Nudibranchs and related molluscs 9780980381382, 9781921833007, 9781921833083, 098038138X

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MUSEUM VICTORIA FIELD GUIDE

Nudibranchs and related molluscs Robert Burn

Nudibranchs and related molluscs

Nudibranchs

iiTambja verconis (Polyceridae), Portsea Pier, Vic. Mark Norman

A Museum Victoria Field Guide to Marine Life

Nudibranchs and related molluscs Robert Burn

Series editors Mark D. Norman Robin Wilson Melanie Mackenzie Other titles in the series Crabs, hermit crabs and allies Sponges Published by Museum Victoria 2015 © Text copyright Museum Victoria © Images copyright Museum Victoria unless otherwise noted. Museum Victoria has made every effort to obtain copyright and moral permission for use of all images. Please advise us of any errors or omissions. Museum Victoria Publishing GPO Box 666 Melbourne Vic. 3001 Australia Tel + 61 3 8341 7370 or 8341 7536 [email protected] Museum Victoria www.museumvictoria.com.au Dr J. Patrick Greene chief executive officer Dr Robin Hirst director, collections, research and exhibitions Dr Mark Norman head, sciences printed by Everbest, China original design by Propellant Typeset by the printer’s drawers and Belinda Smullen Communication Design

An introduction to marine life Barnacles Shrimps, prawns and lobsters

National Library of Australia Cataloguing-in-Publication data: Burn, Robert. Nudibranchs and related molluscs / Robert Burn. Includes bibliographical references and index. Nudibranchs--Australia, Southeastern. Other Authors/Contributors: Museum Victoria. ISBN: 9780980381382 (pbk.) 9781921833007 (e-pub) 9781921833083 (e-pdf) 593.40994 front cover image Burnaia helicochorda, from south eastern Australia and the North Island of New Zealand, eats small anemones living attached to algae and seagrasses. The genus name honours the writer of this guide; the species name refers to the spiralling digestive gland within the cerata. John Chuk. back cover images Upper: Sagaminopteron ornatum. John Chuk. Second: Mexichromis macropus. Leon Altoff. Third: Thorunna arbuta. Platon Vafiadis. Lower: Flabellina sp. RB1. Platon Vafiadis.

Funding for the Museum Victoria Field Guides to Marine Life from the Australian Government through a grant from its Natural Heritage Trust is gratefully acknowledged. The grant was facilitated by the Port Phillip and Westernport Catchment Management Authority. Publication of the series is funded in part by the Victorian Coastal Council and Parks Victoria. This title within the series was also supported by a grant from the Norman Wettenhall Foundation.

MUSEUM VICTORIA FIELD GUIDE SERIES TO MARINE LIFE These field guides to marine life enable the amateur naturalist, beachcomber or environmental scientist to identify the marine animals most commonly found along the shore or in shallow waters along the coast of the state of Victoria, Australia. South-eastern Australia is characterised by a rich marine fauna, with many species found nowhere else. Commonly, species found along the Victorian coast also occur in Tasmania, southern New South Wales, and along the southern coast of the continent through South Australia and into southern Western Australia. This series aims to cover the common marine animals, and each book deals with a different group. More species live on the Victorian shore and in its shallow waters than are included in each book, and many more inhabit the deeper waters of Bass Strait and beyond. See Further Information at the end of this guide, and Museum Victoria’s website: www.museumvictoria.com.au Museum Victoria encourages individuals to explore the diversity of coastal habitats but discourages unnecessary removal of specimens from their natural environment. Museum scientists are interested in new discoveries, and unusual findings can be reported to the Discovery Centre at Museum Victoria or to Reef Watch Victoria: [email protected]

Reef Watch Victoria is a community-based marine monitoring program for Victoria’s temperate marine environment. Divers and snorkellers conduct regular surveys at their favourite Victorian reef sites using the Reef Watch monitoring kit. For more information, go to: www.reefwatchvic.asn.au The Marine Research Group (MRG) is a branch of the Field Naturalists Club of Victoria that has had a long and productive working relationship with Museum Victoria, actively undertaking research into Victoria’s rich marine life and providing curatorial and survey assistance with Museum Victoria’s extensive marine invertebrate collection. The MRG meets regularly, welcomes new members, and can be contacted at: www.fncv.org.au A large proportion of the images and information presented in the MV field guide series has resulted from the ‘Under the Lens’ and ‘Bioscan’ partnerships between Museum Victoria and Parks Victoria. Along with ports and harbours, Parks Victoria manages marine national parks and sanctuaries throughout Victoria. For more information go to: parkweb.vic.gov.au

CONTENTS



1 3 6 11 14 18 21 21 28 29 30 32 33 34 41

Introduction What are nudibranchs and related molluscs? Concealment Biotic associations External and internal anatomy Reproduction Classification Outline of classification Where do they live? What do they eat? What eats nudibranchs and related molluscs? Partnerships and introduced species About this guide Checklist of included species Nudibranchs and related molluscs

244 Further Information 248 Glossary 252 Index Of Scientific Names

INTRODUCTION

The marine environment of south-eastern Australia is very rich in its diversity of habitats and different kinds of animals. Beachcombers, rock-poolers, divers and photographers always find something of interest, but learning more about their ‘finds’ often proves far more difficult. This guide is about the nudibranchs and their relatives (otherwise known as ‘sea slugs’) that can be encountered in the intertidal and divable subtidal waters of the Bass Strait area.

Phyllodesmium macphersonae (Facelinidae), Portland Bay, Vic. Leon Altoff

‘Nudibranchs and related molluscs’ include bubble shells, sea hares, side-gilled slugs, sap-sucking slugs and sea butterflies (pteropods), as well as the true nudibranchs. All have scientific names derived from Latin or Greek; only a very few (mainly tropical) species have common names, and these are usually generic or broadly applied rather than given to a single kind of animal. Like their distant relatives, the common garden snails (with external shells) and slugs (without shells), nudibranchs and their relatives vary considerably in form, ranging from those with an external shell, into which the animal can retract for protection, to those with complete absence of a shell (except briefly during embryonic development). Freedom from a protective shell has allowed the enormous elaboration of body form and colouration of nudibranchs, much to the delight of observers, divers and photographers. 1

Nudibranchs

In recent years, regional field guides to the marine biota have touched briefly upon the nudibranch fauna of southern and south-eastern Australia. More detailed guides to the nudibranchs and related molluscs of Western Australia, southern Queensland, and the southern Great Barrier Reef are available, and these and other references are listed in Further Information at the end of the guide.

Phyllodesmium macphersonae eats the greyish polyps of a stoloniferan soft coral that encrusts the stems of brown algae in the shallows of Western Port, Victoria. Slight differences in colour and body shape between animals throughout its wider distribution in the temperate and tropical Indo-Pacific suggest a complex of cryptic species awaiting discovery. The species name honours Hope Macpherson (Mrs Jessie Hope Black), Curator of Molluscs, Museum Victoria, 1947–1963, mentor of the writer’s early endeavours in malacology and nudibranch research. This image is of a live 25-mm long specimen from Flinders Pier, Vic., type locality for the species. John Chuk.

2

What are nudibranchs and related molluscs? This field guide covers nudibranchs and related marine molluscs, many of great beauty, which until recently have been classified as ‘opisthobranchs’ (a technical term meaning the gills are towards the rear). They are part of the gastropod (or snails and slugs) class of the phylum Mollusca. The many species that completely lack a shell are known far and wide as ‘nudibranchs’ because of their naked or exposed gills. Throughout the last few decades, textbook concepts of ‘Opisthobranchia’ have been challenged by morphology-based and (more recently) molecular studies. It is no longer clear if any precise distinctions can be made between the major groups of opisthobranchs and pulmonates (air-breathing slugs), but the classification used throughout this work retains the old groups for consistency with the large body of existing literature. However, the reader should be aware that current research, especially DNA-based analyses, will soon cause scientists to adopt an entirely new classification for nudibranchs and related molluscs as we come to better understand evolutionary relationships. Thus, although much of the literature refers to ‘Opisthobranchia’ (and that is the concept that also forms the foundation for this field guide), we now know that this assemblage is nothing more than an artificial grouping of gastropod molluscs. The group includes (usually marine) slugs or snails with limpet-like, bivalved or bubble shells showing tendencies towards reduction or internalisation of the shell, having a more or less twisted internal anatomy but externally a bilaterally symmetrical body with at least one pair of head tentacles or a headshield. There are, however, many exceptions to this format, and it should be noted that the true bivalve molluscs are in the separate class Bivalvia. Rather than helping to clarify evolutionary relationships, or even having descriptive value, the ‘Opisthobranchia’ concept is of historical and—to many of us—emotional attachment. For a current technical discussion of the issues, consult Schrödl et al. (2011) in Further Information. For those using this guide, higher classification and phylogeny are of little consequence; their aim is to identify animals at the genus and the species level, a task rendered endlessly fascinating by the often bizarre shapes and magnificent colouration that make nudibranchs and related molluscs so attrac3

Nudibranchs

tive to rock-poolers and diver-photographers. More than 400 species (different kinds) of these gastropod molluscs are known to occur within the Bass Strait region, at least half of which are found from the intertidal zone down to SCUBA-divable depths. In form, these animals vary greatly. Some have strong, snail-like shells into which they can withdraw for protection, a few of these even closing the shell opening with a horny operculum (trap-door or lid) attached to the rear upper side of the tail. At the other extreme are those with external gill-like structures or appendages (cerata) along each dorsal side of the body. Defensive features incorporated into the body surface include nasty-tasting acidic glands just below the skin, and undischarged stinging cells (nematocysts) taken up from their food and stored in special receptacles (cnidosacs) at the tips of the lateral appendages.

Flabellina sp. RB2 closely resembles other Flabellina species occurring in the Bass Strait area, but is distinguished by the rounded, non-tentacle-like corners of the anterior foot. Flabellina species eat hydroids, especially Eudendrium, some of which can be seen in the upper left background. 18-mm long specimen, Portsea Pier, Vic. John Chuk.

4

A number of other marine animals are sometimes called sea slugs, or are mistaken for them. Three common southern Australian gastropods (Lamellaria ophione, Lamellaria australis and Onchidella nigricans) look very much like oval dorid nudibranchs (nudibranchs of the family Dorididae). All three, however, lack the rhinophores and gills that project from the mantle of dorid nudibranchs.

Not all ‘sea slugs’ are nudibranchs

The air-breathing gastropod Onchidella nigricans (Pulmonata: Onchidiidae), Flinders, Vic. Chris Rowley.

The gastropod Lamellaria australis (Gastropoda: Velutinidae), Flinders Pier, Vic.

The flatworm Cycloporus sp. (Platyhelminthes: Euryleptidae), Portsea Pier, Vic. John Chuk.

The sea cucumber Australostichopus mollis (Echinodermata: Holothuroidea: Stichopodidae), Port Phillip Bay, Vic.

Glenys Greenwood.

Mark Norman.

The slug-like or worm-like sea cucumbers (in the tropics, bêche-de-mer), also known as sea slugs, are actually holothuroid echinoderms related to seastars and sea urchins. Polyclad flatworms, too, are sometimes mistaken for nudibranchs, but are usually very soft, very flattened, and underneath lack the very distinct muscular crawling foot. 5

Nudibranchs

Concealment Many nudibranchs and related molluscs are very good at not being seen. They can be astoundingly similar to the host animal or alga on which they live (and often on which they feed). None dress up the body to hide; instead, most use colour and body shape to blend into the background Austraeolis ornata (Facelinidae): adult specimen with conspicuous dark-green and bright-green digestive mosaic for camouflage. glands, subtidal, Portsea Pier, Vic. John Chuk. Most bubble shells live within the substrate (i.e. are infaunal); hence their body colour is whitish and unimportant. The relatively few epifaunal (living on the substrate) bubble shells are often extravagantly pigmented animals, seemingly not afraid of predators. Sea hares and sacoglossans (sap-sucking sea slugs) are most often coloured and patterned to match their algal foods. Many sacoglossans are totally green, gaining their pigmentation from the green algae upon which they feed. Pigmentation of the majority of aeolid nudibranchs is greatly influenced by their hydroid food, the colour of the digestive gland indicating their choice of species. Interestingly, small (