Above: Doto rosacea, displaying all the standard Dendronotina characteristics –  large rhinophoral sheaths, branching appendages arranged down the lateral sides of the notum and feeding upon hydroids.

DELVING INTO DENDRONOTINIDS

If you have been reading the NudiNotes and Book Chapters that have been posted so far on NudibranchDomain you will have seen the word dendronotinids used for a group of nudibranchs. This NudiNote serves to explain that group, and its position in the classification of the sea slugs.

The nudibranchs (order Nudibranchia of the sea slugs) are classified into two major clades: the dorids and the cladobranchs.

The term dorid is derived from Greek mythology, Doris being a sea nymph or sea goddess. The term was also associated with the fertility of the ocean. So the word was not applied to the group because of any common anatomical feature.

Above: A four pack of dorid nudibranch representatives. Clockwise from top left: The phanerobranch Roboastra luteolineata; The cryptobranch Halgerda aurantiomaculata; The cryptobranch Chromodoris lochi; The porostome Phyllidia varicosa.

The cladobranchs, however, are characterised by the possession of a branched digestive gland. The term clado from the Greek klados, meaning branch or branching was first combined to form the term cladohepatica referring to this branched digestive gland. This name was originally applied by the taxonomist Bergh (late 1800’s early 1900’s) who divided the nudibranchs into holohepatica (compact digestive gland) and cladohepatica (branched digestive gland) because he considered this character to be the most significant.

Jumping forward almost a century (and over much disputed taxonomy) the term cladobranch was coined by Willan & Morton, 1984 to encompass the grouping we use today (see below). This word, cladobranch, is making reference to the appendages or branches rising up out  of the notum as secondary gills. The second part of this word is different to the first part of the word and doesn’t mean branching as in English but rather is from the Latin branchia or Greek brankhia meaning gills.

The cladobranchs are further divided into three subgroups: Arminina, Dendronotina and Aeolidina.

Above: Representative examples of cladobranch nudibranchs. Clockwise from top left: The Arminid Armina sp. (undescribed); The aeolid Favorinus tsuruganus; The dendronotinid Mariona sp. (undescribed) of Tritoniidae family; The dendronotinid Bornella stellifer of Bornellidae family.

The members of Dendronotina, the dendronotinids, are characterised by the possession of distinct rhinophoral sheaths that rise up significantly from the notum to surround the stalk of the rhinophores, plus, a series of lateral appendages running down the length of the body on the edge of the notum. These may be highly branched and function as secondary gills. Dendro is derived from the Greek dendron meaning tree, a reference to the appearance of the branching appendages on the animals back or notum. Notum is from the Greek noton meaning the back, hence – Dendronotina.

Above: Some examples of the characteristic rhinophoral sheaths of dendronotinids. Clockwise from top left: Lomanotus sp. 7 (undescribed); Mariona sp. 6 (undescribed); Tritoniopsis elegans; Bornella anguilla. Some sheaths are plain whilst others are elaborate and ornate.

Molecular research over the past 10 years has shown that the Dendronotina are not a natural grouping (not monophyletic – see Note b) below), but the name is still currently in use by many for convenience. Most of the families are clustered in the one clade: the families Lomanotidae (e.g. Lomanotus), Hancockiidae (e.g. Hancockia), Dotidae (e.g. Doto & Kabeiro), Bornellidae (e.g. Bornella), Scyllaeidae (e.g. Crosslandia, Scyllaea & Notobryon), Tethydidae (e.g. Melibe & Tethys), and Dendronotidae (e,g. Dendronotus) with just the Tritoniidae family (e.g. Tritonia, Tritoniopsis & Marionia) being in the other.

All of the dendronotinids feed upon cnidarians (possessing stinging cells) apart from the Tethydidae (e.g. Melibe). Most feed upon Hydrozoa, what we call hydroids, but interestingly, one group, the Tritoniidae, feed on Octocorallia or what we call soft corals.

Above: Examples of dendronotinids feeding on hydroids. Clockwise from top left: Doto rosacea; Bornella anguilla; Kabeiro sp.; Lomanotus sp. 4.

The difference revealed between the two groups by molecular sequencing is therefore also reflected in their diet – a very tidy dietary confirmation indeed. In fact, the molecular sequencing by Goodheart et al, 2017 places Tritoniidae as sister to Arminidae (of Arminina one of the other three cladobranch subgroups) many of which also prey on Octocorallia. “It appears that prey preference is particularly relevant for the evolution of Tritoniidae + Arminidae as species within this group prey exclusively on Octocorallia.” (Goodheart et al, 2017).

Above: Octocorallia (soft coral) feeding by Tritoniidae dendronotinids and Arminidae arminids. Clockwise from top left: The Tritoniidae species Tritoniopsis elegans devours the flesh of a soft coral. Exposed sclerites of the soft coral can be seen discarded below the nudibranch; The Tritoniidae species Mariona distincta preying on the soft coral’s polyps; The large Tritoniidae species Mariona rubra consuming whole branches of the soft octocoral Lemnalia sp. by protruding its buccal mass to envelop and then shear off portions with its jaws; The Arminidae species Dermatobranchus ornatus stripping the flesh from the stems of the octocoral Astrogorgia sp. Notice the completely stripped stem just to its right.

Above: Two unusual dendronotinids that don’t quite “fit the mould”. Left: Embletonia gracilis does not possess rhinophoral sheaths. Right: Melibe viridis and the other members of its family, Tethydidae, do not feed on hydroids. Rather they trap small crustaceans under their expanded oral veil.

Notes:
a) Only the well known genera of the families mentioned are listed for orientation and recognition purposes.
b) Monophyletic clades are the goal of taxonomists. It means a group of organisms, including the most recent common ancestor of all those organisms and all the descendants of that most recent common ancestor, only. Such groupings are often referred to as a clade.
c) Where an sp. number is used to identify an undescribed species that number relates to species on this website.

David A. Mullins – January 2023

References:
– Willan, R. C. & Coleman, N. (1984). Nudibranchs of Australasia. Sydney: Australasian Marine Photographic Index.

– Willan, R. C. & Morton, J. (1984). Marine Molluscs. II. Opisthobranchia. Auckland: University of Auckland and Leigh Marine Laboratory.

– Miller, M. C. & Willan, R. C. (1991). Redescription of Embletonia gracile Risbec, 1928 (Nudibranchia: Embletoniidae): Relocation to Suborder Dendronotacea with taxonomic and phylogenetic implications. Journal of Molluscan Studies, 58: 1-12, Figs 1-8.

– Wägele, H. & Willan, R. C. (2000). Phylogeny of the Nudibranchia. Zoological Journal of the Linnean Society; 130, 83–181.

– Pola, M., & Gosliner, T. M. (2010). The first molecular phylogeny of cladobranchian opisthobranchs (Mollusca, Gastropoda, Nudibranchia). Molecular Phylogenetics and Evolution; 56:931–41.

– Goodheart, J. A., Bazinet, A. L., Collins, A. G. & Cummings, M. P. (2015). Relationships within Cladobranchia (Gastropoda: Nudibranchia) based on RNA-seq data: An initial investigation. Royal Society Open Science; 2:150196.

– Goodheart, J. A., Bazinet, A. L., Valdés, Á., Collins, A. G & Cummings, M. P. (2017). Prey preference follows phylogeny: evolutionary dietary patterns within the marine gastropod group Cladobranchia (Gastropoda: Heterobranchia: Nudibranchia). BMC Evolutionary Biology; 17:221

– Gosliner, T. M., Valdés, Á. & Behrens, D. W. (2018). Nudibranch and Sea Slug Identification: Indo-Pacific – 2nd Ed. New World Publications, Jacksonville, Florida.