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Conclusions, cont

• Possible reasons for selection for larger rubble
  (circumference gt 79 cm)
• Stability
• Less predation
• More inhabitable area

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Conclusions, cont

• Availability of most favorable substrate is not
  constant
• Organisms first select the most suitable habitat
  until the disadvantages outweigh the advantages
  of a less suitable habitat
• Some O. erinaceus may have been forced to less
  favorable habitat

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Conclusions, cont

• Factors affecting habitat selection
• Interspecific and intraspecific competition
• Availability of microhabitat
• Suitability of microhabitat
• Three-dimensional nature of rubble

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Acknowledgements

• Dr. Goodwill
• Dr. Winget
• Dr. Bruner
• Dr. Oba
• MSC Division
• Milena Acevedo, Ari Cheney, Amy Dewsnup, Lydia
  Garner, Esther Lee, Wendy Roehr, Mike Shipman,
  Kelly Yoshimura
• Jon Buchanan - the click meister

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Questions????