What would eat a spiny urchin?!

One thought on “What would eat a spiny urchin?!”

  1. Hi there,

    I wanted to add to this thread because my paper was cited at the start as an example of how there is a common misconception that Diadema only have a few predators. Within my paper I draw heavily on the Randall paper that lists the range of fishes that predate on urchins, and also discuss the effects of invertebrate predators in the Discussion. Indeed I use Randall’s data (on the percentage of fish of each species that contained urchins spines within their stomachs) to weight the biomass of predators inside and outside the marine reserve in order to reflect the fact that some species feed more heavily on Diadema than others. I think the Randall data are interesting because in only 6 species did more than 20% of individuals contain urchin spines (at a time when urchins were much more abundant than they are now). This suggests a hypothesis that while a range of species may feed on Diadema, potentially only a few species feed on them at a sufficient rate to regulate their populations. There is also an interesting question of the number of species that can feed on urchins of different sizes – I suspect that most of the species listed by Randall can take juvenile urchins, but perhaps only a subset can feed on large adults.

    The comment about the few specialist predators in my paper (which incidentally, as the rest of the paragraph shows, was not a statement by me but a cited statement from Pinnegar et al, 2000) was a reflection on the potentially different effect of Caribbean marine reserves on urchins compared to in the Indian Ocean. In the Indian Ocean, Tim McClanahan and others have demonstrated that reserves can increase the abundance of urchin predators, and reduce damaging urchin plagues. Obviously urchin plagues have not been an issue in the Caribbean since the mass mortality in the 1980s, although the Sammarco data from Jamaica suggest that this may have been a problem before that time (at least in some habitats). The issue in the Caribbean is rebuilding Diadema abundances while simultaneously trying to rebuild fish communities that include urchin predators. Urchin population dynamics are complex, poorly understood, and influenced by a range of variables, but it seems likely that the abundance of a few key predators (few possibly being relative compared to the number of predators of, say, a larval fish recruiting to a reef) may be an important top-down control of Diadema densities.

    Cheers,
    Alastair

    Pinnegar JK, Polunin NVC, Francour P, Badalamenti F, Chemello R,
    Harmelin-Vivien ML, Hereu B, Milazzo M, Zabala M, D’Anna
    G, Pipitone C (2000) Trophic cascades in benthic marine
    ecosystems: lessons for fisheries and protected-area management.
    Environ Conserv 27:179–200

    Like

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