(via the Monterey Bay Aquarium)
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
Spiders are amazingly sophisticated animals, and probably the premiere complex adaptation of modern spiders is the ability to spin silk. They have multiple internal glands that can produce multiple kinds of silk — webs contain different kinds, from structural strands to adhesive strands, and other kinds are used for spinning egg cases and for wrapping prey — and they are sprayed out through small spigots mounted on swiveling spinnerets, which are modified opisthosomal (abdominal) limbs. Obviously, these detailed features did not spontaneously appear all at once, but had to have evolved progressively. A couple of fossils have recently been described that reveal a) silk spinning is ancient, from at least the Permian, but that b) these early spiders did not have the full array of modern adaptations.
Here is a pair of fossils: Permarachne novokshonovi, from the Permian in Russia, and a more recent specimen, and Palaeothele montceauensis, from the Carboniferous in France. Both are eight-legged arthropods, and if you saw one scuttling about now you wouldn’t hesitate to call them spiders. There are some differences, though: Permarachne in particular shows a little less tagmosis, or fusion and specialization of segments, than we usually see in spiders, and it also has that prominent flagellum (which is completely different from a bacterial flagellum!), a long segmented ‘tail’ covered with sensory hairs that was probably a sense organ; it has no sign of a web-spinning function.
What about the production of silk and webs in these old spiders? Here’s another specimen, Attercopus fimbriunguis, a 376 million year old fossil. It’s a little less dramatic because these are fragments of cuticle that have been carefully extracted by dissolving the rocky matrix with acid; it means, unfortunately, that it is more fragmented, but the advantage is that now we can zoom in microscopically and see far more detail in the structure. What we can now see in pieces of the ventral plates of the opisthosoma are small spigots, and in a few cases, there are even strands of spider silk still extended from these pores. In F, there’s also a nice shot of a chelicera (fang) from the spider — it’s wicked sharp, but the small holes seem to be preservation artifacts, and there’s no sign that venom secretion, another important spider adaptation, has evolved yet.
One of the critical observations here is very simple: no spinnerets. These spiders did not have the modified limbs with sets of spigots that we see nowadays, but instead, had a series of spigots arrayed across the bottom of the abdomen. They almost certainly were not able to make webs: what they could have done was produce sheets of silk, of the kind that could be used to make egg cases or wrap around prey. These are another example of a transitional fossil, forms that have only some of the capabilities of a later organism.
(via Cheshire, who promises to have his own post on this paper soon.)
Selden PA, Shear WA, Sutton MD (2008) Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order. Proc Nat Acad Sci USA 105(52):20781-20785.
Squid don’t just make sperm: they package it up into fairly elaborate little torpedoes called spermatophores, which are either handed to the female with a specially modified arm called the hectocotyl arm, or squirted onto her with a penis. Once on the female (or a male, it really doesn’t matter), the spermatophore everts, forming a structure called the spermatangia, in which all the packed sperm uncoil, ready to do their job, and the whole mass is anchored to the target with a cement body. These structures do show species-specific differences, but here is one example from Heteroteuthis dispar.
Now the curious observation: squid are often captured festooned with spermatophores and spermatangia, and in many cases, the spermatangia may be imbedded deeply into the musculature of the animal — so it’s not simply as if the spermatophores are lovingly placed in an appropriate orifice, they are piercing the female (or the male, again, they don’t care that much), tearing deep into the interior. The question is, how do they get in there?
A few simple observations have revealed the answer. Spermatophores can be triggered by a gentle squeeze, at which time all of their fertilization machinery will fire. Here are some photos of some spermatophores going to work on a squid carcass.
(A) Placement of spermatophores on a dead male specimen of Moroteuthis ingens (mantle length ~300(Read the caption carefully. That’s a human triggering sperm to ejaculate into a dead male squid. It’s gay necrophiliac bestiality! You don’t see that in the papers every day.)
The answer is that spermatophores also release digestive enzymes and actively burrow into the target tissue. Squid sperm show an aggressive persistence and vigorously active assault on the female body that our own pathetic human emissions lack…I feel a little inadequate, but I’m sure women are a bit relieved.
Another interesting observation is the function of the squid penis. It seems to be less an intromittent organ than a kind of hose to direct the ejaculations onto the female. In natural situations, unlike the photographs above, it is responsible for initiating the spermatophore reaction. Each spermatophore has a threadlike extension of a surrounding membrane, and tugging on that triggers the reaction. It’s like a squad of paratroopers leaping out of a phallic airplane, each attached by a static line that yanks the rip cord as they emerge.
Hoving HJT, Laptikhovsky V (2007) Getting under the skin: autonomous implantation of squid spermatophores. Biological Bulletin 212: 177-179.
If I weren’t so heavily invested in the cephalopods, siphonophores would be a great alternative.
(from Mark Whittaker’s personal collection of dive photos)
(from Strugnell, JM, Rogers AD, Prodo PA, Collins MA, Allcock AL (2008) The thermohaline expressway: the Southern Ocean as a centre of origin for deep-sea octopuses. Cladistics 24:1-8)
