Back to the Blackwater

I’ve done virtually no open water swimming so far this year. I did a quick dip in Sandycove a few weeks ago which I wrote about in My first sea swim of the year… but nothing else since. I’ve been trying to concentrate on my fitness, etc. in the pool and worry about the open water when it [eventually] warms up enough to be able to do proper swims. For the last few years, Dave Mulcahy and I have been aiming to be back in the Blackwater for about St. Patrick’s Day. Though it has been a cold March so far, Dave and I decided that we’d better get in before we put it off for another month!

Photograph – Owen O'Keefe

First river swim of the year in the Blackwater on St. Patrick’s Day.

So just before the parade on Sunday, we headed down to the Rowing Club where we got changed. It was to be my first time to use the new slipway, which is not quite finished but suitable for use. Dave had already had the privilege of using the new slipway, and was among the first to do so, during the Fermoy Christmas Day Swim last December. It was a dry, bright morning but a cool one at the same time. There was a slight westerly breeze, which was a nice change from the chilling north-easterly which we’d had for the last week or so. Another nice change was being able to walk all of the way into the water, a much easier entry than we previously had!

My first few strokes felt good, even if I found it a little hard to take a breath. I decided that it would be better to lift my head up for a few strokes and then go back to normal swimming again. After a minute or two, my hands and feet became quite sore, as did my face. I felt like getting out but then thought of my ice swimming friends and other, colder swims that I’d managed before and decided to keep calm and keep swimming. After about 200 m, which took about 6 minutes swimming against the current, I decided that I’d done enough and decided to head back to the slipway at my ease.

Photograph – Owen O'Keefe

Dave on the new slipway at the Rowing Club on St. Patrick’s Day 2013.

Dave turned at the same time as me and we arrived back at the slipway having swum for about 10 minutes. Dave and I estimated the water temperature at about 6ºC so I was pleased enough to have done 10 minutes at that temperature. Hopefully, it will start warming up soon and we will be able to do longer swims!

As Nuala Moore often reminds me, the swim isn’t over until you are dry, dressed and regaining body heat. On this occasion, I managed to get dressed reasonably quickly (this was a great achievement for me) and my feet, which were very painful on exiting the water, starting to warm up pretty quickly. Things didn’t go so well for Dave, however, who discovered that his pants were locked into the Rowing Club! He then had to endure the walk of shame back to his car and drive home to ask Brigid, his wife, to collect them from the clubhouse after the parade. He won’t make that mistake again…

It was an enjoyable swim, all in all, and it feels good to have finally been in the river. It can only get warmer from here and, hopefully, I’ll start to see the results of my winter “quality rather than quantity” training in the pool.

Freshwater Pearl Mussel

If you read Donal Buckley‘s blog, you might notice a lot of physical and biomechanical references. If you read Karen Throsby‘s blog, you’ll notice a lot of sociological analysis of open water and English Channel swimmers in particular. As you go on reading my blog, you might find out more about the zoological and ecological side of swimming. This post is actually an essay that I wrote on one species of freshwater pearl mussel that I’ve become familiar with from swimming in the River Blackwater, though it’s usually empty shells that I come across. I’m sure that most will find this a bit boring but others might find it interesting so here it is anyway (in time, I will get around to writing about some of the more “exciting” species that swimmers encounter):

Margaritifera margaritifera (Linnaeus, 1758)


The phylum Mollusca is among the most diverse of the animal phyla in terms of morphology (Passamaneck et al., 2004). Molluscs exhibit a great variety of body forms, but there is one external feature that distinguishes molluscs from other members of the animal kingdom, i.e. the mantle. The mantle encloses many of the vital organs of molluscan organisms and is specialised for a variety of purposes in the various mollusc groups, e.g. cephalopods such as squid use their mantle for jet propulsion (Pörtner, 2002), but the mantle of most molluscs is used for the secretion of a hard, calcareous shell.

Sharma et al. (2012) described the class Bivalvia as the second largest class of the phylum Mollusca. The bivalves are characterised by a laterally compressed body enclosed by two hinged valves (shells) and the lack of the radula-like tongue common in other molluscs (Sharma et al., 2012). This group includes clams, oysters, scallops and mussels and is distributed over a global range.

The subclass Palaeoheterodonta is distinct from other bivalves in that both valves are of the same size and shape and the hinge teeth are arranged in a single row. There are two orders within the paleoheterodonts: Trigonioida (marine) and Unionoida (freshwater). A common family among the unionoids is the family Margaritiferidae. Byrne et al. (2009) found that the most widespread margaritiferid species found in Irish rivers is Margaritifera margaritifera, commonly referred to as the freshwater pearl mussel or “river oyster” in certain areas.

English: Freshwater pearl mussel (Margaritifer...

Freshwater pearl mussels (Margaritifera margaritifera) in the Navarån Västernorrland river in Sweden.

Biology: Form & Function

The external morphology of the adult freshwater pearl mussel is similar to that of most other freshwater bivalves. The entire body is enclosed between two kidney-shaped brown or black coloured valves (Degerman et al., 2009). The body is laterally compressed and the posterior end is elongated. The radial depression on the surface of the shell is called the sulcus. Mature specimens can measure 12 cm to 15 cm in length (Skinner et al., 2003) but Degerman et al. (2009) reported that a specimen measuring 17.6 cm was recorded in the county of Jämtland in Sweden.

The valves articulate at a dorsal hinge, which is secured by a ligament. Just ventral to the hinge is the umbo; this is the oldest and thickest part of the valve and growth rings can be seen radiating from it (Nyström et al., 1995). A cross section of a shell reveals three discrete layers; the periostracum, the prismatic layer and the nacreous layer, which lines the internal surface of the shell and is better known as mother-of-pearl (Jacob et al., 2008). The shell is secreted by the epithelial cells of the mantle (Jacob et al., 2008) and serves to protect the mussel’s delicate ctenidia (gills) and other soft body parts from predators and debris moving in the water.

All of the mussel’s soft body parts are located between the two shells and enveloped by the mantle. The mantle’s edge has three distinct pallial lobes; the outer lobe, which secretes the shell, the sensory lobe and the inner lobe, which is muscular. At the posterior end, the inner pallial lobes of the left and right mantles separate to form the incurrent (inhalant) and excurrent (exhalant) siphons. The excurrent siphon is dorsal to the incurrent siphon. Water and food particles are inhaled into the mantle cavity via the incurrent siphon and water and wastes expelled from the mantle cavity via the excurrent siphon (Degerman et al., 2009).

Inside the mantle cavity, a pair of large, feather-shaped ctenidia can be seen extending posteriorly from the visceral mass. The ctenidia of M. margaritifera are specialised for two purposes. The primary function of the ctenidia is to provide a surface for gaseous exchange (Carroll & Catapane, 2007). The respiratory surface area is maximised by a large number of ctenidial plates extending from a central ctenidial axis. The flow of water over the ctenidia is maintained by the movement of cilia on the surface of the ctenidial plates (Carroll & Catapane, 2007). Water flows from the ventral incurrent chamber to the dorsal excurrent chamber, counter to the flow of the blood, thus maximising gaseous exchange. The cilia and grooves on the surface of the ctenidia also play a role in the selection and sorting of food particles from inhaled water (Ward et al., 2003).

Margaritifera margaritifera, like all bivalves, has an open circulatory system. The heart has three chambers (two auricles and one ventricle) and is enclosed by a pericardium. The haemolymph (blood) leaves the heart via a posterior aorta and an anterior aorta. The oxygen-carrying glycoprotein in the haemolymph of M. margaritifera (and most other molluscs) is haemocyanin, and unlike haemoglobin in the erythrocytes of other animals, haemocyanin is suspended freely in the haemolymph (Streit et al., 2005). Gaseous exchange occurs across the ctenidial plates of the gills.

As is the case with other filter-feeding bivalves, M. margaritifera has a complete, though relatively simple, digestive tract. Food particles collected on the gills move proximally towards the mouth, where they are further sorted by the labial palps, before entering the mouth (Ward & Shumway, 2004). M. margaritifera lacks the radula-like tongue characteristic of most molluscs. The mouth and oesophagus are ciliated and serve merely to direct food particles to the stomach (Helm et al., 2004). Mechanical digestion in the stomach is accomplished by a crystalline style, projecting into from an associated sac. This sac is ciliated and the rhythmic motion of the cilia causes the style to rotate, grinding food particles against the gastric shield located on the wall of the stomach (Helm et al., 2004). These food particles then pass to the sorting caecum and enter the digestive diverticula for further digestion. Following digestion, waste products are egested into the excurrent chamber via the anus and are expelled into the water via the excurrent siphon.

The three largest muscle masses of M. margaritifera are the two adductor muscles and the foot. The adductor muscles’ function is to close the shell of the mussel and/or to hold it closed (Itoh et al., 2007). The anterior adductor muscle is located near the mouth, while the posterior adductor muscle is located just posterior to the visceral mass on the dorsal side. The foot of M. margaritifera is recognisable as a large muscular mass projecting anteriorly from the ventral surface of the visceral mass. Its function is to allow the mussel to burrow into the substrate. It achieves this by first extending longitudinally into the substrate, then expanding transversely to form an anchor and finally contracting longitudinally, pulling the entire animal into the substrate (Germann et al., 2011).

Life History & Ecology

Reproductive circle of the Freshwater Pearl Mussle

Reproductive cycle of the freshwater pearl mussel.

Margaritifera margaritifera is widely known for its longevity. Bauer (1987) found that the main maximum life expectancy of European specimens was 93 ± 9 years. However, it is thought that, given the right conditions, they can live for more than 200 years. One specimen, believed to be as much as 280 years old was found in Sweden (Degerman et al., 2009). The reason for this longevity becomes clear when one considers the reproductive strategy of the mussel.

Margaritifera margaritifera is normally dioecious (Moorkens, 1999) but does not exhibit any external sexual dimorphism (Bauer, 1987). Given certain environmental cues, the male mussel releases sperm aggregates into the water and they are then carried by the water current to the female (Degerman et al., 2009). The fertilised eggs develop on the gills of the female for period of a few weeks before being released into the water as larvae known as glochidia (Skinner et al., 2003). A female can produce several million glochidia each time she reproduces, which can amount to as many as 200 million glochidia if she lives out a normal life span (Bauer, 1987).

Glochidia resemble adult mussels but their shells are open as they drift, suspended in the water column (Skinner et al., 2003). The glochidia are parasitic and must encyst on the gill filaments of a juvenile salmonid fish, e.g. Salmo salar or Salmo trutta, if they are to survive (Skinner et al., 2003). Having survived this stage, the glochidia release from the gills of their host and drop to the riverbed, where they burrow into the substrate and develop for a further 5 years (Moorkens, 1999) before returning to the surface. Mussels reach maturity after about 20 years and once they reach this stage, there is a high probability that they will survive into old age (Bauer, 1987). Bauer (1987) also noted that there is no post-reproductive stage in the life cycle of M. margaritifera, meaning that adults can continue to reproduce into old age.

Margaritifera margaritifera inhabits riverine systems across Eurasia and North America, where it can be found partially buried in the substrate (Skinner et al., 2003). Degerman et al. (2009) described, in detail, the habitat requirements of this species. They point out that populations of M. margaritifera are only known in fast-flowing, clean rivers with stable substrata and where there is a healthy stock of host fish. Skinner et al. (2003) noted that the ideal substratum for this species is coarse sand or fine gravel. They also note that this substratum must be stable and that there should be little or no silt in the interstitial spaces. Shallow water near riffles is favourable as it is more oxygenated and there is less sedimentation.

Hastie & Young (2003) cited Ziuganov et al. (1994) in describing the relationship between M. margaritifera and salmonid fishes as mutually beneficial: the salmonids receive clean water and spawning beds in return for hosting the parasitic glochidia of the mussels. While juvenile mussels may be at risk of predation by fish, adult mussels generally have no natural predators (Degerman et al., 2009).

Margaritifera margaritifera is listed as critically endangered worldwide (IUCN, 2012) and critically endangered in Ireland (Byrne et al., 2009). The widespread and direct exploitation of M. margaritifera for its nacreous pearls and the overfishing of its host fishes have had a long-term effect on the species. More recently, the pollution of rivers and the construction of man-made dams has led to the destruction of much of the mussel’s habitat (Watters, 1995). Extensive conservation projects are underway in Ireland, the UK and across Europe (Skinner et al., 2003).

English: Stamp from Deutsche Post AG from 2002...

Freshwater pearl mussel on a German stamp in 2002.


  • Bauer, G., 1987. Reproductive strategy of the freshwater pearl mussel Margaritifera margaritifera. Journal of Animal Ecology, 56, 691-704.
  • Byrne, A., Moorkens, E.A., Anderson, R., Killeen, I.J. & Regan, E.C., 2009. Ireland Red List No. 2 – Non-Marine Molluscs. National Parks and Wildlife Service, Department of the Environment, Heritage and Local Government, Dublin, Ireland.
  • Carroll, M.A., Catapane, E.J., 2007. The nervous system control of lateral ciliary activity of the gill of the bivalve mollusc, Crassostrea virginica. Comparative Biochemistry and Physiology, 148A, 445-450.
  • Degerman, E., Alexanderson, S., Bergengren, J., Henrikson, L., Johansson, B.E., Larsen, B.M., Söderberg, H., 2009. Restoration of freshwater pearl mussel streams. WWF Sweden, Solna.
  • Germann, D.P, Schatz, W., Hotz, P.E., 2011. Artificial Bivalves – The Biomimetics of Underwater Burrowing. Procedia Computer Science, 7, 169-172.
  • Hastie, L.C., Young, M.R., 2003. Conservation of the Freshwater Pearl Mussel: 2. Relationship with Salmonids. Conserving Natura 2000 Rivers, Conservation Techniques Series No. 3, English Nature, Peterborough.
  • Helm, H.H., Bourne, N., Lovatelli, A., 2004. Hatchery culture of bivalves. A practical manual. FAO Fisheries Technical Paper, No. 471.
  • Itoh, H., Ishii, Y., Watari, T., Tokuda, N., Tsuchiya, T., 2007. Recordings of shell movements and tension changes in vivo in adductor of bivalve. Comparative Biochemistry and Physiology, 148B, 351-352.
  • Jacob, D.E., Soldati, A.L., Wirth, R., Huth, J., Wehrmeister, U., Hofmeister, W., 2008. Nanostructure, composition and mechanisms of bivalve shell growth. Geochimicha et Cosmochimicha Acta, 72, 5,401-5,415.
  • Moorkens, E.A., 1999. Conservation Management of the Freshwater Pearl Mussel Margaritifera margaritifera. Part 1: Biology of the species and its present situation in Ireland. Irish Wildlife Manuals, No. 8.
  • Nyström, J., Lindh, U., Dunca, E., Mutvei, H., 1995. A study of M. margaritifera shells from the River Pauliströmsån, S. Sweden. Nuclear Instruments and Methods in Physics Research, 104B, 612-618.
  • Passamaneck, Y., Schander, C., Halanych, K., 2004. Investigation of molluscan phylogeny using large-subunit and small-subunit nuclear rRNA sequences. Molecular Phylogenetics and Evolution, 32, 25-38.
  • Pörtner, H.O., 2002. Environmental and functional limits to muscular exercise and body size in marine invertebrate athletes. Comparative Biochemistry and Physiology, 133A, 303-321.
  • Sharma, P., González, V., Kawauchi G., Andrade, S., Guzmán, A., Collins, T., Glover, E., Harper, E., Healy, J., Mikkelsen, P., Taylor, J., Bieler, R., Giribet, G., 2012. Phylogenetic analysis of four nuclear protein-encoding genes largely corroborates the traditional classification of Bivalvia (Mollusca). Molecular Phylogenetics and Evolution, 65, 64-74.
  • Skinner, A., Young, M., Hastie, L., 2003. Ecology of the Freshwater Pearl Mussel. Conserving Natura 2000 Rivers, Ecology Series No. 2, English Nature, Peterborough.
  • Streit, K., Jackson, D., Degnan, B., Lieb, B., 2005. Developmental expression of two Haliotis asinina hemocyanin isoforms. Differentiation, 73, 341-349.
  • Ward, J.E., Levinton, J.S., Shumway, S.E., 2003. Influence of diet on pre-ingestive particle processing in bivalves: I: Transport velocities on the ctenidium. Journal of Experimental Marine Biology and Ecology, 293, 129-149.
  • Ward, J.E., Shumway, S.E., 2004. Separating the grain from the chaff: particle selection in suspension- and deposit-feeding bivalves. Journal of Experimental Marine Biology and Ecology, 300, 83-130.
  • Watters, G.T., 1995. Small dams as barriers to freshwater mussels (Bivalvia, Unionoida) and their hosts. Biological Conservation, 75, 79-85.
  • Ziuganov, V., Zotin, A., Nezlin, L., Tretiakov, V., 1994. The Freshwater Pearl Mussels and Their Relationships with Salmonid Fish. VNIRO, Russian Federal Research Institute of Fisheries and Oceanography, Moscow.

River Blackwater explored on “Abhainn” tonight…

At 19:30 tonight, the second episode of Series Four of “Abhainn” will be broadcast on RTÉ One. This programme will follow the course of the River Blackwater in Munster from its source in the Mullaghareirk Mountains on the Cork/Kerry border to where it enters the sea at Youghal Bay on the Cork/Waterford border. Swimming in the Blackwater in Fermoy will feature about halfway through. Here is the trailer for the series:

The programme will be broadcast in Irish but there will be English subtitles. Cláracha Gaeilge are doing a great job with these series about Ireland’s rivers. Past episodes can be found on RTÉ Player and the one about the An Laoi (the River Lee) is well worth the watch of you get a chance.

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Provisional date set for the Martin Duggan Memorial Swim 2013…

Image – Wikipedia

Crosóg Bríde | St. Brigid’s Cross

First of all, a happy Lá Fhéile Bríde – Saint Brigid’s Day – to everyone! Today, 1st February, is the feast day of St. Brigid and, traditionally, the first day of the Irish spring. The winter is pretty much over in Ireland by this day and this is what the festival originally celebrates, though it has been Christianised over the last millennium. There are some interesting traditions around the festival, including Féile na Brídeoige, but by far the most widely practiced is the making of the Crosóg Bríde or St. Brigid’s Cross from rushes. Irish schoolchildren are quite skilled in making these crosses but it’s a skill soon forgotten!

The Martin Duggan Memorial Swim is the only swimming event on the Blackwater. An annual swim has been held in the Blackwater at Fermoy since 1999. In 2009, the event was named the “Martin Duggan Memorial Swim” in memory of Martin Duggan Jnr., a fourteen-year-old boy who drowned in the Blackwater on 12th July 2007. This is a 2 km freshwater event and is open to all swimmers over the age of 12 years who have competitive membership of governing body-affiliated clubs. Wetsuits are permitted.

Logo – Owen O'Keefe

Friday, 14th June 2013 – Fermoy Rowing Club, Ashe Quay, Fermoy, Co. Cork

This year’s swim will be held on the evening of Friday, 14th June at Fermoy Rowing Club on Ashe Quay. There are a lot of works, including the construction of two new slipways, underway on the Quay but the event organisers are hopeful that these will be completed in time for the swim. More details are to come on this soon…

Entries will be taken through the event’s website and will be open well in advance of the swim. To keep up to date, keep an eye on the event’s website as well as it’s Facebook page and on Twitter @SwimBlackwater. All contact details for the event organisers are listed on the event’s website also.

Tá leagan Gaelainne de shuíomh idirlín na h-eachtra ar fáil chomh maith agus beidh gach píosa nuachta fén eachtra eisithe as Gaelainn agus as Béarla araon.

Another Big Swim for 2013

In July and August of last year, I did three long swims down the River Blackwater: 18.6 km from Fermoy to Ballyduff, 15.0 km from Ballyduff to Cappoquin and 26.4 km from Cappoquin to Youghal. In late August or early September of this year, I hope to swim each of these in one go, i.e. to swim from Fermoy Rowing Club to Front Strand, Youghal. Given similar river and tidal conditions to last year’s swims, I should complete this 60 km swim in about 12 hours. There is a question, however…

The second half of this swim is quite straightforward – one can swim unimpeded from the tidal limit (just below Lismore) to the sea at Youghal. However, the first half of the swim is not so straightforward – at both Fermoy and Clondulane, there are weirs which must be crossed, and at various other points there are rapids where a swimmer might have to stand up and walk for a while. This raises two issues if the swim is to be ratified:

  1. Can a swimmer walk across weirs and rapids without the swim being declared invalid as a “marathon” swim? This is a complicated question because all of the established marathon swims have their own rules. Before I attempt this swim, I will have to come up with a set of solid rules that outline how a swimmer can cross these obstacles without the swim becoming invalid.
  2. How should the swim be supported (in terms of safety and feeding) and how should it be observed? Only kayaks/canoes would be suitable for the first part, while only a decent sized boat would be suitable for the second half. How can the observe carry out their duties properly from a kayak/canoe and how can they transfer to the boat at the half-way point? Would two observers be better?
Photograph – Maeve Mulcahy

Getting ready to slide down the western end of Clondulane Weir!

I’ve already had some good feedback from Donal Buckley, Conor Power, Niek Kloots, and Steven Munatones on these issues. If you have any ideas or opinions on the above questions please do get in contact with me – it would be very much appreciated!

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