The intertidal rocky shore 

identifying molluscs of the rocky shore 

By Dr J Floor Anthoni (2007) 
www.seafriends.org.nz/enviro/rsmoll.htm
This page identifies the common molluscs found on the New Zealand intertidal rocky shore: octopus, limpets, snails, shells, chitons, nudibranchs and more. Several species can live out of the water for long periods, but most need to be submerged at least most of the time.
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Molluscs
Molluscs are soft-bodied animals but most have a hard shell for protection. Molluscs come in a vast variety of form, from inkfish (like octopus) and seaslugs to sea snails and clams. 

 
inkfish
Inkfishes are strange molluscs whose foot has split into arms, each stuck with many suckers. Their mantle has become a breathing bag covering their internal gills. Whereas other animals are designed according to the head-body-legs concept, inkfish are body-head-arms animals. Inkfish are of the crawling type (octopus) or swimming types (cuttlefish , squid and even a swimming octopus). Most amazing is their jet pipe, a loose flap of skin, possibly once part of the mantle, which they use for breathing out of and as a handy tool for digging and also as a jet pipe for swimming. Crawling octopus pull themselves along by their arms, but they can swim quite swiftly using their jet pipe. In rock pools one does not find the swimming varieties of squid and New Zealand has no cuttlefish species.
a small sand octopus (Octopus Gibbsi)
f036900: a small sand octopus (Octopus Gibbsi) [wheke] has taken refuge in a shallow tide pool. Finding an octopus is a real treasure. Often one can see the surface of a rock pool 'breathing' as the octopus exhales through its jet pipe. Octopus feed on crabs and clams. As hungry predators, octopus need much food, reason why they do not stay permanently in rock pools.
reef octopus Octopus cordiformis
f035330: The other common octopus is the reef octopus (Octopus cordiformis, Macroctopus maorum) [wheke].The reef octopus does not dig dens in the sand but seeks an existing cavity in the rock, preferably with a very narrow opening through which it slides out of view. Whereas the sand octopus feeds mainly on molluscs, the reef octopus feeds mainly from crustaceans like crabs and crayfish. The reef octopus is more grey/purple whereas the sand octopus is brown/orange. The reef octopus has vanished from many places around the North Island.

 
sea hares
Sea hares have large flaps covering their backs and gills. They also have large 'ears' for which they are called 'hares'. They also have large mouth flaps with which they can explore a wide swath for edible weeds. Being so vulnerable without a large protective shell, they are nonetheless not predated upon, presumably because they taste foul. All sea hares are voracious grazers and prolific spawners. Like octopus, sea hares can squirt ink when frightened. All sea hares carry a remnant of a shell under their folds. By opening the folds gently while sliding one's finger over its skin, one can feel the thin shell. If the sea hare co-operates, it can even be seen.
For a parade of NZ sea hares, visit www.seafriends.org.nz/indepth/sea_hares.htm.
black-tailed sea hare (Aplysia dactylomela)
f050511: the black-tailed sea hare (Aplysia dactylomela) or variable sea hare  is brown to greenish with a flower pattern on its skin. When frightened it can squirt a dark purple ink. This sea hare lives for only one year but can grow over 1kg large.
skin folds of a black-tailed sea hare
f050507: closeup of the skin folds of a black-tailed sea hare shows it polka-dotted inside. Also a small 'siphon' is formed in the middle, through which water is exhaled. Note the black tail for which this sea hare is named.
little seahare (Aplysia parvula)
f050421: the little sea hare (Aplysia parvula) is purple-brown with light spots and has a black or checkered margin. It is much smaller than the black-tailed sea hare. When frightened, it can squirt a light purple ink.
egg mass of the little sea hare Aplysia parvula
f025832: the neatly arranged egg mass of the little sea hare. By comparison, that of the black-tailed sea hare is messy and more yellow in colour.
the julian seahare (Aplysia juliana)
f025909: the julian seahare (Aplysia juliana) is not commonly found in the intertidal as it prefers calm water and the food found in sheltered shallows. It can also be smooth brown. It does not slide smoothly but extends and contracts like an inchworm.
black sea hare (Aplysia brunnea)
f035525: the black sea hare (Aplysia brunnea) is found from Wellington south. Although it looks entirely black, its scientific name calls it brown. It is a large sea hare, comparable in size to the black-tailed sea hare.
hairy sea hare (Bursatella leachii)
f005000: the hairy sea hare (Bursatella leachii) (Bursatella glauca) [rori tararua] lives in calm waters while feeding from detritus. It loves places where raw sewage enters the sea.

sea hare lookalikes


shield limpet (Scutus breviculus)
0609173: the shield limpet (Scutus breviculus) [rori] may look like a black blob. The mantle over its back can be parted gently to show its large shield-shaped shell. Also called ducksbill limpet because of the shape of its internal shell.
f050629: a young shield limpet (Scutus breviculus) under water, sliding over bryozoan mats. The shield limpet is a detritus feeder and thrives in areas where raw sewage is dumped.
dark onchidella (Onchidella nigricans)
f010818: the dark onchidella (Onchidella nigricans) is a secretive slug that comes out of its hiding places to graze detritus from rocks and seaweeds by night. Here it is seen cleaning necklace weed during an early morning low tide. It can be mottled, green, brown or black. It always homes back to its own sleeping spot, deep underneath large boulders. It has a tough skin.
butterfly chiton (Cryptoconchus porosus)
f036408: the butterfly chiton (Cryptoconchus porosus) or leatherback chiton is shown here because it can easily be mistaken for a seaslug. This chiton has its eight hard shells underneath a thick leathery skin. (see lower down the page). The 8 plates of this chiton are narrow, with gaps inbetween.

nudibranchs


Nudibranchs (naked gills) are the sea slugs of the sea, a crown of evolution. Finding them in rock pools is always exciting. Little is known about them because they do not occur in vast numbers. Most are thought to be specialist feeders, depending on a specialised diet.
mottled side-gill slug (Pleurobranchaea maculata)
f050525: the mottled side-gill slug (Pleurobranchaea maculata) has external gills under its righ-hand mantle, here just seen. It feeds on detritus and is often found in degraded habitats.
egg mass of Pleurobranchaea  maculata
f039700: finely spiralling egg mass of the mottled side-gill slug attached to the inhalant tube of a parchment tubeworm (Chaetopterus variopedata). Such an egg mass may contain tens of thousands of eggs.
black doris (Dendrodoris nigra)
f039702: the black doris (Dendrodoris nigra) is completely black with a thin red margin.
variable doris (Aphelodoris luctuosa)
f039818: the variable doris (Aphelodoris luctuosa) has many colour variations between white and black, but always has a yellow margin.
gem doris (Dendrodoris denisoni)
f050207: the gem doris (Dendrodoris denisoni) (D. gemmacea) is variable in coulour but always has blue spots, black-lined gills and an off-white margin.
gem doris (Dendrodoris denisoni)
f039736: a brown colour variant of the gem doris (Dendrodoris denisoni).
gold-lined doris (Chromodoris aureomarginata)
f038822: the gold-lined doris (Chromodoris aureomarginata) is a precious find on the rocky shore.
clown doris (Ceratosoma amoena)
f032722: the clown doris (Ceratosoma amoena) has bright orange spots on white, and purple gills and antennas. Here it is seen mating 'top-to-toe'.
lemon doris (Dendrodoris citrina)
f050233: the lemon doris (Dendrodoris citrina) is a yellow doris with a mottled skin.
lemon doris? (Dendrodoris citrina ?)
f050228: lemon doris? (Dendrodoris citrina ?). Four variants are shown here that all vie for the name lemon doris.
lemon doris? (Dendrodoris citrina ?)
f050535: lemon doris? (Dendrodoris citrina ?)
lemon doris? (Dendrodoris citrina ?)
f040034: lemon doris? (Dendrodoris citrina ?)

 
bivalves
Bivalves have two shells, often almost identical in size and shape. Most bivalves burrow in sand but those that can live on rocks, must be capableof attaching themselves, and in some cases detaching themselves as well, to move somewhere else. Mussels do this by means of byssus threads. The animal's foot which is normally adapted to burrowing in mud or sand, has a groove in which it can secrete a mysterious glue that sets once released. The foot extends very far out from the shell, often at night, to attach the end of the thread to a nearby object. The mussel can release all of its threads instantaneously by discarding the 'knot' where they all join.
small red lamp shells (Calloria inconspicua)
f019007: small red lamp shells (Waltonia inconspicua, calloria inconspicua) are not real molluscs but they have two shells and filter the water for food. Brachiopods (arm-foot) have a base shell connected to a short flexible foot. Hinged on the base shell is the top valve and inside is a lophophore, a ciliated filter bracket that sieves the water for food.
flea mussels (Xenostrobus pulex)
0511068: flea mussels (Xenostrobus pulex) are very small mussels that form dense patches on the shore, serving as food for many predators like the oyster borer and purple shore crab. Flea mussels do not grow larger than 10mm but some reach double that size. They do not live long.
greenlipped mussels (Perna canaliculus)
f221825: greenlipped mussels (Perna canaliculus) or greenshell mussels are common on the West Coast where they are keenly sought after. On the rocky shore the odd individual can be found and inside marine reserve perhaps banks of these delicious mussels. It needs rich phytoplankton. This mussel can become over 10 years old and some 18cm in length.
blue mussel (Mytilus edulis)
f980808: the blue mussel (Mytilus edulis) is more common on the South Island. It is a delicious mussel with tender flesh, but does not propagate as well as the greenlipped mussel and does not grow larger than 7cm. It is also easily damaged during harvesting and scrubbing.
bearded mussel (Modiolus aerolatus)
f050807: the bearded mussel (Modiolus aerolatus) or brown mussel is solitary and often difficult to spot as it allows itself to be grown over by other organisms.
cardita shells (Cardita brookesi)
f019013: cardita shells (Cardita brookesi) attached to the underside of a rock, by their byssus threads. These shells can crawl and even swim. The red, orange, brown and grey patches are all carpet sponges.
little fan shell (Chlamys zelandia)
f050535: little fan shell (Chlamys zelandia) [kopakopa] is attached to the rock by means of byssus threads like mussels do. On right a lemon doris nudibranch. The brown patches are encrusting sponges.
oyster borers (Lepsiella scobina)
f215206: large oyster borers (Lepsiella scobina) on a native rock oyster (Sacostrea cucullata) [tio], while attempting to drill holes. The small oyster above has already been drilled into, as the small conical hole shows. The top half of the native oyster is almost the same size as its bottom half, and is usually convex. Oysters are bivalves that have lost their foot as they are cemented by their left shell to the rock. The rock oyster is limited to the North Island.

 
 
univalves
Univalves have only one shell which gives them a very flat profile and a very big foot for attachment. This shape can survive the most turbulent of water. NZ has three species of abalone (paua, earshell) and all three are very strong.
Limpets are also strongly attached to the rock, especially to their homing patch where they sleep and wait for the tide to return.
paua (Haliotis iris)
f036111: the rainbow paua (Haliotis iris) is the largest abalone, growing to 25cm in the cold waters of the South Island. In warmer waters it does not grow well. The paua photographed here sits on its homing spot. Notice how its surroundings, including the hard pink paint have been grazed smooth. The outside of this animal has been ground down and polished by pebbles. Paua are voracious grazers, capable of scraping the hard pink paint, the calcium of which they perhaps use to build their shells with. Inside is rich iridescent blue/green.
paua (Haliotis iris)
f032402: a paua's grip on the rock is so strong that it is almost impossible to remove it without damaging it. This paua came off because the rock gave way. Paua have short tentacles all around their bodies. When exploring and feeding, they extend a large mouth flap like sea hares do. Paua are very strong grazers, not only eating the fine hairy algae but also small macro algae like stalked kelp. When attacked by a sea star, they flick their bodies around, forcefully and literally run away at speed.
yellowfoot paua (Haliotis australis)
f027220: the yellowfoot paua (Haliotis australis) [hihiwa karariwha] (85mm) or silver paua sleeps by day and forages by night. It is perhaps the fastest running mollusc known, capable of 'running' over the tops of waving seaweeds. It also navigates with excellence back to its homing spot in a crevice.
virgin paua (Haliotis virginia)
f019035: the virgin paua or white foot paua (Haliotis virginia) [marapeka] (50mm) is the smallest of the three, its outside often red and inside bright silver. It is a shy animal, hiding under rocks by day and foraging only by night. Here it is seen on a bryozoan mat next to two lamp shells.
radiate limpets (Cellana radians)
f009517: the radiate limpet (Cellana radians) [ngakihi] can have many patterns but it is always a smooth, flat limpet with some radial patterns and its top highly off-centre.
radiate limpet (Cellana radians)
0608229: radiate limpet (Cellana radians) has many forms.
0703048:  (Patelloida corticata) (14mm) 
0703054: (Patelloida corticata) very variable in shape
ornate limpet (Cellana ornata)
0608234: a siphonaria zelandica looking like an ornate limpet (Cellana ornata) but the two spawn capsules above identify it.
f018928: grooved limpet (Tugali elegans) [ngakihi] can cover its shell almost completely with its mantle.
circular slipper shell (Sigapatella novaezelandiae) and ribbed slipper
f051608: circular slipper limpet (Sigapatella novaezelandiae) [ngakihi kopio] and ribbed slipper limpet (Maoricrypta costata),  both native.
ribbed slipper shell (Crepidula costata).
f051617: ribbed slipper limpet (Crepidula costata, Maoricrypta costata). [ngakihi hiwihiwi] Native. Note the small male on its back towards the top left. Eggs are brooded under the female's shell.

 
multivalves
Chitons have eight shell parts (valves), hinged together and surrounded by a flexible girdle. They are extremely hardy and able to lay flat in any contour of the rock, thus minimising their exposure to waves.  They stick so well to the rock that one would not be able to dislodge them without damaging them. Chitons are also called coat-of-mail shells resembling the overlapping plates of body armour on medieval knights.
Most chitons have numerous tiny holes through their valves (shell plates). Through these holes they may extend fine tentacles that can be withdrawn. These tiny tentacles are sensitive to light.
noble chiton (Eudoxochiton nobilis)
0609160: the noble chiton (Eudoxochiton nobilis) has a smooth red ro green girdle. Their foot is always orange underneath. The transparent worm is a ragworm or bristleworm (Flabelligera affinis).
large giant chiton (Guildingia obtecta)
f034116:  a large giant chiton (Plaxiphora obtecta) (Guildingia obtecta) in deeper water is encrusted by pink paint. It has a scruffy hairy girdle and can grow to 9cm long. It is found in the photic (light) zone from mid tide to about 15m depth. When not encrusted it has smooth dark green valves with a pale stripe down the crest.
snakeskin chiton (Sypharochiton pelliserpentis)
f009518: the snakeskin chiton (Sypharochiton pelliserpentis) [papatua] is found mainly in the intertidal. Its skirt looks like a snake's skin, with scales. Here it lives amongst the sheet barnacles which also grow on top of it.
blue-green chiton (Amaurochiton glaucus)
050610: the blue-green chiton (Amaurochiton glaucus) can be blue to green with many different patterns. it is a beautifully marked chiton. It hides underneath stones by day, to come out only at night.
0609171: the violet chiton has a skirt of variable colours and a narrow band of usually violet valves (Notoplax violacea)

f036920: (Onithochiton neglectus ?)
f019001: the large chiton is the violet chiton Notoplax violacea. The green chiton with the white crest is the maori chiton Ischnochiton maorianus. At the top the tree-ring chiton and an unidentified orange chiton or limpet.
tree-ring chiton (Onithochiton neglectus)
f018924: the tree-ring chiton (Onithochiton neglectus) has variable colours from a pink girdle and yellow valves to a purplish girdle with green valves. Its valves have a tree ring growth pattern.
maori chiton (Ischnochiton maorianus)
f051632: the maori chiton (Ischnochiton maorianus) is light coloured with many colour variations and even stripes as shown here. It has a light coloured margin.
leatherback chiton (Cryptoconchus porosus)
f036408: leatherback chiton (Cryptoconchus porosus) [karimoan] has its eight shells (valves) covered with a tough, leathery skin. It has an amazing colour variety from even brown, black and olive to flamed patterns like the one shown here, in green and orange. It is also called butterfly chiton. (Why?)

f018917: a variety of chitons. From left to right: the tree ring chiton, green chiton, violet chiton and a radiate limpet.

 
 
 
snails
Most snails have coiled shells that consist of two parts: the shell proper and the closing disc (operculum), but some snails have lost their operculums (cowries, bubble shells). This shape has been most successful in nature as it provides for permanent protection and adequate mobility. 
Grazing snails can be recognised from their rounded shells and rounded openings. The rounded foot of grazing snails allows them to move from side to side while scraping algae from the substrate by means of their rasp-tongue (radula).
Predating and scavenging snails are all whelks, characterised by an oval opening. Whelks have feet designed for 'running', to catch up with evading prey and to be the first at a dead organism.

grazers

periwinkle (Austrolittorina antipoda)
0609133: the blue-banded periwinkle (Austrolittorina antipoda) [ngaeti] is a very small snail with a thin shell, capable of living out of the water for months. It grows slowly and lives for many years. Here it is seen in a spawning aggregation on a sheltered high of the reef. A larger species brown with a lined shell (Austrolittorina cincta) is found on the West Coast and the South Island. When fully retracted, the periwinkle hangs on by a dried glue of mucus.
f009535: black nerita (Nerita atramentosa) [matangarahu] is well adapted to living out of water for prolonged times. It is the 'crash helmet' of the shore, extremely hardy and able to survive the worst of wavewash. In the picture also small grooved topshells Melagraphia.
grooved topshell (Melagraphia aethiops)
0608231: the grooved topshell (Melagraphia aethiops) [maihi] is found mainly in mid-tide, living where it is half exposed and half submerged. It is easily recognised from its off-blak brown colour and grooves. Perfect shells also show an intricately engraved pattern, particularly around the mouth where new shell is formed.
catseye snail (Turbo smaragdus)
0612056: the catseye snail (Turbo smaragdus) [ataata] has a green-white closing disc, resembling a cat's eye. It has a smooth, rounded shell, except when young when it has parallel grooves, see top right corner. When smaller still, its shell has nodules. The white shells are those of the rock oyster (Sacostrea cucullata). In the centre a small Cominella maculosa.
f036235: the Cooks turban shell  (Cookia sulcata) [karaka] is the largest grazer on the reef, with a very heavy shell. It does not like to be out of the water but can sometines be found in a rock pool. Here it is showing its foot and snout, and a variable triplefin is resting on its back.
closing disc of Cooks turban snail
f049627: the Cooks turban snail also has a thick closing disc, which makes it almost impregnable. Yet spotties know how to flip it over, waiting for the shell to expose it soft tissues. 
opal top shell (Cantharidus opalus)
f040336: the opal top shell (Cantharidus opalus) [matangongo] (up to 40mm) is a very beautiful shell with gradients of colour from red to blue to green. It normally feeds on kelp but can be found in rockpools. Here it is feeding on a young flexible weed (Carpophyllum flexuosum).
purple top shell (Cantharidus purpureus)
f051620: purple top shell (Cantharidus purpureus). (20-30mm), has weak spiral ribs and can vary in colour from purplish green to bright red.
green top shell (Trochus viridus)
f051614: the green top shell (Trochus viridus) [tihipu] is normally found subtidally but may survive in stable rock pools.

predators


oyster bohrer (Lepsiella/ Haustrum scobina)
f009603: the oyster borer (Haustrum scobina) (Lepsiella scobina) [kai kai tio] is a small whelk with a grooved and ribbed spotted shell. It is named after its European lookalike which specialises in drilling holes in oyster shells. The NZ oyster borer is smaller and lives mainly from sheet barnacles and flea mussels.
dark rock whelk (Haustrum haustorium)
f050400: the dark rock whelk (Haustrum haustorium) [ngaeo] lives mainly under water where it predates on any grazing snail. Here it has just caught a catseye snail and is turning it over to allow its radula to drill into its prey. It has a very large aperture.
dark rock whelk (Haustrum haustorium)
f032406: a dark rock whelk has captured a catseye snail and has rotated it face-on. Now it drills into the snail's tissues while slurping the 'soup' through its siphon. The snail is fully covered to prevent spills that could attract other predators or scavengers. Note the white slipper limpet (Maoricrypta monoxyla) on its side, with 3 growth rings.
white rock whelk (Dicathais orbita)
f051604: the white rock whelk (Dicathais orbita) [hopetea] competes with the dark rock whelk for the same kind of food in the same habitat. This whelk also predates on mussels.
f051408: Haustrum lacunosis (Lepsithais lacunosus) (50mm) is a large white whelk, similar to Dicathais orbita but has deep grooves with growth lines and a frilled aperture. Found from Cook Strait south. 
monoplex whelk (Monoplex australasiae)
f018905: the monoplex whelk (Monoplex australasiae) has a beautiful opening and is unusually haired on an external skin (periostracum). It normally lives well below tide but may occasionally be found in a deep rock pool 
f039701: monoplex whelks lay their eggs inside a solid egg capsule, which once the eggs have hatched, remains a mysterious object for a long time. The yellow sponge is the boring sponge Clione cellata.
triton whelk (Charonia capax)
f011619: the triton whelk (Charonia capax) is the largest whelk of the shore. It predates on starfish and sea urchins but does not like to be out of the water. Although its outer shell is often worn, its opening is smoth and beautifuly coloured. The animal itself is red.
trumpet whelk devouring a sea urchin
f018903: a trumpet whelk devouring a sea urchin. It also eats the test and spines to help make its massive shell. Note the little holes in the shell, drilled by tiny burrowing worms.
red charonia whelk (Charonia rubicunda)
f018404: the red trumpet whelk (Charonia rubicunda) has a lighter shell with brown lines. It is also smaller.
trumpet whelk about to eat an arm of a prickly star.
f017234: a red trumpet whelk at the moment of pinning down an eleven-armed prickly star. The star escapes by sacrificing an arm. Note that one of its arms has already been eaten and is now regrowing. The whole arm is ground down and eaten.


scavengers


spotted cominella whelk (Cominella maculosa)
0612054: the spotted cominella whelk (Cominella maculosa) (5cm) is more common in muddy habitats, but may occasionally be found on the rocky shore, particularly on the West Coast.
f050418: the red-mouthed scavenging whelk (Cominella virgata) seen laying eggs in a three-some. Most snails are hermaphrodites, being both male and female in one animal. By laying eggs together, they can be fertilised by the others.
lined whelk (Buccinulum lineum)
f051603: the lined whelk (Buccinulum lineum) is found mainly below low tide or in stable rock pools.
egg capsules of the red-mouthed scavenging whelk
f050419: egg capsules of the red-mouthed scavenging whelk, closely spaced together underneath a large stone.

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