Introduced shore crabs: pest or not?

One of the most common crabs at Vancouver Island's shores: the purple shore crab, Hemigrapsus nudus.


The purple shore crab belongs to the Grapsidae, a family of crabs with a rather square carapace, usually not more than a few centimeters wide.


The typical environment of the purple shore crab: mud, sand, little rocks and debris. They prefer the upper tidal zone, are quick and well adapted to living out of the water.


After turning over a rock I saw up to 25 crabs getting away. It is very important to gently return the rock. Otherwise animals and seaweeds living upon or under the rock will die. Grapsid crabs are swift and not aggressive, however you frequently find specimens with amputated legs or claws. Maybe because they are very abundant and are fighting for space. And a lot of crab species are cannibals.






This is obvious a relative of the purple shore crab: the Asian shore crab, Hemigrapsus sanguineus (NL: blaasjeskrab). It is a recent import (since 2003) on the Dutch shore. It originates from Japan's surrounding waters. It is not as abundant as his relative: the penicillate shore crab, Hemigrapsus takanoi (NL: penseelkrab).

A pest or not?
The penicillate shore crab has replaced the common shore crab, Carcinus maenas, on the Dutch upper tidal shores. However, the much bigger common shore crab is still abundant.


The common shore crab (Carcinus maenas).

In Canada and the USA, where the introduced common shore crab is called the European green crab, it is as in the Netherlands: it has difficulty competing with native grapsid crab for space under rocks (G.C. Jensen, 1995).
The common shore crab is considered a pest in South-Africa: 'it poses a threat to many local molluscs. When it invaded the east coast of South-Africa, it caused millions of dollars of damage to the shellfish industry' (G.M. Branch, 2005). A quotation from Australian Marine Life by G.J. Edgar (1997): 'the species, introduced from Europe in the nineteenth century, is an active predator and has probably affected the populations of a number of local animal species.'

Massive: the sunflower star

Picture yourself as a clam lying on the bottom of Deep Cove bay, Vancouver Island. This starfish approaches you. He likes you, as food. But hey, you're a big and strong clam. So no worries.


Wrong! This is the sunflower star (Pycnopodia helianthoides). Full-grown it can be 1 meter wide, have 24 arms and 15.000 tubefeet and weighing in at about 5 kg.


This specimen was ‘only’ 60 cm.


A juvenile of 6 cm.


A few of the more than 10.000 tubefeet. Have you ever been drunk? Than you know how hard it can be to control your locomotion. How does the sunflower controls that many feet? And he is quick: up to 160 cm in a minute.


Juvenile starfish can look quite different from adult specimens. This one looks even stranger: he has has lost a few arms and is regenerating them (the smaller arms). He walks next to an ochre star (Pisaster ochraceus). N.B. I don't think he has lost his arms: he is growing new ones (click here for a new post about Pycnopodia).

If you want to read more about the starfish of Vancouver Island, here is an excellent book:
Sea stars of British Columbia, Southeast Alaska and Puget Sound, 2000. Philip Lambert. ISBN 0-7748-0825-X. ubcpress.ubc.ca. It is cheap (Can $ 27 = € 19) and describes 43 species of starfish. I think there’s no place on earth with that many species of starfish!

Deep Cove bay

I found the plumose anemones (see previous post) on a jetty at Deep Cove bay. The jetty and bridge are overgrown with foolish mussels, Mytilus trossulus (photo above), sea anemones, hydroids, sponges and seaweeds.


Deep Cove bay is in the vicinity of Sydney. We stayed at the Gazebo bed and breakfast, in a village between Victoria and Sydney. A perfect spot for nature and culture-lovers. Have a look at: www.gazebo-victoria.com


The bay is as sheltered as the rocky shore beneath the Wickaninnish is exposed. Because of the exposure both shores are inhabited by different creatures. However, some species like the ochre star (Pisaster ochraceus) and the red rock crab (Cancer productus) are found on both kind of habitats.


The ochre star (Pisaster ochraceus) resting and searching for prey on the bridge.

The jetty seen from above. A lot of people have no idea of the creatures on and under the jetty. For example the sunflower star; a starfish that can grow to 1 meter, with 26 arms and 15.000 tubefeet! More about this starfish in a next message.


The screw of a ship overgrown with hydroids, a colony of animals related to sea anemones.


Even ropes are footage enough for sea anemones, hydroids, seaweeds etc. As you can see, the water was a bit misty and muddy.

I have snorkelled two times at Deep Cove bay for more than 4 hours. After 2.5 hours I swam to the beach to drink and eat something and to get warm: the water felt a bit warmer than at Tofino. At Tofino I measured a water temperature of 10˚ Celsius!

The giant plumose anemone

It can grow up to 1 meter high: the giant plumose anemone (Metridium farcimen). The specimens I saw were not higher than 25 cm. It’s one of my favourite sea anemones because of its fluffy appearance, created by its hundreds of tentacles.


Compare the tentacles of the giant plumose anemone with the green surf anemone (see one of the previous messages).
The thinner the tentacles the smaller its prey. So the plumose anemone needs a lot of tentacles to catch plankton. The green surf anemone lives of individual organic and much bigger prey, like fish, crabs etc.


The short plumose anemone, Metridium dianthus (NL: zeeanjelier), another Metridium-species you can find at the west coast of Canada. This is a Dutch specimen. The two species look very much alike. The big difference is its size. The short plumose grows up to 20 cm, the giant to 1 meter. Find a real big one and you're sure of the species!


Since 1974 I own a cold water sea aquarium. It hosts mostly sea anemones (27 species) and a few starfish, crabs and fish. I have collected the animals myself from all over the world (Europe, New Zealand and South Africa).

Since two years it’s more difficult to take them home because of the regulations at airports: you are only allowed to take home flasks containing no more than 10 cc of liquid. But I still managed to take a few sea anemones and starfish home.
Before we got to the security check I dumped the seawater in a toilet. To my surprise they didn't ask me to open the bucket, after it went through the rontgen scanner. At a store we bought a few bottles mineral water. In the plane I mixed the mineral water with the aquarium salt I brought from home (divided in portions of 1 liter in serviettes), till it was salt enough (I had to taste it...). The animals responded well and the only thing I had to do to keep them well, was to shake the bucket a bit now and then.

The man sitting next to my wife was looking at us as if we where doing something very illegal. But I understand: who is mixing white stuff in serviettes with mineral water in a plane?


The beautiful plumose on this picture is now my guest.

The Lewis's moonsnail and other shells

This is the Lewis's moonsnail (Euspira lewisii). It lives in and on sand where it preys on bivalves like the Pacific gaper (Tresus nuttallii) and other bivalves. I saw this beautiful snail at Deep Cove bay, Vancouver Island.


It’s the biggest moonsnail I have ever seen: up to 14 cm across, shell only.


It grasps its prey with its sticky foot. Then he drills a hole in it, pours an acid in the shell that dissolves the weak tissue of the bivalve. Then he sucks it up.
The moonsnail has a gigantic foot: as you can see the foot is much bigger than the shell itself. For those who do not realise: the shell is the external skeletal structure of this animal (as our internal bones).


Here you see a shell that has an unfinished hole in it. It was attacked by a moonsnail. But he stopped drilling. Maybe because he realized it was already empty? Or he had to leave it because of an approaching sunflower star? In the Netherlands we frequently find bivalves with two, three or more - sometimes unfinished - holes.


This thing that looks like a broken vase are eggcases of the Lewis’s moonsnail. You can see a helmet crab hiding under the eggcases.


After portraying for a few pictures the Lewis’s moonsnail decided he had enough of me and, using his foot as a plough, dug himself in the sand. They frequently go ‘underground’ in search for prey.


The shell of the Pacific gaper (Tresus nuttallii).


This is the siphon, a retractable tube like organ, of the Pacific gaper. With the upper tentacled part he inhales water for food (plankton) and oxygen. With the other - lower - part he emits the 'waste water'. Now you understand why the moonsnail has to dig himself in to search for this kind of prey.

Fish: sculpins and gunnel

From above it looks like a tadpole, in profile it looks like a scorpionfish. There is a fish called the tadpole sculpin, but this is the buffalo sculpin (Enophrys bison). It belongs to a large family of fish called the Cottidae. There is quite a difference in appearance between the approximately 36 species found on the west coast of Canada. See the next pictures of the tidepool sculpin.


The tidepool sculpins (Oligocottus maculosus) I saw were not bigger than 8 cm, the buffalo sculpin can reach a length of 37 cm.




The brown Irish lord (Hemilepidotus spinosus), another sculpin. Brown Irish lord? What's in a name!
Easily overlooked, because he is well camouflaged between the weeds.




The saddleback gunnel (Pholis ornata) was curious, but when he realised what was looking at him, he jumped away and disappeared in the weeds.


Take a good look.

Orcinus orca: the killer whale

Orcinus orca, the scientific name of the orca or killer whale. It means something like the creature from hell.


If I am not mistaken, this is Ruffles. He is the oldest male (they say 60 years old) of the pod. He is called Ruffles for the shape of his dorsal fin (see next picture).


Our captain knew the orca’s by heart because of the form of their dorsal fin. The upper left orca is Ruffles.


This is a male from a resident group of more than 50 orca’s.


Resident orca’s hunt mostly for salmon. Transient orca’s are the ones hunting whales.


We had a wonderful trip on a zodiac with a captain that knew – as he said and did – when to shut up. Just what I like: I want to enjoy them in all quietness.
For some free advertising: www.emeraldsea.ca

The captain trying to catch salmon to lure a bald eagle.


He did catch a lingcod, Bassozetus elongatus, that he released. No good for luring a bald eagle.


The bald eagle.