01 Geitodoris planata

The ample mantle covers the whole body and extends well beyond the foot. Its underside and the dorsal surface of the foot have characteristic dark freckles and blotches.

Menai Strait, Wales, July 2013.

Full SPECIES DESCRIPTION BELOW

Sets of OTHER SPECIES at: www.flickr.com/photos/56388191@N08/collections/

 

Geitodoris planata (Alder & Hancock, 1846)

 

Authors: Ian Smith (text) and Malcolm Storey (shore work and images).

Synonyms: Doris planata Alder & Hancock, 1846; Discodoris planata (Alder & Hancock, 1846);

Current taxonomy: World Register of Marine Species www.marinespecies.org/aphia.php?p=taxdetails&id=139580

Vernacular name: Millennium wratslak (Dutch)

 

GLOSSARY BELOW

 

Description

The body grows up to 65 mm long or, exceptionally, up to 120 mm in the Netherlands (van Bragt, 2004). The ample mantle covers the whole body and extends well beyond the foot fig. 01 flic.kr/p/2o4DLjT . It often assumes a low profile similar to that of a planarian worm. The wide mantle skirt is often almost horizontal fig. 02 flic.kr/p/2o4yHKJ , even when the body is humped up fig. 03 flic.kr/p/2o4CzQV . The mantle is covered with tightly packed, rounded, spiculose tubercles which give it a hard and crisp feel. It has various shades of, usually dull, yellow fig. 04 flic.kr/p/2o4CzMi , brown fig. 05 flic.kr/p/2o4DuY9 , orange-brown, pink-brown fig. 02 flic.kr/p/2o4yHKJ or purple-brown fig. 06 flic.kr/p/2o4DRWK & fig. 07 flic.kr/p/2o4CzJc . Up to about sixteen papillate, whitish, acid glands fig. 07 flic.kr/p/2o4CzJc are usually arranged subdorsally along the flanks fig. 08 flic.kr/p/2o4DuP6 and are often surrounded by a pale stellate patch fig. 09 flic.kr/p/2o4B4PS . The underside of the mantle characteristically has distinct, dark marks of reddish brown fig. 10 flic.kr/p/2o4B4Mx or purplish brown fig. 11 flic.kr/p/2o4DuE3 , but sometimes there are few fig. 12 flic.kr/p/2o4yHmC or no marks .

The rhinophores have a smooth, stout, translucent basal half speckled fig. 13 flic.kr/p/2o4DRBr or blotched fig. 06 flic.kr/p/2o4DRWK with purplish brown. The upper half has about twelve or thirteen yellowish lamellae with brown markings fig. 13 flic.kr/p/2o4DRBr . They can retract completely into pits with irregular marginal flaps which can fold over and conceal them fig. 14 flic.kr/p/2o4DRAE .

The tripinnate gills fig. 15 flic.kr/p/2o4CzkG are whitish with brown markings. Each has a stout main stem. They are often arranged into a left and a right group fig. 16 flic.kr/p/2o4DRyA . They can retract completely into the branchial pocket fig. 09 flic.kr/p/2o4B4PS which has peripheral flap-like valves fig. 16 flic.kr/p/2o4DRyA & fig. 17 flic.kr/p/2o4yHdm which can close over them.

The head has well developed linear oral tentacles flanking the mouth parts which expand greatly for feeding fig. 18 flic.kr/p/2o4Cz9Q .

The foot has a rounded, bilaminate anterior fig. 18 flic.kr/p/2o4Cz9Q ; the upper lamina is cut centrally and can extend forward of the lower lamina. The sole and dorsal surface of the foot vary from yellowish white fig. 12 flic.kr/p/2o4yHmC to dark orange fig. 11 flic.kr/p/2o4DuE3 . The dorsal surface of the foot often also has dark marks fig. 01 flic.kr/p/2o4DLjT & fig. 11 flic.kr/p/2o4DuE3 similar to those on the underside of mantle. It may occasionally protrude very slightly when in motion fig. 09 flic.kr/p/2o4B4PS , but is otherwise of much less extent than the mantle. When horizontally lit with a strong flash, internal viscera may be visible through the sole fig. 19 flic.kr/p/2o4B4su .

 

Key identification features

Geitodoris planata

1) Up to 65 mm long (exceptionally 120 mm in Netherlands).

2) Dull yellow or shade of brown, generally less colourful and variegated than Doris pseudoargus.

3) Rounded spiculose tubercles fig. 03 flic.kr/p/2o4CzQV , smaller than largest ones of D. pseudoargus, much bigger than digital caryophyllidia on Jorunna tomentosa. Animal feels hard and crisp.

4) Dark marks on underside of mantle fig. 11 flic.kr/p/2o4DuE3 . (Occasionally absent ).

5) Up to 12 whitish papillate acid glands on sides of mantle, often in stellate pale patch fig. 08 flic.kr/p/2o4DuP6 .

6) Foot much smaller than mantle fig. 01 flic.kr/p/2o4DLjT .

7) Head has well developed linear oral tentacles fig. 18 flic.kr/p/2o4Cz9Q .

8) Gills whitish with brown pigment marks; often formed into a left and a right group fig. 16 flic.kr/p/2o4DRyA .

9) Spawn a broad, undulate ribbon laid in a spiral attached to substrate by one edge. With age, colour may change white to yellow and spiral becomes looser. Ova arranged in ribbon randomly; not in discernible lines fig. 20 flic.kr/p/2o4DRgM .

 

Similar species

Doris pseudoargus fig. 21 flic.kr/p/2o4CyZw

1) Up to 120 mm long, half grown ones similar size to G. planata.

2) Mantle usually yellow or buff-orange, often with large blotches of bright colours.

3) Variously sized, rounded, spiculose tubercles give stiff unyielding feel to mantle, but G. planata even stiffer.

4) Underside of mantle lacks dark markings (may have slight staining at edge) fig. 22 flic.kr/p/2o4DRbS .

5) No whitish, papillate, acid glands on sides of mantle.

6) Foot not much smaller than mantle.

7) Head small with vestigial oral tentacles.

8) Translucent gills often tinted purple or lilac.

9) Spawn a broad, undulate ribbon laid in a spiral attached to substrate by one edge. With age, colour may change white to yellow and spiral becomes looser. Ova arranged in transverse lines fig. 23 flic.kr/p/2o4CyVo & fig. 24 flic.kr/p/2o4CyFk .

 

Jorunna tomentosa fig. 25 flic.kr/p/2o4yGv9

1) Up to 60 mm long, similar size to G. planata.

2) Mantle sandy, buff, or whitish; any other marks few, small and dark.

3) Very small digital caryophyllidia cover mantle densely; velvety appearance, soft feel.

4) Underside of mantle lacks dark markings fig. 26 flic.kr/p/2o4CywC .

5) No whitish, papillate, acid glands on sides of mantle.

6) Foot not much smaller than mantle. Extends well behind mantle when in motion.

7) Head has well developed linear oral tentacles

8) Whitish or yellow gills with no opaque pigment marks encircle brown anus. Base of extended gills surrounded by raised collar.

9) Spawn a broad, strongly crimped ribbon laid in a spiral attached to substrate by one edge. Ova arranged randomly fig. 27 flic.kr/p/2o4DQEw .

 

Habits and ecology

G. planata occurs at LWS on rocky shores and on oyster beds and sublittorally to 45 m. It is well camouflaged on stone by its dull colours and lack of shadow from its flat profile fig. 28 flic.kr/p/2o4B3GM & fig. 29 flic.kr/p/2o4DgH3 . It feeds on sponges such as Hemimycale columella, Mycale rotalis and Mycale micracanthoxea. It exudes defensive acid from whitish papillate glands on the flanks of the mantle. When disturbed, it tends to remain flattish while it contorts, rather than contracting tightly like Doris pseudoargus. Like other nudibranchs, it is a simultaneous hermaphrodite. In north-west Europe it mates and spawns from late spring to late autumn fig. 29 flic.kr/p/2o4DgH3 . The spawn mass is a broad, undulating ribbon laid in a spiral attached to the substrate by one edge. It is white or yellow, probably changing with age. The ova are arranged randomly in the ribbon; not in discernible transverse lines fig. 20 flic.kr/p/2o4DRgM .

 

Distribution and status

The range of G. planata has expanded this century and now includes Norway, Britain, Ireland, Netherlands, Brittany, Spain and New England (USA), GBIF map www.gbif.org/species/2292467 . A population explosion, including very large specimens, around the year 2000 was observed in the Oosterschelde, Netherlands (van Bragt, 2001, 2004 & 2013). Its timing led to its Dutch name, "Millennium wratslak". By 2005 numbers had declined, but it is still present in the Oosterschelde.

Finds have also increased in Britain and it is found widely on rocky coasts, but it may still be scarce in south-east England, UK map NBN species.nbnatlas.org/species/NBNSYS0000175442

 

Acknowledgements

For use of images I thank Jim Anderson, Anne Bignall, Rokus Groeneveld www.diverosa.com , John de Jong www.jojodive.nl , James Lynott, Sheilah Openshaw, Allan Rowat, Malcolm Storey www.bioimages.org.uk and Stefan Verheyen.

 

References and links

Alder J. & Hancock A. 1846. Notices of some new and rare British species of naked Mollusca. Annals and Magazine of Natural History 18: 289-294, pl. 4

www.biodiversitylibrary.org/page/2319713#page/312/mode/1up

 

Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society. Family 1 Plate 8 www.biodiversitylibrary.org/item/131598#page/118/mode/1up

 

Rudman, W.B. Geitodoris planata in The Sea Slug Forum www.seaslugforum.net/find/geitplan (accessed December 2022)

 

Thompson, T.E. & Brown, G.H. 1984. Biology of opisthobranch molluscs 2. London, Ray Society.

 

van Bragt, P.H. 2001. Geitodoris planata - population explosion in Holland, on the Sea slug Forum www.seaslugforum.net/message/5125

 

van Bragt, P.H., 2004. The sea slugs, Sacoglossa and Nudibranchia

(Gastropoda, Opisthobranchia), of the Netherlands. Vita Malacologica, 2: 3-32, plates 1- 10.

 

van Bragt, P.H., 2013, Millennium-wratslak, Geitodoris planata (Alder & Hancock, 1846), Soorten, Stichting Anemoon, Netherlands. Accessed 4 November 2022

www.anemoon.org/flora-en-fauna/soorteninformatie/soorten/...

 

Glossary

caryophyllidia = digital tubercles with terminal knob surrounded by coronet of spikes, like a clove (spice), on some dorid sea slugs.

 

cephalic = (adj.) of or on the head.

dorid = a sea slug in the infraorder Doridoidei; usually with gills and rhinophores on the dorsum; often shaped like half a lemon or grape.

 

lamellae = small plates on rhinophores, or leaflets of gill.(sing. lamella)

mantle = (of sea slugs) sheet of tissue forming part or all of notum.

notum = dorsal body surface.

papilla = (pl. papillae) small cone-shaped protrusion of flesh.

papillate = shaped like a papilla or bearing papillae.

plankton = animals and plants that drift in pelagic zone (main body of water).

rhinophores (pl.) = chemo-receptor tentacles on top of head of nudibranch.

tripinnate = (of gill plume) threefold branching; “boughs, branches, twigs” in one plane like a feather.

 

spicule (in dorid seaslugs) = small, slender, sharp-pointed feature mainly composed of calcite (CaCO3) and brucite (Mg(OH)2) .

 

stellate = arranged in a radiating pattern like that of a star.

 

veliger = shelled larva of marine gastropod or bivalve mollusc which swims by beating cilia of a velum (bilobed flap).

 

 

Done