Dale Fort Blog Number 11

27 08 2012

This blog contains some interesting stuff about limpets.

It’s concerned mainly with the common limpet Patella vulgata

 

Classification:

PHYLUM: Mollusca     CLASS: Gastropoda   Subclass: Prosobranchia

Family: Patellidae    Genus: Patella  Species: vulgata

Limpets are among the commonest and well known of intertidal creatures. They are found all over Europe from The Mediterranean to Western Norway.

Feeding:

They are grazing animals, feeding on a variety of organisms like red green and brown algae (though they exhibit a preference for green) and lichens.   They scrape these up from the rocks using a rasp-like tongue called a radula.

The Radula is made of chitin and is impregnated with iron oxides to make it harder.  It wears as it scrapes and a new bit is pushed out.  The Radula can be 11 times longer than the body of the limpet (it coils up inside).  Limpets graze when the tide is in or at night or when it’s damp and cool or when it’s raining.   They have to walk over the top of their food to eat it so they only feed on seaweeds when the weeds are very small (usually not noticeable to us humans).  You can often see zig-zag scrape marks on the rocks where they have been feeding, if you have good hearing you can actually hear them scratching.

 The mature large seaweeds we see on rocky shores are therefore the ones that were lucky enough to escape being grazed by limpets.  An oil spill or similar thing that results in the death of lots of limpets can have a profound effect on the shore.  Usually you would see a “green flush” of green algae, doing very well without the attentions of limpet grazers.  Limpets have a big effect on what the shore looks like.

Predators:

As members of the plankton they presumably get eaten by other plankton and filter feeders.  They are not in the plankton for very long (see Reproduction and Dispersal) and once they settle are available to both seashore predators when the tide is high and terrestrial predators when the tide is low.  I suspect they are easy prey for things like shore crabs when very small and there are records of dogwhelks attacking them too.  Settled baby limpets are probably eaten by fish like the common blenny as well.  Once they get bigger with a thicker shell, they are less vulnerable to the creatures just mentioned.  They are however eaten by gulls and oyster catchers.  When I had the privilege of living in a small room overlooking the shore at Dale Fort I observed oyster catchers sliding their bill under the edge of the victim’s shell, turning their head and flipping the limpet over.  They would then slide their bill under the soft parts inside, detach them from the shell and swallow.  Herring gulls had a less subtle approach.  They would club the shell with their bill until it broke and then eat the contents.  Some birds carry the limpet to solid ground before eating, I found limpet remains on the parapet at Dale Fort only yesterday, left there (presumably) by a bird.

Behaviour:

Limpets can move considerable distances on their grazing missions 1 to 2 metres in a few hours is not uncommon.  Many limpets walk off grazing and then return home to the spot on the rock they started from.  This behaviour is called homing.  It is not known how they do this.  Some say they leave a slime trail which they follow back (how does it persist under water?) Some say they have a knowledge of the micro-topography of their bit of rock stored somehow in their simple molluscy brains (more neural swellings really), some say they use satellite navigation.  Whatever it is, a lot of them do it.  Some do not, nobody knows why; but you can say that if it homes it can be pretty sure of finding an unoccupied space when it has finished grazing.  Animals also scrape their own bit and grow to fit it exactly and the bare bit of rock is known as their “home scar”.  If you fit your bit of rock exactly you can probably hold on tighter and lose water less readily when the tide is out.  You can tell which way around the animal is without pulling it off the rock (you’ll probably kill it if you do this) because the head is directly underneath the apex (pointy top) of the shell.  Interesting investigations can be done on the orientation of limpets on seashores.  For instance, do they all face the same way?  Do they face into or away from the waves?  Is there a difference in orientation between shores of different exposure?  Is there a difference in orientation between different shore heights?  And so on and so on.

Limpets live from the upper part of the sub-littoral zone up to the upper shore.  They are well protected against wave action possessing a thick conical shell and they can hold on as tightly as limpet.  They adhere more tightly than they would by air pressure alone.  There is a chemical bonding effect as well, produced by a

reversible reaction that converts the mucous on their foot from a lubricant into a sort of natural super-glue.  This is achieved very rapidly by enzymes that join chains of muco-polysaccharides together.

Respiration:

They also have a cunning ruse that allows them to respire on the upper parts of shores when the tide is low.  The mantle cavity (between the body and the shell) houses the gills (ctenidia).   When the animal is emersed the gills do not collapse but remain supported by water trapped in the mantle cavity, thereby providing a large surface area for gas exchange.  The animal can loosen off its grip on the rock and raise its shell just enough to allow gases to diffuse in and out but not so much as to let the water escape.

 

Shell shape:

Limpet’s shells vary in shape from shore to shore and from part of shore to different part of shore.  A common observation (at least in text books) is that limpets of exposed shores are pointier than limpets of sheltered shores.  The explanation given for this (by J.H. Orton in the Journal of the Marine Biological Association of the UK in 1929) is that in exposed conditions the limpets spend more of their time with their foot muscle contracted.  The mantle (which secretes the shell) surrounds the foot and is  therefore pulled downwards and inwards a lot.  The animal therefore grows up tall and thin instead of short and squat.

 

Some biologists scoff at this and one of the most knowledgeable limpet-men of all (Bill Ballantine) reckons limpets are simply fatter and grow faster on sheltered shores because they have more food available and longer feeding time because of the generally more favourable abiotic environment.  Limpets do very well on exposed shores and usually there are more of them per unit area than on sheltered shores, so maybe intraspecific competition could play a part here as well?  Also there are more barnacles on exposed shores, so maybe there is less room to grow outwards (interspecific competition).

Reproduction and dispersal:

Limpets reproduce in autumn (in West Wales).  Like humans, bouncing up and down gets them sexually excited.  (In limpets it’s the stormy Autumnal waves that do it not other limpets).  Unlike many humans, lower temperatures also jazz them up.  The males eject cream coloured sperm and the females eject green eggs into the water.  Fertilization is external.  Fertilized eggs turn into trocophore larvae which live in the plankton for a few days.  When the larvae are about 0.2mm long they settle out on the shore and grow into limpets.  When they settle they have no sex (neuter), after a while they all turn into chaps and then after about two years about 40% turn into girls.  A typical population would be about 60% males, 20% females and 20% either immature or spent (knackered).  This is called protandric hermaphrodity and is quite common in the zany, crazy world of the invertebrates.  P. vulgata can live for a long time, older individuals might 15 to 20 years of age.

There are 3 species in UK waters: Patella vulgata  the common limpet (which has a greyish coloured foot)(see above), Patella  ulyssiponensis (formally P. aspera) (which has a yellowish coloured foot) and Patella depressa  (which has a black foot).  If you look on the middle shore and avoid rockpools they’ll mostly be P. vulgata
 

Watch out for the next blog where we may return to historical matters

 





Dale Fort Blog Number 10

7 08 2012

 

Imagine a lump of matter with a mass so huge that its gravitational pull distorts the very fabric of space-time.  If you could find this warp in space (or wormhole), you might use it to defy relativity and visit unfeasibly  distant parts of the cosmos

   Adventurous readers may be disappointed to find The Dale Fort Wormhole Project slightly more prosaic.  The wormholes we investigated were not massive warps in space, nor were they the burrows excavated by worms themselves.  We were interested in the holes dug in muddy shores by people seeking bait for fishing.

People have been concerned about the impact of bait-digging on the soft-sediment shores of Milford Haven for at least 40 years.  The Countryside Council for Wales decided that an investigation might be appropriate and they employed Dale Fort Field Centre to carry it out.

  The aims of the study were to find out:

Which species were being collected ?

Where they were being collected from ?

What was the intensity of collection?

Did collecting vary seasonally?

Who was doing the collecting ?

How they were doing it ?

How abundant were the bait species ?

Where did the local bait shops obtain their stocks ?

What Species are Collected and from Where?

   Ragworms and lugworms are the two main species dug for in Milford Haven.  Ragworms are by far the most popular.

Easily the most important site for ragworms is The Gann followed in a distant second place by Angle Bay.  There is evidence of small scale digging at Sandy Haven, Gelliswick, Pembroke Dock and Pennar and a miniscule amount at Lawrenny and Llangwm.

Lugworms are harvested on a much smaller scale than ragworms and at fewer sites.  The main lugworm digging site was at Angle Bay, followed by Sandy Haven.  A few holes were found at The Gann, Gelliswick and Pennar.  Anecdotal information from bait-diggers suggested that sites like Saundersfoot and Pembrey were better for lugworms and more frequently dug for this species.  The only site where there was evidence of collection of razor clams was on the Dale village side of The Gann Flats.  Crabs were collected only from Sandy Haven and Dale.

How Intense is the Collecting and is it Seasonal?

We used a Garmin GPS (Global Positioning System) to map precisely the locations of bait-holes.  At the same time, holes were aged on a scale of 1 to 4, based on the nature of the sediment and the structure of the hole.  This work concentrated on the main sites: The Gann and Angle Bay.  We mapped and aged a total of 35,536 holes.  26,615 of these holes were in The Gann.

 

 

 

 

 

 

There were so many holes in The Gann that we divided the area into 50x50m grid squares and mapped the density of holes (as in the diagram above).  A typical grid square towards the central, most intensively dug area might easily contain over 500 holes.

We used the hole-age data to determine the proportion of holes of different ages to shed some light on the seasonal intensity of digging.  This data seemed to indicate intensive digging towards the end of the winter period, followed by a reduction in digging intensity until autumn.

Who is Doing the Collecting and How do They Do It?

Dale and beaches on the north side of the Haven are mainly visited by diggers from Milford.  We know of only one fulltime professional digger and he says he digs up to 2000 worms a day from The Gann.  Angle is visited by people from the Pembroke/Tenby area.  The less productive sites such as Gelliswick and Pembroke dock/ Pennar are only visited by people who live locally.  The summer period finds occasional holidaying anglers digging on the flats.  For the most part holiday makers probably purchase bait from shops rather than dig it for themselves.  The survey team met only two holidaying anglers over the whole period of field work.

Ragworms and lugworms are usually dug with a fork.  Peeler crabs are collected by turning over stones and boulders.  Razor clams are collected by skilled and fast use of a fork which may be helped by encouraging the animals to the surface by sprinkling salt onto the burrow holes.

How Abundant are the Bait Species?

   The highest density of ragworms was found at Sandy Haven Pill. This site also possessed the worms of lowest average weight of all 9 sites investigated.  The second highest ragworm densities were found at The Gann.  The mean weight of ragworms at The Gann was much higher than at Sandy Haven.  The animals sold by bait farmers seem to weigh 5-6g each. This is similar to the mean weight at The Gann.  It would therefore seem reasonable to suggest that this is the size anglers most desire.  The high density and high mean weights of ragworms found at The Gann, together with ease of access account for its huge popularity among worm digging enthusiasts.

Where Do the Local Bait Shops Obtain Their Stocks?

   We surveyed all the bait shops in West Wales and discovered:

Each shop sells an estimated average of 1265 worms per week

50% of shops sell ½ farmed and ½ locally dug worms

The number of locally dug worms sold per week is 7274 or 48.5 Kg

(= 378248 per year or 2522Kg per year)

Conclusions

If The Gann was populated by fluffy pink seaweed-bunnies that bait collectors impaled on hooks to lure bunny-guzzling dolphins to their doom, there would be much public concern for both the bunnies and the dolphins.  It’s hard to elicit a similar degree of sympathy for worms.  But The Gann is a Site of Special Scientific Interest (SSSI).  It’s (still) an extremely diverse sediment shore.  This diversity depends hugely upon its variety of sediments, some fine, some coarse and some in between.  As well as worms, it’s of vital importance for wading and other birds; it’s home to many interesting seaweeds and molluscs; and it’s got over 26,000 large holes in it.  The effect of all this digging is to mix and homogenise the sediments.  This is bad news for species that need specific conditions (E.g. Coarse sediment).

Species that can cope with these changes can do really well.  One such adaptable species is the ragworm.  It can feed on almost anything in a number of different ways.  It can actively prey on other creatures, it can feed directly on the sediment, it can spin a mucous net and filter food from the water and it can tolerate a variety of physical conditions.  Studies carried out at The Gann in 1960 and 1992 indicate a decline in species richness, coupled with a massive increase in the abundance of ragworms.  You might say that the area has been transformed inadvertently into a giant ragworm farm.

Return here for Blog Number 11 which will probably be another episode in the history of Dale Fort