Okenia nodosa

Okenia nodosa (Montagu, 1808)

Okenia nodosa @ Strangford Lough, Co Down, Northern Ireland. 20 Oct 2005 by Bernard Picton

Taxonomic Note
Paz-Sedano et al.(2024) conducted a comprehensive phylogenetic analysis of the family Goniodorididae, which includes both genus Goniodoris and Okenia. Their findings indicated that G. nodosa, and therefore the genus Goniodoris, should be synonymized with the older genus, i.e., Okenia. However, the morphology of the remaining species of Goniodoris matched better with Pelagella, a genus that already includes species previously assigned to Goniodoris. Thus, Paz-Sedano et al. (2024) tentatively transferred species of Goniodoris to Pelagella, except for Okenia nodosa, until more species are sequenced.

Synonyms

• Doris barvicensis Johnston, 1838
• Doris elongata Thompson W., 1840
• Doris nodosa Montagu, 1808 (original)
• Goniodoris emarginata Forbes, 1840
• Goniodoris nodosa (Montagu, 1808)

Description
O. nodosa reaches a maximum recorded body length of 27 mm (Thompson & Brown, 1984). The dorsal mantle is capable of fully covering the body, but it is often reflected or raised, exposing the body. Its surface is characterized by numerous raised mounds, each capped with a small white marking. In adults, these mounds are irregularly distributed across the mantle, whereas in juveniles (~2 mm in length), tubercles may align along the medial ridge to form three longitudinal ribs. The prominence of these mounds varies with age and reproductive condition, and in some cases, they may be absent. The external appearance of O. nodosa is largely influenced by its translucent body, which makes many of these features difficult to discern. The only visible pigmentation consists of fine white or yellow-white stippling, along with larger pigment spots. The stippling is absent posterior to the rhinophores, creating the impression of shallow depressions, while a transparent, stipple-free circular area behind the gills gives the illusion of an open pore. Yellow pigmentation, when present, is most pronounced on the oral tentacles and along a medial band on the metapodium. In juveniles (~2 mm in length), the mantle’s fine stippling is often predominantly yellow. The internal organs are visible through the translucent body, with the viscera appearing pink or yellow and, in some cases, a bluish spermatheca discernible on the right side. The rhinophores consist of a short, smooth, translucent basal stalk, and in adults, they bear thirteen to fourteen incomplete lamellae on the distal 70% of their length. These lamellae do not extend across the anterior face of the rhinophore. The rhinophoral apex is short, bluntly rounded, and projects slightly beyond the lamellae. Rhinophores are entirely translucent white or yellowish, with sparse white pigment flecks that become denser distally. The branchial plume comprises up to thirteen unipinnate, transparent, and colorless gills with thickened midribs, arranged in a circle around the cylindrical, white anal papilla. In larger specimens, three additional smaller gill plumes may be present at the posterior margin of the main circle. The head bears a ventral, yellowish mouth surrounded by broad, flattened, translucent oral tentacles, which are speckled with opaque white or yellow-white pigmentation distally. The oral veil is faintly developed. In dorsal view, the head is usually covered by the mantle unless the mantle is retracted or reflected. The foot is oblong with parallel lateral margins, a broad anterior region with rounded corners, and a posterior end that tapers to a rounded point. The ventral surface is translucent white, with the central viscera visible as a pinkish hue. Propodial tentacles are absent. Dorsally, the foot is translucent white, scattered with small, opaque white spots. The tail (metapodium) extends beyond the mantle and features a distinct yellow medial band.

Biology
O. nodosa inhabits the lower shore and sublittoral zone, occurring at depths of up to 120 m. Despite most Okenia feed on bryozoans, O. nodosa has been reported to feed on ascidians. Phylogenetic results confirm that O. nodosa does not belong to a different genus but is a rather specialized Okenia not feeding on bryozoans. Juveniles primarily feed on polyzoans, including Alcyonidium polyoum, Callopora dumerili, and Flustrellidra hispida, while adults consume various ascidians, such as Diplosoma listerianum, Botryllus schlosseri, and Dendrodoa grossularia (Thompson & Brown, 1984). As a simultaneous hermaphrodite, O. nodosa engages in reciprocal copulation. Mating typically occurs with individuals positioned head to head, aligning their genital apertures, which are located behind the right cephalic tentacles. Alternatively, pairs may mate with one individual lying prone and the other supine, maintaining genital contact. This species produces a long, cylindrical spawn mass that is either white or rosy in color. The spawn is attached to the substrate and may vary in curvature, from nearly straight to a coil of approximately one and a half turns. Each mass contains between 14,000 and 32,000 eggs. Fertilization occurs during deposition, utilizing sperm stored from previous copulation within the bursa copulatrix. In the British Isles, the primary spawning season extends from January to May, with an additional spawning period in autumn under favorable conditions that might promote rapid growth and early maturation of the spring cohort (Thompson & Brown, 1984). The shelled veliger larvae develop as plankton before settling on the substrate and undergoing metamorphosis into the benthic adult form. Adults typically die after spawning in May, rendering the species inconspicuous during summer until juvenile recruitment begins in August.

Etymology

• Okenia. In honor of Prof. Dr. Ludwig Gilbert Lorenz Oken, (1779-1851), German natural philosopher, editor of the magazine “Isis” and publisher of the book “Lehrbuch der Naturgeschichte”.
• Nodosa. From Latin, tied into many knots, full of knots, knotty.

Distribution
O. nodosa occurs from Norway and Faeroe Islands to Galicia, Spain (Trigo et al., 2018). It is common all round Britain and Ireland on hard substrates.

More pictures

Bibliography

1246567 Okenia nodosa 1 eunomia-revista-en-cultura-de-la-legalidad 50 creator asc 1 13010 https://opistobranquis.info/wp-content/plugins/zotpress/
Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.

Further reading

• AsturNatura
• Biodiversity Heritage Library
• DORIS
• Encyclopedia of Marine Life of Britain and Ireland
• Estran 22 - Faune et flore de la zone de balancement des marées en Côtes d'Armor
• European Sea Slugs by Morddyn
• Flickr pictures
• Interim Register of Marine and Nonmarine Genera
• NCBI GenBank
• OBIS - Search by Taxon
• Scottish Nudibranchs
• World Register of Marine Species

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