Of Isopods and Whelk Algal Gardens - Observation of the Week, 3/27/24

Our Observation of the Week is this isopod (potentially Exosphaeroma kraussi) on a Smooth Plough Shell snail (Bullia rhodostoma), seen in South Africa by @penel1!

“I happened to be climbing the mountains above Llandudno Beach (about 5 km from where I live in Hout Bay, Cape Town) on the Southern Apostles,” recalls Penelope Brown. 

The day was hot so afterwards I dropped down for a swim. The tide happened to be very low and the sea calm and very cold (due to intense upwelling). I had time before going home for lunch so I did a bit of iNaturalisting in the intertidal zone…see other photos on iNat at the time! And that is when I noticed the isopod / sea louse on the shell of one of the Bullia (plough shells). I was intrigued, so I took a few photos, posted them and then promptly forgot about them until the smiley sandy beach ecologist, Linda Harris (I don't know her), got all excited and proposed it as the Observation of the Day…To be honest, not being a very skilled iNaturalist participant, I did not even know that there was something like that!!  

Linda Harris (@linda_harris) is a South African marine scientist who specializes in coastal ecosystems, and as Penelope said she was quite excited by this observation. I reached out Linda for more information about the species involved, and any insights she might have into what was documented in the photos. She tells me that the snail is a type of whelk which can be found in the intertidal areas of sandy beaches, migrating along the beach as the tides change. 

These plough shells are scavengers with an exceptional olfactory system that allows them to detect even the faintest scents of beach-cast fish, jellyfish and bluebottles. They are so well adapted to the erratic supply of food that they can consume enough from one meal to last them for 18 days. However, they also have “algal gardens” on their shells on which they can forage when food is scarce. Because of their lifestyle of burrowing in and emerging from sand in the swash zone, with constant, relatively fast water movement around them, only the snails themselves can graze on their algal gardens. No other animal has previously been observed foraging on the algal gardens growing on these snails’ shells. That’s what makes this observation so unique. 

The plough shells must have been in the sheltered, shallow pool of water long enough for the isopod to detect the source of food and to start grazing on the algal garden. I haven't had a chance to key the isopod out yet, but I suspect this is one of the rocky shore isopods because I don't recognise it as a (at least, common) beach species. It’s the first time an interaction like this has ever been observed.

Penelope (above, on Table Mountain) grew up on a farm in South Africa’s Eastern Cape and credits the freedom she had to explore her surroundings for instilling her an interest in nature. “Walking with my father, she says, “we used to  sometimes listen to the ‘grass grow’ or sit quietly watching a buck nibble the vegetation, and when having tea in the forest with my mother, we'd leave little bits of chocolate cake for the fairies…it was magical!”

She ended up studying zoology at university, “on the advice of some senior students at my residence, because that department was ‘way more fun’ than the botany department (not really a good reason, but good enough as it turned out)” but ended up spending a lot of time with plants anyway, researching phytoplankton production and bloom dynamics in the the southern Benguela off the Cape Peninsula and diving in kelp beds. Then,

being increasingly intrigued by the ‘fynbos’ in of the Cape Floral Region while exploring the very different (cf Eastern Cape) mountains, the vlakte (plains) and coast of the Western Cape, my interests veered more into the terrestrial biota ... and this is where, later in life, iNaturalist eventually provided a space to 'formalize' my interactions with the incredibly biodiverse region in which we are privileged to live. (I actually started with iSpot with Tony Rebelo, a SANBI botantist friend, and dedicated local curator of iSpot and now on iNat, and a wonderful advocate for it!) 

I enjoy using iNat especially when I am on my own, and can immerse myself in it and bumble along happily in nature. I use it wherever I am, on and off, as it is a good record of where I have been and what we saw there. However, more specifically, I am tending to use it more and more for recording the locally indigenous plant species, and also for invasive alien plants, in our catchment (the Baviaans river catchment between Skoorsteenkop and Constantiaberg) which our community group is systematically clearing of invasive alien vegetation.

(Photo of Penelope taken by Judy Jepson. Some quotes have been lightly edited for clarity.)

- Linda says she based a lot of what she wrote on research by A.C. Brown, which you can find on Google Scholar. She also recommends checking out page 14 of this paper [PDF] about whelk algal gardens

- way back in 2016, @oryzias‘s observation of an isopod attached to a fish was an Observation of the Week!

Posted on March 27, 2024 07:01 PM by tiwane