MERMAID'S PURSES
by Jim Hall

with additions by Andy Horton

Regular rockpoolers are likely to have come across 'Mermaid's Purses', containing the eggs or young of the Lesser-spotted Dogfish, Scyliorhinus canicula, lying amongst the debris on the tideline. These egg capsules that have been dislodged after being laid by the adult female dogfish are doomed to perish. Some, if not most, of the capsules are empty. However, on many occasions I have found a live embryo inside, some of them nearly ready to break free from the capsule.
 

Empty Dogfish egg case washed up on Shoreham Beach, Sussex

Early Attempts

My early attempts at hatching these embryos were largely unsuccessful*. On one early occasion, the young dogfish seemed unable to free itself, so I cut off the end of the purse and released the fish. Unfortunately, after reeling around helplessly for five days, the young fish eventually perished.  I did not know why this happened, but one problem was to get the fish to feed itself orally after many months living on the egg yolk.

Further Successes

Eight weeks later, another small dogfish was born, very pink at first like a baby child, then it darkened slightly after three days. This one was quiet like the previous success, and when it was transferred to my main 50 gallon (227 litre) tank and it reached 8 cm long in two months and thrived.

Young Dogfish just released from the purse. It is helpless at first and just lies around on the floor of the aquarium.

During early August 1993, I travelled to the Scilly Isles and when I returned I found one of the nine developing egg capsules in my tank empty. I had assume the dogfish had managed to extract itself from the purse and had been attacked. The newly hatched dogfish are very soft and vulnerable for a day or two.
 

Early Expulsions

I decided to transfer the remaining eight purses to a separate tank set aside for the purpose of rearing young fry. I also decided to expel one of the embryos a little early to see if it would survive. The baby dogfish lay quietly for a couple of hours, and then moved around normally with a tiny egg yolk still attached. The fish was healthy, so encouraged by this success, I released a further two dogfish that were 'very close' and they lived as well.

 These fish did not look for food, as they were still feeding from their yolk, but I felt that they would harden up sooner and be more used to their surroundings by the time they were ready to attempt oral feeding.

Feeding

The other six purses were all within two weeks of expected birth and over the following weeks I took the risk of expelling them all early.  They were all bombarded with newly hatched brine shrimp and small mysids and they all survived and after five weeks were transferred to the main tank with the bigger fish. In the large tank the ten hatchlings all thrived, with the first born reaching 18 cm, the second 13 cm and the remaining eight at 10 cm by September 1994.

Tips

The egg yolk when viewed through the purse needs to be reduced to the size of a 'match head' before expulsion is attempted. If the yolk sac is any larger than this, death is probable because the egg yolk will almost certainly rupture on the aquarium gravel. The yolk sac needs to be small enough to allow the baby dogfish to swim around comfortably. When the dogfish emerges it is always sluggish and will most likely to lie on the bottom looking decidedly ill. After a few hours it is likely to perk up.
 The egg capsule is about 6 cm long by 2 cm wide. The baby dogfish are about 8 cm at birth.

Egg Capsules

The female dogfish will lay about 10 eggs per month during the breeding season from November to July. They are laid about two at a time in areas of quite strong currents with the capsules carefully attached by long elastic threads to seaweeds or rocks.
It is possible to find 'Mermaid's Purse's that have come adrift just after laying and to keep the purses in the aquarium through the whole of their development. It begins as a tiny yolk in the purse and after about one month a tiny dogfish can be distinguished. After about seven months the dogfish has almost absorbed the yolk and completely fills the purse.

Raising the Dogfish

From February to October 1994, I expelled eleven embryos of which nine survived to 13 cm, and seven of these were distributed to various aquaria. In my six foot (2 metre) tank I have two specimens that reached 20 cm by November 1995.
 The dogfish will eat all the normal food of mussel and white fish consumed by the other fish in an aquarium without crabs or sea anemones.

 In late 1994 and 1995, 'Mermaid's Purses' were not so easily found on the south Wales coast with only three brought home from the strandline, with minute embryos just visible. Most of the purses were found on Swansea beach after a period of bad weather.  However, this beach is now cleaned by specially adapted tractors which clear the debris on the strandline.

Biological Notes

The Dogfish is a common small shark in the shallow waters all around the British Isles, although not so prevalent off the east Scottish coast. It is also found in more southerly seas and throughout the Mediterranean. It lives on or near the bottom and its back is a sandy brown colour with small dark brown spots. Adults reach about 65 cm in length.
 

Lesser Spotted Dogfish

 Its natural food consists of small crustaceans especially hermit crabs off Sussex, but it will eat worms and molluscs.

# The larger egg cases of the Greater Spotted Dogfish, Scyliorhinus stellaris, and the cases of the Thornback Ray Raja clavata, Undulate Ray Raja undulata, the Skate, Raja batis, or any of the rarer true skates and rays of the family Rajidae could conceivably be discovered. When laid they are a translucent olive-brown and the developing embryo can be seen. If they are washed up for a long time on the strandline they turn black. The dogfish purposes should have at least some of the thin tendrils that were originally used to secure the purse. However, the broader skate's purses have a long curved spike at each of its four corners.

At the beginning of 1996, a week before the Sea Empress oil spill on 15 February, I was lucky enough to obtain two purses of the Greater Spotted Dogfish from Mike Batt (Tenby Aquarium) who found them washed up on the shore at Lydstep, near Tenby in south Wales. At the same time young newly hatched fish were also found on the shore. These two purses remain alive in my aquarium and are expected to hatch around Christmas.

 I was also able to obtain an egg case of the Greater Spotted Dogfish from Weston-super-Mare Sea Life Centre. This purse was 12 cm long by 4 cm wide, excluding the tendrils, and the dogfish successfully hatched from the purse in October at a size of about 13 cm and is thriving on diet of mysids and white fish.

 The Greater Spotted Dogfish is also known as the Bull Huss and the Nursehound.

* the Dogfish would squirm around inside the purse, but was unable to break free.

Egg Capsules of Rays & Sharks

 
6 June 2009

Egg Cases of the Lesser Spotted Dogfish (top) and Undulate Ray

One Undulate Ray eggcase looked in good condition from Shoreham Beach, Sussex, and was placed in an aquarium in case the embryo was still alive.
 

12 March 2008 This large eggcase was discovered on my local beach (Waterville, Co. Kerry, Ireland).. The size can be discerned from the tennis ball. It is large enough to be the eggcase of the endangered Skate, Dipturus (=Raja) batis,  Report by Rosemary Hill

21 January 2008
 

The usual debris was on the Shoreham Beach strandline including a Ray's, Raja sp., eggcase illustrated above.
It measured 67 mm long and this was probably the eggcase of an Undulate Ray, Raja undulata. It width was 48 mm.
The horns were 40 mm and 45 mm long.

Adur Coastal 2008
 
 

21 December 2006 Egg Cases on Constantine Bay beach, north Cornwall Left:      Ray Right:    Dogfish Photograph by Amanda Bertuchi

16-17 January 2006
Twenty four egg cases of the endangered Skate, Dipturus (=Raja) batis, were  discovered on the Sandside shore near the Dounreay nuclear power plant, Caithness, west of Thurso and Scrabster Harbour and John o'Groats on the northernmost coast of mainland Scotland, the first records reported to the Shark Trust and the first records on the mainland Scotland since these egg case occurrences have been recorded.
 

The egg cases were estimated at between 23 to 28 cm long and 13 to 16 cm wide in a dried state.
 

Egg Capsules of Rays & Sharks (Link to the Web Pages)
BMLSS Mermaid's Purses
January 2005 Report
Egg Capsules of Rays & Sharks (Link to the Web Pages)

January 2005
 

Skate Egg Capsules from the Orkney Isles Photograph by Richard Land These large egg case washed up on the shores of the Orkney Isles, north of mainland Scotland. The large size of these egg cases means they are almost certainly* the cases of the endangered# Skate, Dipturus (=Raja) batis. The width of the purse on the right seems to be about 127 mm.  Over a hundred egg cases were washed up.

These egg cases are washed up every year. The local school children collected twenty in 2004.  The photograph shows the much smaller ray egg cases for comparison.  Notes: The lengths (wet when discovered) varied between 270 - 300 mm in length with a width of between 140 mm and 160 mm.  These egg cases lacked the external horns present on ray egg cases and these ones appear to be internal. Divers report the egg cases resting on the bottom at between 7 and 20 metres in depth over sand or sand and gravel bottoms. These are scallop grounds. When lifted the egg cases falls to the bottom and stays there. Large adult Common Skate are observed by divers on these grounds, but no juvenile skates have been spotted.  There is a problem inamuch that these egg cases do not match a known photographed egg case of the Common Skate. This is being investigated. It is possible that the original photograph is incorrect and these "hornless" egg cases are correct for Common Skate?   Or, more likely, the horns have broken off?   Information provided by Richard Land

*Not thought to be either the egg cases of the Norwegian (or Black)  Skate, Dipturus nidarosiensis, or the Long-nosed Skate, Dipturus oxyrinchus. # The Common Skate is now absent from most of the seas around the British Isles. There is a small population caught off Mull (SW Scotland, islands) but the Orkneys remain a stronghold for an unimportant commercial catch of mixed Skate (Common and Black Skate).

Egg Capsules of Rays & Sharks (Link to the Web Pages)

Egg Case Identity Sheets (Shark Trust)
 

Raja undulata egg case (by Andy Horton)
Identification by Peter Bor (Egg Capsules of Rays & Sharks)
 
 
 
 
 
 
 
 

Species	Length     mm	Width     mm	Horns
Raja undulata	72-90	42-52
Thornback Raja clavata	50-90	34-68.5
Common Skate Dipturus batis	106-245	50-145
Lesser Spotted Dogfish Scyliorhinus canicula	49-70	15-30	N/A
Blonde Ray Raja brachyura	100-143	58-90
Nursehound Scyliorhinus stellaris	100-130	35-45	N/A

Reference (Link)
 
 

On 25 March 2005, a damaged ray eggcase from the coast at Shoreham measured 40* mm long, 34 mm wide (dry size) with one "horn" measuring 50 mm at the longest attachment point or 40 mm when measured to the length of the purse. These small purses are often washed up on the Sussex coast.  It looks like a small specimen of Raja undulata, the Undulate Ray eggcase which are sometimes, 80 x 50 mm. The Thornback Ray egg purses, Raja clavata, seem to have shorter horns, but this needs confirmation. The size only matches the Thornback though. (*Damage meant it could possibly be longer.)  There is a possibility of a smaller Spotted Ray, Raja montagui, eggcase?

* NB:  When the egg cases are placed in water they expand in size (John Knight). It is these larger sizes that are measured as they correspond to the size when they are submerged.

What does an eggcase look like? (Shark Trust link)

Procedure for ID (Bucket Science)

Original report on: http://www.thisiscornwall.co.uk
More information on:
Cornish Wildlife
Vince Smith's One-List/Cornish Wildlife
Send a message to the list at: CornishWildlife@onelist.com

Regards,

Catriona Hassey
Dingle Oceanworld
Co. Kerry
Ireland
 

Dogfish hatched (image)
 
 

EMail for the BMLSS

Dogfish (by Len Nevell)
Egg Capsules of Rays & Sharks

Fish Page
BMLSS Shark Page
International Marine News