Matt J. Nimbs

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The Aplysiidae is a small family of  sea slugs that consume marine plants and are commonly known as ‘sea hares’. Populations can boom when their food sources are abundant and they may play an important role in the control of algal growth.

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At present, there are 11 genera in the Aplysiidae but there is likely to be some synonymy among them.

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Some species may represent a cryptic species complex but are presently recognised as mere colour morphs (Yonow 2000; 2012) such as the widely distributed, circumtropical species Aplysia parvula Mörch, 1863. Conversely, the pelagic Stylocheilus longicauda (Quoy & Gaimard, 1824) and benthic Stylocheilus striatus (Quoy & Gaimard, 1832) may represent different colour morphs of the same species which appears to vary according to geographic location or ecological niche.

There are 23 nominal species of sea hare in Australian waters, however the enumeration of the aplysiids in this region suffers considerable insecurity through a history of uncritical taxonomic descriptions. For example, the original description of Aplysia sydneyensis G. B. Sowerby I, 1869 was based solely on its vestigial internal shell, whereas other widely-distributed taxa were described using specimens sourced from a single location. This makes identification of specimens collected from elsewhere, where regional variation may exist, extremely difficult. The putative east coast endemics Aplysia denisoni E. A. Smith, 1884 and Aplysia cronullae Eales, 1960 have not been re-described since their original description and are likely to be synonymous with Aplysia extraordinaria (Allan, 1932) (R. C. Willan, pers. comm.). These tenuous early descriptions mean that there is scope for taxonomic rationalisation, particularly with the advantage of modern molecular analysis.

Further taxonomic uncertainty stems from an assumed circumglobal distribution for some taxa. Only two species, Aplysia dactylomela Rang, 1828 sensu lato and A. parvula sensu lato, have been genetically analysed so far and both were both found to comprise separate species in the Atlantic and Indo-Pacific Oceans (Alexander and Valdés 2013; Gosliner, Valdés, and Behrens 2015, p. 101). Furthermore, sea hares exhibit high ontogenetic variation; there is wide-ranging intraspecific variation in some species (particularly body colour); and, some taxa exhibit high intraspecific plasticity in body shape.

The only published key to the Australian sea hares was produced by Bebbington (1977). However, it was developed solely using preserved animals collected by T. E. Thompson in 1968-1969 and during its development, the author never examined a living specimen. As a result, the key is likely to lack important identifying features that would have been more apparent in life.

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Aims

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The aims of this study are to: (a) photograph, in situ, and collect specimens of various taxa at various stages of ontogenic development (this may vary in response to the requirements of each research question as below); (b) compare the internal and external morphology among and within taxa at various ages; and, (c) analyse and compare molecular data for each species to resolve their respective taxonomic questions; (d) use both morphological and molecular data to determine the phylogeny of the Australian Aplysiidae; (e) analyse the phylogeny of the family globally; and (f) develop an updated interactive, electronic key to the global Aplysiidae.

Changes to the taxonomy and phylogeny of the Aplysiidae resulting from this study may have important implications for the conservation status of some species, and for further research into aplysiid biochemistry and neurological systems. Further taxonomic questions may also arise.

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Research questions

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The bulk of the project will comprise taxonomic analyses based on the following research questions:

1. Are Aplysia extraordinaria (Allan, 1932) from New South Wales and Aplysia gigantea G. B. Sowerby I, 1869 from Western Australia conspecific? (This component of the proposal has already been awarded a research grant from the Malacological Society of Australasia).

2. Are Aplysia sydneyensis G. B. Sowerby I, 1869, Aplysia sowerbyi Pilsbry, 1895, Aplysia oculifera A. Adams & Reeve, 1850, Aplysia cronullae Eales, 1960 and Aplysia extraordinaria conspecific?

3. On the taxonomy of Stylocheilus, Bursatella, Barnardaclesia and Notarchus:

   1. Are Stylocheilus longicauda and Stylocheilus striatus conspecific?

   2. Is Stylocheilus striatus merely a juvenile stage of Bursatella leachii de Blainville, 1817?

   3. Are Bursatella, Stylocheilus, Barnardaclesia and Notarchus congeneric?

   4. Is the recently discovered (http://www.seaslugforum.net/find/19160) black, Bursatella sp. from Western Australia a distinct species or a form of Bursatella leachii?

4. Do the tropical and temperate colour forms of Aplysia parvula represent two distinct species?

5. Does Petalifera petalifera (Rang, 1828) contain four subspecies as described by Baba (1959)?

6. Are the two undescribed Australian Phyllaplysia spp. on Sea Slug Forum (Rudman, 2000), three species on Nudibranch.com.au (Cobb & Mullins, 2016), the single species on Roboastra.com.au (Riek, 2013) and the single species in IPNSSI p. 103 (Gosliner et al., 2015) new taxa?

7. What is the phylogenetic structure of the Aplysiidae?

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Methods

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Methods will vary as determined by each research question, but will generally follow contemporary molecular and morphological taxonomic process. Living specimens will be collected, where possible, from the field. The advice and assistance of local citizen scientist volunteers will be sought. If possible, two specimens of each species at each stage of development (to be determined) will be preserved, one for molecular analysis and one for morphological examination. Specimens not able to be obtained from the field will be sourced from museums and other repositories.

Molecular analysis (sampling; DNA extraction, amplification and sequencing; sequence analysis; model selection, phylogenetic analysis and species delimitation) will be carried out at the National Marine Science Centre, Coffs Harbour. In most cases, two mitochondrial loci (COI and 16S) and two nuclear loci (H3 and 28S) will be used in DNA analyses (Elderkin et al. 2016). With complex or cryptic taxa, the integrated systematic method (combined morphological, molecular and ontogenic data) as described by Ekimova et al. (2015) will be used. External and internal morphology will be explored and documented. Collected specimens will be lodged with a museum, gene sequences with GenBank, and results will be prepared for submission to quality journals that focus on molluscan taxonomy.

The freely available LucID-based software will be used to develop an online key. Data will be drawn from the various morphological analyses from this project as well as the existing literature for those taxa that are adequately described. Where possible, photographs will be used to illustrate the morphological features used in the key. It will appear in a similar design and format (and form part of a planned expansion) of the existing NudiKey v1.0 (available for preview online at http://keys.lucidcentral.org/keys/v3/NudiKey/. This key is soon to be made public upon publication of the manuscript NudiKey: an illustrated, interactive identification key to the families of Australian heterobranch sea-slugs (Mollusca: Gastropoda) which is currently under review with the journal Spixiana.

 

Expected outcomes

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Many of the research questions posed in this proposal are drawn from comments in the literature. Some of these questions are long-standing taxonomic issues which have either been neglected or haven’t had the advantage of modern molecular methods. As a result, the outcomes of this project are likely to result in some synonymy amongst Aplysia, possibly in Stylocheilus and other genera and descriptions of new species in Petalifera, Bursatella and Phyllaplysia. The development of an interactive key will allow potentially complex information to be accessed by a user base that extends beyond the scientific community.