A spongy habitat

By Danielle Ludeman

The world is full of organisms, living on organisms, that are living on other organisms.  You just have to take a moment to think about the complexity of life that can occur to start to appreciate all of the life around us.  Take a tree in your front yard – at first glance you may just see a tree, but when you start to look closer you notice the bird nest that will be home to baby chicks in the spring, and the squirrel that runs up and down the branches.  Then you notice all the different types of moss, lichen, and mushrooms that are growing on the tree.  And upon closer inspection you realize that this creates even more space for a variety of spiders and insects to thrive.  And we can keep going on and on to include all of the life that we need a microscope to see. And this is just on a single tree!

This summer, while doing some field studies at Bamfield, I began to appreciate all of the life that can be found on a single sponge.  Now it is well known that sponges can be very important habitat for many organisms, with some species being obligate commensals of sponges, meaning they can ONLY live on a sponge to survive.  But when I started to look closer at some of the sponges in my studies, I began to realize just how many other organisms call a sponge its home!  One species of sponge in particular – Suberites sp.  that I collected off of Brady’s beach – seemed to have a surprise guest visiting every time I looked at it!  I managed to photograph a few of these, and thought I would share these with you in the slideshow below!

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Baby Urchin!

By Nicole Webster

This guy speaks for himself, and is another prize from Ceratostoma foliatum (leafy hornmouth) cleaning:

Credit: N Webster

Its a baby urchin! It is too small to determine the species, but there are three common urchin species found in this regions, the purple (Strongylocentrotus pupuratus), red (S. fransiscanus) and the green (S. droebachiensis) sea urchins. Colour is the main way to tell them apart, although some greens can be a bit purple-ish, and the reverse may also be true. Spine length and tube foot arrangement can also be used to try to distinguish species.

Here you can see the joints of where the spines attach to the test, and the tiny ossicles inside the tube feet ends. Credit N. Webster

Each spine on an urchin is independently moveable, and separated from the body (test) of the urchin by a small joint of tissue with muscles. In the presence of a predator, they can all point towards it to increase cost of attacking one.

I can hear you say aww! from here.

Baby Crepidula!

By Nicole Webster

As tedious and eye-straining as it is to clean my Ceratostoma foliatum shells of epibionts (So I can get accurate weights), I keep coming across all sorts of wonders.

Yesterday my prize was a tiny slipper limpet (Calyptraeidae). It is too small  (3mm in length) to be sure of an identification, but Crepidula sp. seems most likely due to the shape of the ‘shelf’ on the inside of the shell.  Once I peeled him off (chipping his shell), he motored around the dish a good pace, making photography difficult. Once I flipped him upside-down, he struggled valiantly, so I still had a hard time capturing his charm.

Aww! (note the prominent protoconch (larval shell) at the apex) Credit: N Webster

“Let me up!” (a clear view of the large, filtering gills) Credit: N Webster

This is the empty shell showing the typical ‘slipper’ shelf of the shell. Credit: N Webster

Snails in the family Calyptraeidae are not true limpets, but have convergently evolved the same low profile, high expansion shells good for sticking strongly to the substrate. The limpet morphology has evolved at least seven times (depending on the phylogeny you use) in Gastropods.

Calyptraeids are sequential hermaphrodites, most famous for the stacking Crepidula fornicata, in which many smaller males stack on top of a larger females in a mating aggregation. As the males grow, they transform into females and can start their own stack. This little guy is probably too young to be sexually mature, and is perhaps more properly addressed as ‘it’.

The gills are so large and prominent because most Calyptraeids are filter feeders, using their gills to trap plankton, and sucking it in their mouth on a stream of mucous from their radula.

Cool Papers! Part 2: Hermit crabs with a not-so ‘hermit’ lifestyle

By Danielle Ludeman

‘Hermit’ crabs seemingly implies that they are just that – hermits – living alone in their own (second-hand) shell, apart from society (i.e. their kin).  But are they as alone as their name implies?

According to a new study by Mark Laidre, terrestrial hermit crabs are forced to socialize if they want to find a new shell to move into! Carrying around giant shells on land would be heavy, and so terrestrial hermits, unlike marine hermit crabs, modify their gastropod shells by eroding the interior and creating a larger and more lightweight home. This also takes time, and so hermits will move into previously modified shells, if they are available, when it is time to move into a larger one.  To look at the consequences of this modification, Mark Laidre placed hermits in either 1) new unmodified shells or 2) previously remodeled shells of the same diameter, and found that only the hermits in the shells passed down from other hermits survived! The hermits in the unmodified shells could not fit into the small amount of space available, and thus they did not have full protection against ant attacks. There are virtually no unoccupied remodeled shells in the population, so when it comes time to moving on up into a bigger shell the hermits benefit from being around others if they want any hope in finding a previously remodeled shell!

Laidre calls this remodeling of the shell ‘niche construction’ (the process in which an organism changes its environment).  According to the fossil record, this ‘niche construction’ has been taking place in terrestrial hermit crabs for millions of years! Laidre argues that that is ample amounts of time for such a trait to start to drive social behavior, so that these remodeled shells become a form of ‘ecological inheritance’, where modification of the environment is reused by many successive generations.  Unlike genetic inheritance, ecological inheritance can be passed on to individuals that are unrelated.  These remodeled shells, therefore,may be a driving force for social dependence, even among unrelated individuals.

As it turns out, the need to find a new shell when a hermit crab grows drives another form of social behavior as well!  When a new shell becomes available in the environment, many hermit crabs gather round forming something called a ‘vacancy chain’. The hermit crabs will essentially line up from biggest to smallest (albeit in a not-quite so elegant fashion), taking the next biggest shell as it becomes vacant!

Mystery Arthropod

By Nicole Webster

My squishy mystery on first inspection ~1mm in length Credit: N Webster

While sorting out my hatchlings (Nucella lamellosa and N. ostrina) I came across something bizarre. A tiny red dot to the eye, a grotesque writhing blob under the microscope. What is it? Although difficult to see, it clearly has segmented appendages in the anterior region, making it an arthropod. If it was just a segmented body, then it could be an annelid (polychaete) worm, which would not have surprised me considering their amazing diversity of shapes and colours.

Ventral view showing body segments. Length 1mm. Credit: N Webster

Past that I’m rather unsure. It may have two pairs of antennae, meaning its not an insect.

Dorsal view, segmented antennae visible. Length ~1mm Credit: N Webster

Why would I even think its an insect at all? I found it in the ocean! Well there’s a rather cool fly in the genus Oedoparena whose larvae eat barnacles. I have been feeding my hatchlings barnacles, so perhaps. It doesn’t look quite right though when compared to the only drawing I could find from  the Habits and Life history of Oedoparena glauca (Diptera, Dryomyzide), a Predator of Barnacles:

5th instar of Oedoparena glauca. Much larger and more uniform morphology than my specimen. From Burger et al. 1980

The invertebrate world is full of wierd and wonderful beasts, and there’s no cheat sheet. I wish I knew what it was!