Sunday, December 28, 2008

Strange Little Spirula

We can safely say that Spirula is an unusual coleoid cephalopod with a ventrally curved ("endogastric") planispiral shell, a vestigial radula, a photophore on the tip of the mantle, and oegopsid (cornea lacking) eyes (Warnke and Keupp 2005, Warnke 2007, Young 1996). That's about the limit of what can be said concisely...


The internal planispiral shell of Spirula, taken from scamazine's flickr. While this shell looks similar to those of the extinct ammonites, it curves ventrally rather than dorsally. This illustration shows how the shell fits into the animal.


Spirula was formerly considered to contain several species (see here) but now only S. spirula is valid. Spirula has two disjunct populations in the Atlantic and Indo-West Pacific Oceans; a molecular study of intraspecific variation suggests that individuals from Fuerteventura and New Caledonia are distinguished to a degree typical of separate species (Warnke 2007). The lifespan of the species is only ~20 months, making gene flow appear to be unlikely between the populations, but individuals closer geographically have not been investigated yet (Warnke 2007). With the sparse molecular and morphological data on Spirula variation, much more work needs to be done in order to (re-)establish multiple species.

So now that we haven't resolved the number of Spirula species, what exactly is it? Spirula certainly is a unique looking cephalopod and many sources (e.g. Wikipedia) place it in a monotypic order. Warnke and Keupp (2005) suggested that Spirula is the most basal decapodiform and further suggest that it can be used to study ammonite development! There are prominent morphological distinctions between the groups - ammonites tend to have shells with exogastric coils (heteromorphs are of course exceptions) and four prismatic layers while Spirula has a shell with endogastric coiling and two prismatic layers (Warnke and Keupp 2005). However, the initial chambers (protoconch) of ammonites and Spirula show morphological similarities and the mode of mineralization appears to be the same (Warnke and Keupp 2005). Warnke and Keupp (2005) cite prior preliminary molecular evidence to support the notion of Spirula being the most basal decapodiform and imply that similarities are plesiomorphies. Later molecular studies contradict the placement used by Warnke and Keupp (2005) but the issue of Spirula/ammonite similarities being homologies or homoplasies is still unclear. Spirula development will certainly be an interesting topic to investigate regardless.

More recent molecular analyses suggests that Decapodiformes consists of two orders (rather than 4) - Sepioidea (containing Sepiidae, Myopsida, Sepiolidae, Idiosepiidae, Spirula) and Teuthoidea (Strugnell et al. 2006). Spirula consistently grouped with Sepiidae and it should be noted that the proposed clade has the synapomorphies of sperm placement in females and the structure of the statoliths and tentacle clubs (Lindgren and Daly 2007, Strugnell et al. 2005). Their next nearest relative is either Myopsida or Sepiolidae, but either way Spirula is fairly nestled within the Decapodiformes (Lindgren and Daly 2007, Strugnell et al. 2005). The earliest probable member of Sepioidea is the early Carboniferous Shimanskya which is either a member of the Spirula-lineage or another taxa that convergently lost the nacreous layer (Strugnell et al. 2006). This fossil and the molecular clocks suggest that many clades are far older than previously thought, e.g. Sepiidae was previously thought to have originated in the Oligocene but apparently diverged around a couple hundred million years earlier (Strugnell et al. 2006)! Supporters of the Spirula-is-basal camp could argue that this incredible revision suggests that Spirula is not placed correctly, but other evidence in Strugnell et al. (2006) implies that many decapodiform lineages are similarly ancient. Undoubtedly this will undergo further revision, but it seems very unlikely that Spirula is the most basal of the Decapodiformes.

Despite apparently existing in large numbers, much of the life history of Spirula remains poorly known (Lukeneder et al. 2008). Study of shell isotopes suggests that the juveniles are born in waters >1000 m deep, migrate to warmer waters from 400-600 m as adults, and then migrate back down into deep and cold waters (Lukeneder et at. 2008). It's worth pointing out that Lukeneder et al. (2008) suggest that future studies of the Spirula life history can be applied to ammonites...



References:

Lindgren, Annie R. and Daly, Marymegan. 2007. The impact of length-variable data and alignment criterion on the phylogeny of Decapodiformes (Mollusca: Cephalopoda). Cladistics 23, 464-476.

Lukeneder, Alexander et al. 2008. Stable isotopes (18O and 13C) in Spirula spirula shells from three major oceans indicate developmental changes paralleling depth distribution. Marine Biology 154, 175-182.

Strugnell, Jan et al. 2006. Divergence time estimates for major cephalopod groups: evidence from multiple genes. Cladistics 22, 89-96.

Strugnell, Jan et al. 2005. Molecular phylogeny of coleoid cephalopods (Mollusca:Cephalopoda) using a multigene approach; the effect of data partitioning on resolving phylogenies in a Bayesian framework. Molecular Phylogenetics and Evolution 37, 426-441.

Warnke, Kerstin. 2007. On the Species Status of Spirula spirula (Linne, 1758) (Cephalopoda): A New Approach Based on Divergence of Amino Acid Sequences Between the Canaries and New Caledonia. IN: N. H. Landman et al. (eds.). 2007. Cephalopods Present and Past: New Insights and Fresh Perspectives, 144-155. Springer.

Warnke, Kerstin and Keupp, Helmut. 2005. Spirula—a window to the embryonic development of ammonoids? Morphological and molecular indications for a palaeontological hypothesis. Facies 51, 60-65.

Young, Richard E. 1996. Spirulida Haeckel, 1896, Spirulidae Owen, 1836. Spirula spirula Linnaeus, 1758. Version 01 January 1996. http://tolweb.org/Spirula_spirula/19989/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/



Spirula can also be seen bearing gifts on my Christmas card. Unusually among cephalopods, the buoyant shell puts the animal in a head-down orientation (presumably because it had bottom-based ancestors?). I accidentally depicted it without skin of the mantle.

15 comments:

Three Ninjas said...

Hi, please forgive my ignorance of this subject.

I'm assuming that exogastric means "outside the stomach", so ammonites wore their shells on the outside like snails or nautilids, instead of internally like Spirula. Am I understanding that correctly?

My question is, how do we know that ammonite shells were exogastric? Doesn't Spirula raise the possibility that ammonites had endogastric shells? The only ammonite fossils I've seen consisted only of shells. I haven't seen any fossilized soft parts, at least not that I can remember, so I'm curious as to how we can tell that it was one way instead of the other.

Thanks for your interesting blog, and I'm sorry if this is a dumb question!

Cameron McCormick said...

Mollusc people use the terms endo/exogastric in reference to the relative position of the shell apex and the mantle cavity - I found a
book
which hopefully clarifies this. I think that the description of "dorsal/ventral" placement makes things a lot more clear!

As for internal/external placement, ammonite soft remains and muscle scars have been recorded from inside the body chamber of the shell. It's possible that the two prismatic layers in Spirula are related to the internal placement of the shell, in comparison to the four layers in ammonites.

You certainly didn't have a dumb question - it forced me to re-affirm facts which I had taken for granted.

Three Ninjas said...

Thanks Cameron! That is very helpful indeed.

Unknown said...

Hello there! This is my first comment here I truly enjoy reading through your posts. Can you suggest any other blogs/websites/forums that deal with the same subjects?

Obat Degenerasi Makula Herbal Terbaik Masa Kini
Obat Herbal untuk Menyembuhkan Encephalitis
Obat Herbal untuk Mengatasi Galaktorea
Obat Flu Singapura Herbal
Obat Hemofilia

native american jewelry said...

I am speechless after seeing these pictures! I love them all! I teach kindergarten and I'm going to make a theme, and photographs have given me so many ideas! You are so talented! Thanks!native american jewelry

can i refinance said...

Art & More fertigt Ihren Bilderrahmen oder Ihr Passepartout auch in Sondergrößen. Sonderformate bei Wechselrahmen, rahmenlosen Bildhaltern oder Passepartouts sind jederzeit möglich, wir liefern Ihnen jedes gewünschte Format.can i refinance

JuraganUser said...

Good information, thank u and please visit back my article~!
Cara Mengempeskan Dan Menghilangkan Benjolan Di Kelopak Mata Atas, Cara Menghilangkan Benjolan Di Langit-Langit Mulut Tanpa Operasi,

Anjali Arya said...



Our client can choose any of them as per their convenient. We are also open to send our angels any other place of your choice. Russian Escorts in Vasant Kunj are waiting eagerly to meet you. You need to decide whether you want to meet them at home or at any hotel. If you are facing any challenges in arranging any place, let me inform you that we have tie-ups with 3 start hotels as well as 5 start hotels. Check our other services...
Russian Escorts in Vasant Kunj
Russian Escorts in Vasant Kunj
Russian Escorts in Vasant Vihar
Russian Escorts in Vasant Vihar
Russian Escorts in Vasant Vihar
Russian Escorts in Vasant Vihar

mayaescorts said...

Truly, what you are perusing is all evident. Mahipalpur escorts will get it going for you without a doubt. Our Mahipalpur female escort’s principle point is to give you an accomplice that satisfies both your sexual and feelings. One who can tune in to u and quiet you down and support you. Our organization has young ladies that no other escort specialist co-op can introduce before you. Our lovely Escorts in Mahipalpur are stunning and have delicate bodies that can retain our torment and stress from your body totally.
#mahipalpurcallgirls #Mahipapurescorts #Escortsinmahipalpur
Mahipalpur Escorts
mahipalpur call girls
Mahipalpur Escorts

Ishika Sharma said...

The Bold Call Girls in Kalkaji incorporate housewives, Air master, models, understudies, teenager young ladies, and so on by which you can appreciate each snapshot of delight all through the meeting. These Kalkaji Escorts are delightful as well as gives the unchallenged and cute administrations which can make you even experience passionate feelings for. These administrations are such a great amount of committed to causing you to feel enthusiastic and you can make you each action being energetic.

Ishika Sharma said...

The Best Vasant Kunj Call Girls comes in the best reach and you can peruse for the best on our official site. Pick anything you desire since this time, just you pick is significant for the hookups.

Ishika Sharma said...

For feeling relaxed and happy many men try availing Kalkaji escorts service. Escorts are the best solution when it comes to making your day happier.

Best Trending Products said...

Nice post! This is a very nice blog that I will definitely come back to more times this year!
Bestsofacovers.com provides a collection of the Best Sofa Products, Accessories, Reviews & Recommendations. Such as Sofa Covers, Pet Sofa Cover, Sofa Slipcover, Sofa Accessories & More. We have collected only the best sellers from amazon based on customer reviews and sales.
Best sofa Covers
Cheap sectional sofas under 500
Cheap sectional sofas under 300
Best couch for back support

Delhi escorts said...

Delhi Escorts Service Call now 9810927890
We must secure our everything escorts and Services as a private and scholarly movement that we do make are not just cleanliness to certain the Independent chanakyapuri Escorts and Independent Delhi Escorts of your decision will give the most real. We had a gathering of extremely refined and affirmed Green Park Escortsand young pleasures that not simply you should live with remarkable conditions. Like millions, you can rely on us for that where your everything sort of want and dreams come in genuine with hot hauz khas Escorts. It is safe to say that you are a representative and are intending to take an aerocity escorts? Be that as it may, directly on, in light of the fact that we don't make any talking with any client with us, the upkeep given by Connaught Place escortsService is particularly It is awesome.

Lajpat Nagar Escorts Service said...

Probably the best Females Lajpat Nagar escorts are horny to give you delight. They need you to appreciate the best of sexual joy which knocks your body and socks off. They need you to press their body which gives them the best of suggestive inclination and makes them increasingly horny. Our Call Girl Service in Kalkaji Escorts are attractive and upscale which gives you a genuine sweetheart like organization.

Lajpat Nagar Escorts Service
Kalkaji Escorts Service
Ashram Escorts Service
Paharganj Escorts Service