April marine observations

We continue to have a superb time living on the beach and feeling a part of each day’s natural history. In the morning, we can tell which way the wind is blowing based on the location of the steady stream of cormorants traveling from guano platform to ocean. A million of them in a long line, they tend to fly into the wind. We have not yet determined the cues that cause them, on some days, to gather as a huge black blanket on the beach before heading out to sea. One day last week, we witnessed semelparous reproduction: A particular type of flying insect has been accumulating in ever greater numbers – Mr Klein calls them midges, and they are mosquito-sized, with no bite or sting, but a very dark lipid stain when smushed, highly attracted to light. One morning we found thousands of them stuck to the windows of Hotel California, interspersed with spiral egg cases about a cm long, surrounded in gelatinous mucus. By the end of the day, it was clear the insects had made the wrong choice for egg-laying: midges and mucus had all dried up. And we haven’t had to be nearly as vigilant about closing doors before turning on lights since then. Seasonal changes are also evident in the colors on the beach at low tide. When we arrived, the intertidal zone was red with Gracilariopsis (this identification tentative, but coincides with the monograph on Namibian algae published in the 90s). Now much of this has been reduced to perennial holdfasts, with a few female plants bumpy with carposporangia, and the intertidal zone is green with a flush of ulvoids. It’s somewhat embarrassing to admit that we have not yet determined the identity of the less than half dozen species of terrestrial plants living alongside us in the coastal desert. But, we know a bit about their natural history: Many of them accumulate mounds of wind-blown sand, where gerbils and ants then tunnel for security (and presumably food). Since our arrival here, many of these plants have been subtly flowering, more so on the down-wind (N) side. We can also see the dried remnants of annual plants that apparently completed their whole life cycle in the spring before we arrived.
We know that the tide has dropped, leaving dislodged mussels stranded on the beach, when we see the gulls flying up, dropping a large Perna, then following it to the ground to see if it has broken. The introduced clam in the salt pond, Ruditapes decussata (from the Channel Islands), was recently decimated by birds – hundreds of large shells lay broken around along the road. Alan has not been particularly impressed by gulls with such dysfunctional bills that they can’t even get into a dislodged bivalve, but Mr Klein says they’re actually quite smart: along areas of the coast with no hard substrate for dropping and cracking bivalves, they simply place Donax clams on the sand dunes until they gape from the heat and desiccation. While the gulls go for the large Perna mussels, turnstones seem to love the small Semimytilus. These have been washing up in clumps on the beach recently, probably dislodged by waves as the mussel beds have become thick, no longer attached to rock but to an accumulated layer of sand of several cm. It’s easy to see the pattern of patches within the mussel beds at low tide, and we suspect that, given the fast growth and small size of this mussel species, it would be possible to watch patch dynamics over months, rather than the years required by Paine and Levin on the Washington coast!
On the subject of mussels, I spent one illuminating afternoon looking for boring organisms in Perna. The idea came from our desire to begin testing top-down control of Polydora in the salt pond: what really caused the polychaete to essentially disappear in 2004? Was it isopods? Nemerteans? Since we have found so few Polydora in oyster shells – an infection rate of about 2% - we wondered if we could use spionids in mussel shells as a surrogate, then test to see if either isopods or nemerteans caused mortality. We know from walking along the beach that the wrack is full of bored mussels (you know, riddled with holes. You can’t be the other kind of bored when you’re dead!).
So, on a fair to middling low tide, JR walked down to the rocks by the Salt works, dressed in my normal Namibian field gear: bathing suit, wrap-around skirt, Crocs, wide-brimmed hat, long sleeves (it’s either that or a lot of sunscreen). The first thing I noticed was that the upper limit of Perna was just barely above the waves, which is not unusual given the rather small tidal amplitude, just a bit over 1 m. The second thing I noticed was that Perna at its upper limit is not bored, but instead seems to be sand-scoured except at its growing edge, sometimes to the point of having a concave outer shell surface. Finally, I found a few large mussels rolling around in a tidepool that had apparently been dislodged from lower down: they were covered with erect bryozoan epiphytes, red tufts of algae, and obviously bored. I put these in a bit of water in my bucket and carried them off hopefully to the ‘scopes at the hatchery, then spent the next 3 hours noticing a third thing: Most of the eroded burrows in Perna are full of phoronids!
For those of you not completely versed in marine biodiversity, I’ll simply state that Polydora is a polychaete annelid, a segmented worm, in a family characterized by the presence of two long palps on the head. I think I saw 2 long palps once in 15 shells, but was unable to extract any more – and in any case it may not even have been a boring spionid, but rather one that builds its tube in sediment. In contrast, phoronids are in an entirely separate invertebrate phylum. They are soft-bodied, unsegmented worms, with a horseshoe-shaped ring of tentacles on their head – this headdress made them quite unmistakable as soon as I found a shell that still contained live individuals. But then the next question: Did the phoronids make the tubes, or just occupy someone else’s burrow? Our satellite internet access at the beach came in handy once again, as I was able to search on “shell-boring phoronid” and learn that one of the 17 species of phoronids IN THE WORLD – and the smallest one, at that – makes burrows in mollusc shells. All the evidence points towards Namibian subtidal Perna perna full of Phoronis ovalis. It has been reported from a different Perna species in New Zealand, as well as from abalone in Chile. As far as we can tell from beach wrack, only one of the 4 mussel species on this coast hosts Phoronis ovalis, and we gather they are not a problem in aquaculture here. I guess that is good news for the oyster growers, but it puts another hold on our quest to discover the mystery of the missing Polydora.
Another quest we have set ourselves is a better understanding of Venerupis corrugatus, the native littleneck or steamer clam on this coast. We heard early on that this clam was ubiquitous, and indeed we’ve found it in mussel beds, intermixed with intertidal polychaetes, and washed up next to the Walvis Bay yacht club. Most impressively, we saw tiny (1-2 mm) individuals at incredible densities fouling the oyster culture gear in Walvis Bay, apparently a recent recruitment event at exactly the same time that so many oysters were dying in March! Two weeks later, the oyster gear coming out of Walvis Bay had clams around a cm long. This suggested to us that the native clam might be particularly well-adapted to survive low oxygen conditions and grow rapidly, perhaps an untapped aquaculture option! So, we tasted some “big” clams (they seem to get not much larger than 3 cm) with one of our oyster-growing friends and can now pronounce them delicious. How about a new market for Benguela clams?
Well, even though Venerupis corrugatus seems to weather much of what nature dishes out to it, it’s not very resistant to science… or perhaps to the blunders of curious scientists. We collected around 3000 of them in late March from oyster gear coming out of Walvis Bay. They probably got a little bit of initial mistreatment that was not our fault: a freshwater rinse, and a 1-hour car ride in a small tub of water. Then, we distributed the clams into 3 sand or gravel-filled trays and watched them burrow in – at least most of them. We had to leave for Windhoek soon after that, so anchored the trays in a salt pond canal… that reached nearly 30C due to a series of bright, hot days. Half of the clams died. When we returned from Windhoek, we placed the trays back in the hatchery tanks, where another 50% died over several days. Then, we anchored the trays in a different, cooler part of the salt pond canal, just where the water is pumped in from the ocean. Over the next few days, the trays silted up and sank, with another half of the clams dying. However, by this time the surviving clams averaged 15 mm, and we had found live clams throughout the canal that had recruited and grown on their own. This gave us the perfect opportunity to set up our first experiment (as opposed to simply measuring conditions in different locations): we planted out the surviving clams into PVC rings embedded in the sediment, then put bird/fish exclosures around half and cage “controls” (just 2 sides) around the others. Because greater flamingos feed on invertebrates in the sediment, and a variety of waders (curlews, stilts, sandpipers, lapwings) probe for food, we think that these cages may allow us to document their ecological impact on infaunal communities. It’s so exciting to do a flamingo exclosure! And so nice not to have to accommodate 3-meter tides (as in any tideflat exclosure experiment in Willapa Bay) – in fact, the water barely goes up and down depending on how vigorously the pumps are working at the inlet.This experiment went up about 3 weeks ago as a “pilot” to see what would happen to structures – 2 exclosures were apparently trampled by birds, perhaps invisible to them on a dark night; and 1 exclosure (so far) has been gnawed by a hyena! Perfect evidence of the importance of replication.