Hungry Starfish and Digital Photography

On my other truly awful blog, I recently featured an out-of focus photograph of Ham Harbour on the Caithness coast of the Pentland Firth. My reason for being there had been collecting rock-samples from betwixt the tide levels to compare measurements of chlorophyll within the biofilm on the rock-surface using the digital photography with the laboratory-based methods.

Biofilms are almost imperceptible layers of micro-organisms which can form on organic and inorganic surfaces alike. In the case of rocky shores, they provide food for numerous invertebrate species and may contain high levels of micro-algae which, in turn, contain chlorophyll: the green pigment in plants which powers photosynthesis; and absorbs visible red light but is maximally reflective of infrared light. Digital photography which allows computer applications to record levels of exposure to these wavelengths can then be used to back-track to estimate chlorophyll levels, and therefore ‘biomass’

Previously this had been the domain of expensive aerial photography to algal growth on marine surfaces, or certain plants on arable land. With high-quality digital cameras now costing less than a personal computer, the possibility of quickly obtained images from field work which avoid the time consuming process and destructiveness of physically removing rock-samples for laboratory analysis.

Monitoring the experiment was Dr. Angus Jackson, who is lead author on a recent ecology paper discussing feeding habits by the starfish, Patiriella exigua – some call it Parvulastra exigua – on rocky shore line in Botany Bay, Australia. This paper is open access.

In my first, and quite possibly final attempt at scientific journalism, I will give an oh-so brief overview of it.

Jackson’s main interest is in the ecology of shore lines, and foraging behaviour of species which inhabit them . My other truly awful blog also features a geo (pronounced hard-g) in Thurso which shows one of harshest environments on Earth.


P. exigua is an herbivorous starfish found across shore-lines throughout the Southern Hemisphere. Like many other shore species, its principle source of sustenance will come from micro-algae on often imperceptible biofilms exposed at low tide, and a straightforward method of determining consumption will be to examine chlorophyll content of grazed and un-grazed areas.

Jackson et al. appear to have conducted one of the first studies into the grazing habits of P. exigua specifically, and herbivorous starfish generally. They had also noted track marks on the rocky-surface which were assumed to be marks from starfish feeding-patterns.

To cut a long story short, Jackson et al. examined both rocky-surfaces on the shore-line and surfaces which were nurtured in the laboratory with P. exigua allowed to feed. An a priori case was made for the observed track-marks being due to P. exigua, with significant reductions of chlorophyll content recorded within the track marks. It would appear that competition for feeding material in rocky-shores may be greatly increased by herbivorous starfish.

As a side note, and related to my original investigation, Jackson et al. used digital photography in addition to laboratory extraction of biofilm material from the rocky surfaces. Estimated chlorophyll content from the former did not differ significantly from the findings from the latter.

When radio-tagging of migrant animals became cheap and easy, there was a rush to utilize this in all forms of research. Was this all necessary? Could it have been the life sciences equivalent of stamp collecting?

This has not yet occurred with digital photography, and as yet untapped areas of valid research do exist. Such as comparing biofilms on Caithnessian rocky-shores to work previously preformed by Jackson and colleagues in south-east Australia.

Further reading: Patiriella exigua: grazing by a starfish in an overgrazed intertidal system Jackson et al. (2009) Mar Ecol Prog 376 153-63


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4 Responses to “Hungry Starfish and Digital Photography”

  1. Francis Sedgemore Says:

    You asked me privately how truly awful is this “attempt at science journalism”. Let’s just say that unless this piece is radically re-written in order to render it comprehensible by normal human beings, you will surely be hunted down and destroyed by the Sacred Guardians of the PUST (Public Understanding of Science and Technology). You have been warned.

  2. efrafandays Says:

    Thank you, Francis, you pasty-faced old anarchist… on the bright side, I can only improve from this point on.

  3. Francis Sedgemore Says:

    It still ain’t “scientific journalism”, but this is a distinct improvement. That said, statements such as “This is, of course,…” are are known to be annoying to non-scientist readers. It’s a bit like scientists writing in their peer-reviewed papers “It is trivial to show that…”, which often gets up the noses of critical readers. Some educated people may not know what chlorophyll is.

    p.s. I am not pasty-faced, sporting as I do a healthy tan from regular 100-km cycle rides around the North Downs. Which is where I’m off to right now…

  4. efrafandays Says:

    ~*sobs out loud*~

    Okay, *another* modification in addition to a couple of observations from Angus himself.

    In time, you leather-skinned wannabe Bakunin, in time. We all have to start somewhere.

    I need to get out to Bettyhill soon. Not 100 miles, but with the gradients, as much as you’d do.

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