Species species of the Week week # 8 OR Tautonomical Beach Adventure Times!

The faithful among you will recollect that months ago I alluded to an upcoming blog post featuring surprise organisms from my trip to New Zealand.  Well that time has arrived!  I went on a couple of stellar walks in the Waitakere Ranges just west of Auckland. On a hike back along the black sand of Karekare beach, I was stunned to see this breath-taking site (Figure 1):

Figure 1. Oh for God's sake, she's going to make us look at more shells.

I know, right?!

Ok, but look more closely (Figure 2): 

Figure 2. Hmm.  This purple slightly depressed globose shaped shell with countershading reminds me of a blog post I wrote once...

HECK YES! Back when I wrote that post about how awesome the aviator snail/Baggins snail (Janthina janthina) is, I didn't ever expect to actually see it. I wasn't even really paying attention, or reading my field guide closely enough to realize I might see it. When I walked out onto the beach and saw them all strewn about like so much flotsam and jetsam, I had to pick my jaw up off the ground long enough to take some photos!

But, lo! What else have we here (Figure 3)?

Figure 3. Why, we have a planar equiangular spiral shell approximately 3cm at the widest diameter, Charming for sure, but, Emily, why do you show us this beauty? That's right, because it's a tautonym!

Oh no f in way!  This lovely segmented shell is that of: 

Spirula spirula!

A cephalopod ("head-foot", people) resembling a squid(1), the organism itself is actually quite adorable (Figure 4). 

Figure 4. Tiny!  The shells I found were about 2-3 cm in diameter, which means the actual  S. spirula was much larger than this one.  You can see the internal shell toward the top right of the photo, oriented perpendicular to the camera. Photo from deapseanews.com.

Unlike most mollusks, S. spirula(2) doesn't use this shell for protection, but for buoyancy regulation. Much like in the external shell of a nautilus (nope, this one), the chambers are separated by septa(3) and connected by a long cord called the siphuncle  (Figure 5, soooo many figures!).

Figure 5. Shell anatomy of S. spriula ("rams horn shell"). The siphuncle regulates the gas pressure in each chamber, allowing S. spirula to maintain neutral buoyancy at whatever depth it is found.
(Image from http://www.metafysica.nl/ after Clarkson 1979 Invertebrate Paleontology and Evolution)

The animal usually floats head- (and tentacles/arms-) down, and there is a light-sensing organ on the end of the mantle (above the shell in Figure 4). They live deep (~1,000m) during the day and migrate closer to the surface (~100-300m) to feed at night.  This helps them avoid things with eyes that would like to eat them during the day - like fish. This is an EXTREMELY long daily migration: 90,000 body lengths each way! JEEZUM, that is like a human swimming from Philadelphia to New York and back every night, not accounting for traffic!(4)

Actively swimming this distance would require 1x10^23 bowls of Wheaties(5). How does S. spirula manage without the Breakfast of Champions ("...and so on")? Aha! This is where the shell comes into play: S. spirula can reduce pressure in the chambers of the shell, increasing buoyancy, and the animal can pop up like a scuba diver (well, safely, of course), without having to waste energy swimming. Hopping on the elevator instead of taking the stairs - amiright? There's a theory that Sperm whales use spermaceti to help them sink in a similar way(6).

There's some uncertainty about these guys' range. When they die, the shells often float to the surface and can drift really far. So who knows where those shells on the transcendent black sand beaches of paradise-I-mean-New-Zealand came from? Anyone in NZ want to offer me a postdoc? Worth a shot.

References and Miscellany:

(1) Perhaps surprisingly, at least to me, in spite of how similar this shell is in structure and function to that of a Nautilus, molecular evidence suggests that Spirula is more closely related to other squids and octopusses/octopodes than to Nautiluses, and relatively recent addition to the cephalopods. The shell appears to be an ancestral character that has been lost in many of the other cephalopods, but maintained and internalized in Spirula and in a somewhat altered form in Sepiida (cuttlefish).

(2) Incidentally Spirula spirula (Linneaus, 1758) is indeed the only member of the genus Spirula, all other taxa having been synonymized, lending support to my theory of tautonyms - just sayin'.

(3) Yeah, Philadelphia, you know what I'm talking about.

(4) I think. I'm new at these comparison things that science communication people say are so effective at making numbers less abstract. Here's my logic: I figured 900m vertical distance divided by 3cm body length (figure 4) is 90,000 body lengths. A human is roughly 5.75m, 90,000 human body lengths is ~98mi. But looking around to figure what that people have an idea of that is 98 miles is the hard part, and truthfully, google says it's only 96.3mi, but really you get the idea. Holy Michael Phelps, Batman! Actually, that dude is a sprinter, here is the real long distance heroine.

(5) OK, that number I just made up.

(6) Clarke 1970 Nature. 228:873.