1. Lucretia Neff Biology 493 30 November 2001
Distribution of the brittle star Ophiocoma erinaceus (Echinodermata, Ophiuroidea) on a mixed coral rubble reef flat
Lucretia Neff
Biology 493
30 November 2001

2. 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)

3. Introduction con’t
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)

4. Introduction con’t
Fretwell’s 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

5. Introduction, con’t
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

6. 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

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

8. Methods con’t Measured coral rubble by circumference at base
Counted O. erinaceus under each piece of rubble
Measured diameter of central disk of each O. erinaceus

9. Size variation in O. erinaceus

10. Number and size of rubble

11. Number of O. erinaceus per 1000 sq cm of rubble cover

12. % association of O. erinaceus compared to % area covered

13. Average number of O. erinaceus per rubble piece compared to rubble size

14. Number of O. erinaceus compared to available cover

15. Average O. erinaceus size under each rubble

16. 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

17. Conclusions, con’t 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

18. Conclusions, con’t
Possible reasons for selection for larger rubble (circumference > 79 cm)
Stability
Less predation
More inhabitable area

19. Conclusions, con’t
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

20. Conclusions, con’t Factors affecting habitat selection
Interspecific and intraspecific competition
Availability of microhabitat
Suitability of microhabitat
Three-dimensional nature of rubble

21. 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”

22. Questions????