Posted by: Rob Viens | April 21, 2012

Coastal Lagoons and Steamed Mussels

April 21st and Darwin’s entry is again short, focusing mainly on the logistics of the trip:

“Started at day-break & proceeded for some leagues on the former road; we then turned off, being determined to reach the city by the interior line. — Our party was reduced to Mr Lennon, his nephew & myself. — We arrived in the evening, almost without having rested our horses, at the Rio Combrata: this country was much more cultivated. The Venda was beyond anything miserable, we were obliged to sleep on the Indian corn.” (Apr 21)

As noted, the party (now just 3 members) took a more inland route back to Rio, skirting along the landward side of the coastal lagoons.

Macaé excursion – April 21, 1832 (note that routes are approximations based on Darwin’s descriptions)

Map of Darwin's Expedition – April 21, 1832

Coastal lagoons are interesting places. In this case, they are isolated from the sea by barrier beaches that form along the coast (see Lagoons, Beaches, and the Eternal Motion of Sand ). Where the sand bars form an effective barrier (and there is a regular inflow of freshwater), the lagoons typically contain freshwater or possibly brackish (slightly salty) water. However, when a storm breaks through the barrier beach (or if there is not much new freshwater being added) the lagoon can be quite salty.  Since barrier beaches are temporary features, that come and go with the seasons or with big storms, coastal lagoons such as the ones near Rio can fluctuate a lot in the saltiness of the water.  (And rising sea level is another factor that can have a longer-term impact on these lagoons, too.) This change in water salinity provides a challenge for living things.

Most animals that live in water, such as clams and snails, are adapted to either freshwater or saltwater.  There are exceptions – I think of our salmon in the Pacific Northwest that spend part of their life in both environments.  But it is difficult for most species because cells have to carefully balance how salty they are.  Cells adapted to freshwater will lose water (effectively shriveling up) when placed in saltwater. And when cells used to living in saltwater are placed in freshwater, they can take in too much water and explode. So living in water that changes its salinity rapidly requires a special sort of adaptation. Some species can regulate their cell salinity regardless of the salinity of the environment (they are called osmoregulators).  Others, such as mussels, may have to close up tight during times the salinity is not to their liking.  And some, such as snails, might just have to migrate somewhere else.

In the section on the Macaé expedition in Voyage, Darwin says a few things about mollusk life (snails and bivalves) in these coastal lagoons.

“we continued to pass through an intricate wilderness of lakes; in some of which were fresh, in others salt water shells. Of the former kind, I found a Limnæa in great numbers in a lake, into which, the inhabitants assured me that the sea enters once a year, and sometimes oftener, and makes the water quite salt. I have no doubt many interesting facts, in relation to marine and fresh water animals, might be observed in this chain of lagoons, which skirt the coast of Brazil. M. Gay (Annales des Sciences Naturelles for 1833) has stated that he found in the neighbourhood of Rio, shells of the marine genera solen and mytilus, and fresh water ampullariæ, living together in brackish water.” (Voyage of the Beagle)

Lymnaea sp. (from Wikipedia Commons):

Lymnaea sp.

Limnæa (which appears to be Lymnaea) is a variety of freshwater pond snail (gastropod). Species of the Lymnaeidae family are found world-wide, however they seem to be more common in the northern hemisphere. (Humans seem to be rather interested in the Lymnaea, as they are a major carrier of parasites that can be harmful to people and livestock.) They are restricted to freshwater, so I assume they must have to migrate if the salinity of the lagoon changes. Even Darwin finds this enigma interesting, asking the question:

“Is not this fact curious, that fresh water shells should survive an inundation of salt water?” (Zoological Notebooks)

Blue mussels (from Wikipepia Commons):

Mytilus edulis

Solen is a type of razor clam belonging to the family Solenidae, and Mytilus is the name of a genus of mussels (both are saltwater species).  If you have ever ate mussels, you then you know Mytilus – maybe a tasty Mytilus californianus steamed with butter here in the Pacific Northwest, or a Mytilus edulis in a nice bouillabaisse in southern France. (RJV)

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Responses

  1. […] part seems normal enough and Darwin frequently mentions the coastal lagoons in the Rio area (see Coastal Lagoons and Steamed Mussels). However, he quickly takes an interesting philosophical turn and, for Darwin, sort of rambles […]


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