Fiona pinnata, common name Fiona, is a species of small pelagic nudibranch (sea slug), a marine gastropod mollusk in the superfamily Fionoidea. This nudibranch species lives worldwide on floating objects on seas, and feeds mainly on barnacles, specifically goose barnacles in the genus Lepas.
The anatomy of this species is very unusual. It is currently the only named member of the genus Fiona but a 2016 study showed that this species is a species complex. The family Fionidae was expanded in 2016 to include Tergipedidae, Eubranchidae and Calmidae as a result of a molecular phylogenetics study.Cella, K; Carmona Barnosi, L.; Ekimova, I; Chichvarkhin, A; Schepetov, D; Gosliner, T. M. (2016). A radical solution: The phylogeny of the nudibranch family Fionidae. PLoS ONE. 11(12): e0167800. Features that are characteristic of the genus Fiona includeWillan R. C. (1979) "New Zealand locality records for the aeolid nudibranch Fiona pinnata (Eschscholtz)". Tane 25: . PDF the similar-looking oral tentacles and rhinophores; the cerata with a membrane and lacking a cnidosac; a dorsal anal opening; a reproductive system with two genital openings; two jaws with a cutting-edge, and a radula with only one central denticle in each row of teeth. That one denticle has a central cusp and a few surrounding cusps.
Distribution Fiona pinnata is found in all seas worldwide, on many different kinds of floating objects.Fiona pinnata (Eschscholtz, 1831). Sea Slug Forum. Retrieved 17 December 2009.
The type locality is the island location of Sitka, Alaska (Baranof Island), on the extreme northwestern coast of North America.
Taxonomy A 2016 study showed that this species is a species complex, but did not name the segregate species.Trickey, J. S., Thiel, M. & Waters, J., 2016. Transoceanic dispersal and cryptic diversity in a cosmopolitan rafting nudibranch. Invertebrate Systematics 30(3):290. DOI: 10.1071/IS15052
Various names have been created for this species. Limax by Peter Forsskål from 1775 was preoccupied by Johan Ernst Gunnerus in 1770. Alder and Hancock's 1851 name Oithona was preoccupied by a Cyclopoid genus from W. Baird in 1843, so in 1855 they chose instead Fiona from a character in Ossian. Harold John Finlay proposed a new genus Dolicheolis for one of those synonyms Eolidia longicauda in 1927.Finlay H. J. (23 December 1926). "A Further Commentary on New Zealand Molluscan Systematics". Transactions and Proceedings of the Royal Society of New Zealand 57(1927): 320-485.
Description The body is elongated and oblong-elliptical. The length of the body is usually about 20 mm, but the largest reported size of the body is 50 mm.Fiona pinnata (Eschscholtz, 1831). Malacolog Version 4.1.1. A Database of Western Atlantic Marine Mollusca. Retrieved 17 December 2009. Dimensions of a specimen with a total length of 31.7 mm are as follows: 17.7 mm is the body to the tip of cerata, the length of the foot is 14.4 mm, the tail at the end of the foot is 14 mm. The colour of the head and body ranges from white to brown or purple depending on its food.Behrens D. W. (last change: 7 August 2002) Fiona pinnata Slugsite. Retrieved 17 December 2009. The foot is long and lanceolate, rounded in front and produced into a fine point behind. The margin of the foot is thin, fringed and crumpled, except near the head, where it is simple. It is divided in front, but not produced into propodial tentacles.
The cerata are numerous, elongated, with a membranous fringe on the inner sides. Cerata may seem to be without apparent order but they are set in oblique rows containing from four to six cerata. There are also small cerata near the margins of the body. Cerata on the sides of the back are dark brown, each margined with white. The cerata have no cnidosacs. They are particularly compressed towards the base.
Fiona pinnata has no eyes.
The rhinophores are simple and resemble the oral tentacles. They are distant, subulate, tapering and they project outward. They are not retractile, and are without pockets.
The oral tentacles are shorter, thickened at the base, tapering, projecting laterally and horizontally and curved backward.MacFarland, F . M . (8 April 1966). "Studies of opisthobranchiate mollusks of the Pacific Coast of North America". Memoirs of the California Academy of Sciences 6: 354-358. 546 pp. Plate 68, figures 23-28. Plate 70, figures 11-12. The mouth is situated on the inferior surface of the head. The mouth is small and the external lip is divided behind on the median line.
The anus is between the cerata on the right side of the body, and its opening is directing dorsally. The genital opening is separate.
Joshua Alder and Albany Hancock (1851) described the tissues of Fiona pinnata as being very tough and firm.
Digestive system
Digestive system: The channel leading from mouth to the buccal mass is very short and constricted; and, just before it opens into the buccal mass, it receives on either side below, a very slender duct from a large, much folliculated, salivary gland. These glands lie beneath the stomach and extend almost halfway down the body. That on the right side is considerably less than the other, and is somewhat tubular, — distinctly so towards its termination; the one on the left side is much complicated in form, being irregularly and extensively sacculated. The position of these glands is unusual, but there are also other species like Doto fragilis, that open into the channel of the mouth in advance of the buccal mass.
The buccal mass is small, rather long, slender, and irregularly elliptical. There are two corneous plates or jaws at the sides of the buccal mass. It is slightly prolonged behind for the reception of the posterior portion of the radula, and there are muscles are arranged around. Muscles are from dorsal view extensively developed, forming a dense mass, the fibres passing transversely and have their extremities inserted into the dorsal margins of the jaws. These muscles assist in the motion of the jaws. Muscles for moving the whole buccal mass forward are composed of flattened and isolated bands with their extremities attached to the posterior margin of the jaws and to the muscles forming the walls of the channel of the mouth.
Fiona pinnata has two corneous jaws (mandibles), with a denticulate cutting-edge. The posterior portion is flattened. The corneous plates are little short of the size of the buccal mass, and much elongated, well arched and ovate. (When they are entirely isolated, they strongly resemble the shape the valves of a small bivalve of the genus Mytilus). They are smooth, glossy, and of a brownish amber colour, darkest towards the anterior extremity, which gives support to the cutting blade. This is a winglike appendage of no great size, terminating below in a free point, and having the cutting margin arched forward, plain, and nearly at right angles to the general direction of the plate. Above is a small process or fulcrum — the point at which the two plates are articulated. Immediately behind this point there is the dorsal margin of the plates is reflected and expanded into an arched lobe for muscular attachment. The length of the jaw is 2.8 mm. The maximum width and maximum height of the jaw is 1.3 mm.
The radula is supported on a fleshy ridge that rises up from the floor of the buccal cavity, and extends in the antero-posterior direction from the oesophagus towards the anterior opening. The radula is long, linear, and strap-formed, and is composed of semicircular and crescent-shaped denticles (tiny teeth) of an orange colour. There are 40 rows of teeth in radula: the 15 oldest denticles are in the anterior end, with 22 denticles after the angle and three incomplete denticles in the sheath in the posterior end of the radula. The radula formula is 0+1+0, which means that there is only a single central denticle in each row. There is a pointed spine in the centre, and 6 or 6-7 smaller spines on each side of the denticle. There are also sometimes minute spines at the base of denticle's outer margin. All the spines are a little bent, and have their points directed backwards towards the oesophageal opening. The whole length of the radula is 2.6 mm.
The oesophagus is a short and rather slender tube. It leads from the upper part of the buccal mass towards, and opens into, the anterior margin of a distinct pyriform stomach. The stomach has the broad end forward, is placed above the reproductive system, and lies quite in the anterior portion of the visceral cavity. The internal surface of the stomach is not lamellated. The intestine leads from posterior end of the stomach, and is inclining slightly to the right side and passes backwards to the tubular anus. The anus is placed a little to the right of the median line of the back, immediately behind the heart. The intestinal tube is rather short, of equal diameter throughout, and internally plicated longitudinally.
The hepatic apparatus is very peculiar in this animal. The caudal end of the stomach receives two biliary ducts, one on each side of the intestine. These ducts or hepatic canals are nearly as wide as the intestine, and they are diverging as they leave the stomach, very shortly pass into the skin at the sides of the back, where each opens into a wide channel that extends nearly the whole length of the body. The channels receive numerous branches, which communicate with the glands of the cerata, and as they approach the lateral expansion at the side of the body. These channels are subdivided several times and are irregularly disposed. The anterior portions of the great hepatic channels are connected with two folliculated glandular bodies much and irregularly sacculated. These glands are united to the skin, one on each side near the region of the stomach, and probably form the inner walls of those portions of the channels. Hepatic canals are almost entirely within the skin. The hepatic glands are large, nearly filling the cerata. They are slightly and irregularly sacculated, with the inner surface of the investing membrane lined with a dark granular substance; above, this substance is very abundant, forming a dense mass; below, the membrane in some of the cerata is entirely devoid of it.
The arrangement of the hepatic canals differs from that which prevails in the Eolidida. In Eolis, Embletonia, Doto, Dendronotus, Lomonotus, and Antiopa, the principal canals lie free in the visceral cavity, and in all of them there is a median posterior trunk. In these respects Oithona would appear to resemble Hermaa, in which the whole of the hepatic ramifications are apparently connected with the skin, and there are only two principal trunks, which pass down the sides of the back. It is evident, however, that the digestive system alone sufficiently distinguishes Oithona from all the above genera, not even excepting Hermaa. In the central part of the caudal end of the body, behind the ovary, there is likewise a glandular substance, of a reddish colour, folliculated and apparently branched, in connexion with the branches of the hepatic canals within the skin. These branches at the posterior portion of the body probably form a sort of network of tubes across the dorsal aspect. Such perhaps may be inferred from the appearance the branches present when the skin of the back is divided down the median line. See also MacFarland (1966, page 357).
Reproductive system
Reproductive system: There are two genital openings on the right side of the head behind the oral tentacle: the opening for the penis and for the (hermaphroditic) genital pore. The reproductive system and the mucous gland is the same as in the genus Aeolidia. The only difference is that there is a distinct vas deferens in Fiona pinnata.