Amphioxus Oxytocin

For the moment, this page will be mostly speculation.  I intend to come back at some time in the future to flesh it out. 

Citation 6<110 of 276<278 OxRcpDistCite  says "The amphioxus locus encodes a single neurohypophysial hormone, designated as [Ile4]vasotocin. Although it's been given the vasotocin name, presumably on the basis of its structure, my speculation is that its function is reproductive. 

Quoting from:
   Oxytocin Receptors  

    "The earthworm Eisenia foetida is the most primitive species from which an OT-related peptide (annetocin) has been isolated ( 429 ). Injection of annetocin in the earthworm or in leechs results in induction of egg-laying behavior ( 430 ). "  

    "The above shows that oxytocin evolved before the split between the Protostomes   and the Deuterostomes.  What's more, it suggests that the original function of oxytocin-vasopressin was reproductive rather than water-balance.  If so, then the water-balance function of human vasopression evolved after the reproductive function of human oxytocin.  Further, it also suggests that the water-balance function only evolved when the lamprey swam inland to spawn and needed to adjust to the change in osmotic pressure from a salt water environment to a fresh water environment."      

Quoting from:  Oxytocin Receptor Distribution .     

Arginine-vasotocin stimulates smooth muscles from a wide variety of vertebrate species. It can stimulate contraction of oviducts from many jawed fishes and tetrapods."  

    I thought I had references for what follows, but apparently I don't.  So, I'm going to go ahead and offer what I think to be true and, hopefully, come back later with supporting references. 

    The amphioxus lives in densely populated colonies, and, unlike fish schools, these colonies do not perform any sort of mutual defense function.  The predators which feed on the amphioxus are rays which suck up and filter the sand in which the amphioxus burrows.  The amphioxus' defense is to sense the approaching ray and burrow more deeply into the sand.  The function of the amphioxus colony is sexual, rather than defensive. 

    Animals which defend themselves with shells are immobile, and when it comes time to breed, they pump vast clouds of sperm and eggs into the surrounding water.  This is inefficient, since the sperm must find the eggs in a large volume of water.  Freedom from a heavy shell allows the amphioxus, both male and female, to swim to the surface and release their sperm and eggs into a fairly restricted volume of water.  See:   Amphioxus Behavior .  The close proximity of individuals within the colony allows them to coordinate their breeding. 

    It would seem from this that the affiliational function of oxytocin is, at least in the amphioxus and its immediate descendants, essentially sexual.