Science in restricted territory

Dec 11-15
Sometimes, when people find out that I’m a scientist, they ask: “So, what have you discovered?” Usually, I’m at somewhat of a loss to answer this question. It’s not easy to compress the fine details of interaction strengths in a food web into a comment that is both intelligible and compelling. Or I sound vague and undecided in stating – truthfully – that one species can have both positive and negative effects on another, depending on time, place, and what is being measured. Now, however, there’s a clear answer for the “discovery” question: Alan and I discovered a new species, which has never been reported from Africa, but which has strong negative effects on bivalves. Furthermore, there is good evidence that the species is not a new introduction, but simply something that no one bothered to record previously. This ease of discovery is the silver lining of sabbatical in a country where you can count the marine biologists on one hand.

The species we discovered is a phoronid worm that burrows into shell. Phoronids are unsegmented worms, currently included in the same phylum as brachiopods, with which they share a horseshoe-shaped ring of tentacles for filter-feeding. Phoronis ovalis is the only phoronid reported to burrow in shell, and the other dozen or so species of phoronids make free-standing tubes. Very little published literature is available on Phoronis ovalis, but by piecing together papers and websites, it appears to have a temperate distribution that includes Scandinavia, the Mediterranean, Pacific Northwest, New Zealand, east and west coasts of South America, and Japan. It has not been reported anywhere in Africa, including South Africa (where its absence may be genuine, since there is close scientific attention to marine species). But it’s definitely in Namibia. Here are some things we’ve learned about it (let’s call it Phoronis sp., since we can’t be sure it’s P. ovalis): The ring of tentacles is less than 200 microns across, and the total worm length is around 2 mm. The burrows come to the surface at densities of more than 100 per square cm. Shells that are seriously infested with phoronids become porous but thicker, because the host has to repair and re-apply shell on the inside. This takes so much energy that the host itself loses condition! Phoronids are not too picky about host species, as long as they are essentially always under water: we have found phoronid holes in Perna perna (brown mussel, our main study species), Choromytilus (native blue-black mussel), Mytilus galloprovincialis (introduced blue-black mussel), limpets, whelks, and even a barnacle. How could this species have gone unnoticed for so long? More than half the mussel shells on the beach are riddled with these holes! Maybe the answer is best summed up by the comment after our public talk at the Swakopmund aquarium in late November: The only mussel we’re really interested in is the “white mussel”, which scientists call a clam, and then only because we use it for fishing bait.

Well, to quote Dr Seuss again, “If you want to find beasts you don’t see everyday/ You have to go places quite out of the way./ You have to go places no others can get to./ You have to get cold and you have to get wet too.” Ever since we arrived in Namibia, we’ve wanted to see the full range of intertidal sites, but we’ve been limited to the area around Swakopmund because only this 200-km stretch of coast is publicly accessible. To the south, the coast is well protected by the great sand sea, and off limits to all but diamond miners. To the north, most of the Skeleton Coast Park is closed to public access. However, our discovery of Phoronis sp. in Namibia gave us a logical reason to face the administrative hoops and request a permit for beach access in the Skeleton Coast Park. (We have, alas, run out of time for southern sampling, but we have contacts in a few key coastal towns who may be able to report on the presence or absence of phoronid holes there.) After a few false leads, we finally had an in-person meeting with the Director of Parks and Wildlife Management, where we presented our case in a 2-page letter and – Eureka! – walked out with a Skeleton Coast Park permit! We were on our way into restricted territory with no roads, no people, but full of stories and legends to fuel our excitement!

The Skeleton Coast gathered its name and fame from a book by that name, written in the 1950s about events during WWII: a passenger ship ran aground at a point even farther north than our planned sampling, and rescues were attempted by land, sea, and air. Planes crashed, tugs grounded, trucks floundered in sand, but ultimately all but two of hundreds of people involved in the accident and rescue survived. By sea, the Namibian coast is nearly straight, with little in the way of protected harbors, and waves on shore that are large even in calm seas. The Namib desert is essentially devoid of freshwater, a forbidding barrier by land. The desert is breached at intervals by rivers, but these are generally dry, running to the coast only when inland rains are extreme, which happens every couple of decades. Mindful of the difficulties of negotiating such an area, we traveled with Mr Klein, our host at the Salt Company, who knew the coast by air and land from many years of prospecting and mining; we also traveled with one of our students from the Polytechnic, Uapindi Kazahe, who knows people wherever he goes. From Swakopmund (22.6 degrees S), we traveled north about 160 km to a campsite at Mile 108, then a further 200 km to Mowe Bay, where we camped for two nights at the end of the road. From there, on a day trip, we all got into Mr Klein’s Toyota Fortuner, which can ford rivers of sand, climb mountains, and stick to seaside cobble. We followed a track or the beach to our northern sampling site at Rocky Point (19 degrees S). On the return trip to the south, we camped at Torra Bay, totally outclassed by the summer holiday fishers who set up colorful castles of tents, windscreens, and caravans, with all the comforts of home powered by generators. We crossed the dry river mouths (in order from the south) of the Ugab, formed by Paleozoic glaciers; Huab; Koigab; the five mouths of the Uniab, including its current channel harboring reeds, gemsbok, and springbok; Hoanib, with a flamingoed lagoon bordered by a thick layer of snail egg cases; and avoided the quicksand that sometimes marks the mouth of the Hoarusib. Mr Klein had been stopped for weeks by these rivers in other years, so we were thankful for just minimal amounts of rain. (Actually, the weather was remarkably cooperative, particularly the absence of strong southwesterlies that can blow away campers, and rain just as we were packing to leave Mowe Bay).

What does a beach look like when human impacts are really low? The sand was littered with wood and whalebones. Vertebrae like coffee tables. Two-story ribs. The tough part of the back of the skull. A midden of charcoal and mussel shells, along with a ring of rocks, still was visible where native people had homesteaded perhaps a century ago, living in a hut of skins draped on whale ribs. We saw bits of old wooden fishing boats, the crashed airplane, and the top of the tug, swept by waves, along with several other wrecks. We saw the detritus of old mining operations, cleared areas where people had camped, piles of sand and stone, and rusty equipment for sorting beach gravel by size and density. Mr Klein said that Namibia’s large diamond mining company, Namdeb, sold its mining rights quite some time ago, which should have been an indication that prospecting on the Skeleton Coast was fruitless, but nevertheless some mining still goes on.

Most of the Skeleton Coast, and in fact Namibia’s coast in general, is sandy beach. Any species requiring hard substrate is restricted to a few outcrops (although what happens below low tide is something of a mystery: there must be rocks that we can’t see in some places, as indicated by kelp blades at the surface of the water). At the rocky sites we visited along the Skeleton Coast, the limpets and snails reached absolutely enormous sizes – 10 cm diameter, as big as a hand! Some familiar species disappeared: lobster molts and clam shells were not on the beach after Mile 108. New species amazed us: Ghost crabs popped out of their sand burrows by the hundreds and scattered across the beach. 1-cm moon snails with a comma umbilicus showed up on a few beaches and in jewelry for sale at the park entrance, but the comma necklace snail distribution is supposed to end at the southern tip of the continent. To the north, mussels became both smaller and less abundant, and some of the larger individuals were actually drilled – something we’d only seen in small clams and mussels in the Swakopmund area. Of course, we can’t know for certain how much of the variation in intertidal community structure that we witnessed on our trip was due to latitude, and how much to human access. The rock type varied as well: Rocky Point is made of rare red volcanic rock that only reaches the coast in one place. Other sites we sampled were a conglomerate, often eroded into steep intertidal mesas. At still others, layers of rock were turned on end. Most of the sites had been previously sampled by Spanish researchers, and the algal results reported in a monograph by Rull Lluch. But Alan’s extensive searching with Google Earth revealed one more rocky site, north of Terrace Bay in the restricted park area. It was particularly limpet-y, sort of like the bottom of a pink-red non-slip shoe, with lots of grippy bumps. The red and pink came from the apparently unpalatable algae, the bumps from the shells of limpets. Only where the cobble had recently receded could any green be seen on the rocks, more clear evidence of the importance of herbivores in regulating community structure, because the algae colonize these rocks faster than the limpets.

So what’s the story on phoronids? As we sampled north from Swakopmund, the prevalence first increased in mussels, and then as large mussels became less common on the beach, the frequency of phoronid holes also decreased. By the time we reached Rocky Point, we didn’t find any mussels with phoronid holes, although they showed up in a few snail shells. When we discussed our trip with Dr Bronwen Currie at NATMIRC in Swakopmund, she produced some large Perna shells that she had brought back from a rocky site near the Kunene River, the border with Angola: 1 of 10 had phoronid holes! And the marine biologists in South Africa are now sending photographs of snails with small holes in the shell: these may be phoronids, too! So it seems we haven’t quite finalized the entire range of the species, but we’ve found a 200-km stretch of coast where they are particularly common. We also found them buried in layers under about 3 m of silt, sand, and vegetation, now being eroded by waves at the base of a cliff – surely these must be quite old! But we are always warned that a 1-year snapshot of life on the beach is deceiving. That sand could have been deposited just a short while ago during a massive east wind blow. Like any proper “discovery”, ours still has a few mysteries remaining.