86 — Gem nudibranch, Dendrodoris krusensternii

The gem nudibranch (Dendrodoris krusensternii (Gray, 1850)) is found throughout the warm temperate Indo-west Pacific. In New Zealand waters they are found in the northern parts of North Island. They live in shallow water, from low tide down to a few metres and can reach ~90mm in length.

They can be be quite spectacular in colour, with small diamond-shaped patches of bright iridescent blue (sadly missing from the individual pictured below). However, they are also very variable, both in colour and morphology, so they have been given several names over the years. They have suctorial mouth parts and closely related species feed on sponges, but their diet remains little understood.

Nudibranchs have evolved to be shell-less, so they lack the primary protection a shell provides. Therefore they have developed chemical defences to render them unpalatable or toxic to potential predators. Some New Zealand nudibranchs (e.g., Tambja spp.) have the ability to sequester toxins from their prey (to use for chemical defence). However, Dendrodoris denisoni can biosynthesize its own. Being able to do this is an advantage, as it means that the slug not tied to a particular food for the source of its chemical defences.

Dendrodoris krusensternii, Pilot Bay,
Mt. Maunganui, 16/11/16.

Dendrodoris krusensternii, Pilot Bay,
Mt. Maunganui, 16/11/16.

More info:


Grkovic, Appleton, DR, Copp BR, 2005. Chemistry and chemical ecology of some of the common opisthobranch molluscs found on the shores of NE New Zealand. Chemistry in New Zealand. Dec. 12–15.


http://www.seaslugforum.net/find/denddeni

Willan, R.C.; Morton, J.E. 1984. Marine Molluscs Part 2: Opisthobranchia. University of Auckland, Leigh Marine Laboratory, Leigh, New Zealand. 106 Pp.

85 — Helmet shell, Semicassis labiata

There are a few species of helmet shells (Cassidae) in New Zealand waters. The two most common are Semicassis pyrum and S. labiata. Semicassis pyrum is found throughout New Zealand down to considerable depths (~500m), While S. labiata is a shallow water species found only in the north.

They are also found in southern Australia and South Africa and it is thought that this wide distribution is due to a relatively long planktonic larval stage. The larvae swim/float around in the sea for ages and this enables them to be spread far and wide. 

Semicasis labiata are thought to feed on echinoderms, particularly heart urchins (Echinocardium spp.). Oddly enough, I've not been able to find any published data confirming this, just anecdotal information, so at the moment it seems to be assumed. 

This one was found  emerging from the sand just after low tide at Pilot Bay, Mt Maunganui. This the 'insperata' form, which is lighter in colour than typical labiata, and has a row of knobs on the body whorl of the shell.

Semicassis labiata, emerging from the
sand at Pilot Bay, Mt. Maunganui, 17/11/16.

Semicassis labiata, Pilot Bay
Mt. Maunganui, 17/11/16.

Semicassis labiata, Pilot Bay
Mt. Maunganui, 17/11/16.

84 — Comb star, Astropecten polyacanthus

The comb star, (Astropecten polycanthus, Müller & Troschel, 1842) is found in northern areas of New Zealand's North Island. They are also found all over the Pacific in warm seas and can grow to about 15cm across. Comb stars live in sandy sediments, from low tide down to a few metres depth, and can often be seen emerging from the sand on the turn of the tide.

The surface of the comb star (and other starfishes and seastars) are covered with small upward-facing wrench-like appendages called pedicellariae (one of my favourite scientific words btw). Pedicellariae make the upper (aboral) surface of the comb star an unpleasant place to land and settle. They do this by intercepting anything trying to settle and then encrust the surface of the comb star. This is why you don't see starfishes covered in any encrusting organism.

Comb stars have been known to contain poisonous levels of tetrodotoxin, so it's probably not a good idea to try and eat one. This also probably goes some way to explaining why you often see them upside down (in a seemingly vulnerable position), after emerging from the sand at low tide. However, despite this, it has been reported that the trumpet shell Charonia lampas (Linnaeus, 1758) predates them and can in fact sequester the toxins from the comb star.

Comb stars are thought to be predators on sand-living invertebrates (e.g., polychaete worms and molluscs), and a recent study suggests that they are important in the regulation of some gastropods and bivalves.

Astropecten polyacanthus, aboral surface
showing numerous groups of pedicellariae.
Pilot Bay, Mt.Maunganui, 15/11/16.

Astropecten polyacanthus, Pilot Bay,
Mt.Maunganui, 15/11/16.

Astropecten polyacanthus, Pilot Bay,
Mt.Maunganui, 15/11/16.

More info:

http://nzetc.victoria.ac.nz/tm/scholarly/tei-Bio07Tuat03-t1-body-d5.html

NIWA starfish identification guide
https://www.niwa.co.nz/static/extraordinary_echinoderms.pdf

83 — Clown nudibranch, Ceratosoma amoenum

Ceratosoma amoenum, Pilot Bay,
Mt. Maunganui, 15/11/16.

The clown nudibranch, Ceratosoma amoenum * is instantly recognisable with it's bright orange and purple coloration. Clown nudibranchs are sea slugs in the Order: Nudibranchia. The name nudibranch means naked-gill and these gills can be seen at the back of the animal as a circlet of purple feathery tufts. At the front of the animal there are two purple antennae-like structures, called rhinophores and these are olfactory (sense of smell) organs.

Clown nudibranchs are found from temperate southern Australia to northern New Zealand. They can get up ~80mm in length and live from low water down to ~40 metres in depth.

Ceratosoma amoenum, Pilot Bay,
Mt. Maunganui, 15/11/16.

Ceratosoma amoenum, Pilot Bay,
Mt. Maunganui, 15/11/16.

There is considerable colour variation between the New Zealand and Australian populations, with the the Australian animals having smaller orange patches and also occasionally purple dots. These colour differences are probably related to differences in food sources.

Clown nudibranchs feed on sponges and can collect (sequester) the toxins found within the sponges. They then accumulate those toxins in their own tissues for defence.

*(Cheeseman, 1886)











More info:

https://www.niwa.co.nz/blogs/critteroftheweek/149

http://www.seaslugforum.net/find/ceraamoe

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0033479

82 — Dwarf swimming crab, Liocarcinus corrugatus

Liocarcinus corrugatus Pilot Bay,
 Mt. Maunganui, July 2018.

Liocarcinus corrugatus Pilot Bay,
 Mt. Maunganui, 14/11/16.

Liocarcinus corrugatus Pilot Bay,
Mt. Maunganui, 14/11/16. This is what you see,
as they break through the sand.

Liocarcinus corrugatus Pilot Bay,
 Mt. Maunganui, 14/11/16.

Liocarcinus corrugatus Pilot Bay,
 Mt. Maunganui, 14/11/16.

Dwarf swimming crabs (Liocarcinus corrugatus (Pennant, 1777)) have a nearly global temperate distribution (essentially, all over the place in warm waters), which suggests that they have a really long planktonic larval dispersion stage.

Dwarf swimming crabs are a smallish crab (to ~40mm across the carapace). Usually they are sub-tidal, down to ~130m, but occasionally they can be found in sand at extreme low tides, where they emerge out of the sand just after low tide. When disturbed they play dead and lie motionless—they seem quite passive and not aggressive like other paddle crabs (e.g., Ovalipes catharus (White, 1843)). After about 30 seconds (I didn't time it), they suddenly come alive and then very quickly bury themselves.

I do wonder if this playing dead makes them easy prey for seagulls though.

These photos were taken at Pilot Bay, Mt. Maunganui, 14/11/16 and there nearly a dozen of them (spread along ~25m of Pilot Bay), all coming up out of the sand, just after low tide.








































More info:

NIWA crabs guide: Wilkens, S. & Ahyong, S. 2015. Coastal crabs: A guide to the crabs of New Zealand, Version 1.

81 — Sub-fossil trough shell, Cyclomactra tristis

Cyclomactra tristis, Te Makutu
Marine Reserve, 13/11/16.

Cyclomactra tristis (Reeve, 1854) is a medium-sized bivalve found in sheltered muddy estuarine conditions. They live quite deeply buried and can die in-situ with the valves conjoined. This is the case with the examples below, which come from the Te Matuku Marine Reserve on Waiheke Island. These examples were found in some newly exposed (but previously buried) mud and are sub-fossil (quite old).

Cyclomactra tristis, Te Makutu
Marine Reserve, 13/11/16.

There appears to be some debate regarding whether C. tristis is a separate species from C. ovata (Grey, 1843), which is a similar species and lives in similar habitat. It would appear that genetic techniques could assist in resolving this question.

80 — Violet snail, Janthina janthina

I've previously posted regarding the other two common violet snails which occur in New Zealand waters, but I would be remiss if didn't mention the most well-known species. Janthina janthina (Linnaeus, 1758), is one of the most commonly encountered of the violet snails (Family: Janthinidae). They are the largest of the violet snails, reaching ~4cm in size, and have a global distribution in warm seas. Violet snails float by means of a raft of bubbles (held together with what is essentially snot), which they secrete.

Ecologically, violet snails are what is known as holoplanktonic, meaning that they spend their entire lives in surface waters and basically float around at the mercy of wind and current. This is why they wash up on beaches after sustained periods of onshore winds.

Typically J. janthina feed on by-the-wind sailors (Velella velella Linnaeus, 1758) and Portuguese man-o-war (Physalia physalis Linnaeus, 1758). They are in turn preyed on by birds, fishes, and other molluscs. Once they die their shells can sink to sea bed, I have seen one which was collected in a deep-sea sample from ~1800m on the Challenger Plateau, west of New Zealand.

On Auckland's west coast beaches they are commonly seen washed up along the high tide line, but they are less common than J. exigua. On the east coast of the North Island this is reversed with J. janthina being the most common, plus J. globosa is occasionally also seen.

Janthina janthina, South Piha, 10/11/16.

The same animal, transported to a mid-tidal
rock pool. Note the bubble raft and the
orientation of the shell.

Janthina janthina, Papamoa Beach, 2/10/16.

79 — Mysids (opossum shrimps)

Mysids or opossum shrimps are very important in freshwater and marine ecosystems, as they are food for a wide variety of animals. They are named after ventral pouch, where they keep their eggs and brood their offspring. There are several species in New Zealand waters from freshwater, to marine and then from shallow waters down to over 1000m. The deep-sea mysids are important food for deep-sea sharks (Etmopterus baxteri Garrett, 1957 and eels (Diastobranchus capensis, Barnard, 1923).

Mysids are thought to feed on detritus, algae and other small invertebrates. I believe the mysids pictured below are Tenagomysis novaezealandiae Thomson, 1900, but I'm not certain (I'd need to have dead ones and a look under a microscope to id them with confidence).

They were amongst rocks at the base of Mount Maunganui, opposite Matakana Island. Each mysid is about 20mm in size.

Swarm of mysids, Mt. Maunganui, 2015.

Swarm of mysids, Mt. Maunganui, 2015.

Swarm of mysids, Mt. Maunganui, 2015.

Swarm of mysids, Mt. Maunganui, 2015.

Swarm of mysids, Mt. Maunganui, 2015.
This clip is slowed down to half speed.