Predators & defenses
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  Bioluminescence
  Considered in this section is bioluminescence, while other topics on predators & defenses including, including SEA-STAR & NUDIBRANCH PREDATORS, BURROWING, SWIMMING, and TOXIC CHEMICALS/UNPALATABILITY, can be found in other sections.
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Research study 1
 

sea pen Ptilosarcus gurneyi with luminescence depictedSCUBA divers at night know that both sea pens Ptilosarcus gurneyi and sea pansies Renilla amethystina (koellikeri) respond to touch with greenish-yellow flashes or waves of bioluminescence.  The light originates in special luminescent cells that, in sea pens, are located in both autozooid and siphonozooid polyps.  In comparison, in sea pansies the luminscent cells are located only in the siphonozooid polyps.  The author notes a tendency in both species for light to be directed into the water current. By its production in the polyps and, thus, its outward direction of illumination, a communicative function is suggested, although defense cannot be ruled out.  Morin 1976 p.629 In, Coelenterate ecology and behavior (Mackie, ed.) Plenum Press, NY.

NOTE  bioluminescence in Ptilosarcus gurneyi is generated when a luciferin-type compound coelenterazine is oxidised in the presence of a luciferase-type enzyme.  The reaction products are coelenteramide + carbon dioxide + LIGHT.  The light travels along the rachis at 26 cm . sec-1 (at 20OC).  Davenport & Nicol 1956 Proc Roy Soc Lond B 144: 480; Shimomura & Johnson 1979 Comp Biochem Physiol B 64: 105

 


The flashes depicted here are actually waves of light
that travel the length of the rachis from bottom to top 0.3X

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But communication with what?  and for what purpose?  Here are a few possibilities for the function of the bioluminescence, some obvious; others, not so obvious. Think about them, then CLICK HERE to see explantions for them. Ideas from Morin 1976 p. 629 In, Coelenterate ecology and behavior (Mackie, ed.) Plenum Press, NY. 

A means of communication with conspecifics, e.g., to synchronise spawning or to warn of approach of predators. 

A means to attract planktonic prey. 

A means to scare off predators. 

A type of “burglar alarm”, in that predators of an animal attacking the sea pen may be attracted to the scene and eat the attacker. 

A kind of aposematic (warning) coloration. 

NOTE  lit. “away signal” G., referring to bright, easily visible colours displayed by toxic animals (and some plants) that convey a “do not eat, for I am unpalatable and will make you sick” message to predators with colour vision.  A familiar example of aposematic colouration is the black and yellow markings on bees and wasps

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Research study 2
 

Bioluminescence in sea pansies Renilla amethystina (koellikeri) originates in both siphonozooids and autozooids.  The light manifests itself as a wave or series of waves radiating over the rachis from any point of stimulation, with many different patterns (see depictions below). In fact, under strong repetitive stimulation (as from 100msec shocks of 80v) the colony may enter a self-propagating excitatory state, “frenzy”, in which waves of irregular and constantly changing form course over the rachidial surface for up to an hour (at 20-25OC). The multidirectional spread of luminescence assumes that a non-polarised nerve net in the rachis is involved.  The author of the study estimates that over 6m of nerve net are required to conduct excitation across an average-size colony during the passage of a wave of light emission.  Interestingly, in summer the autozooids retract upon being touched and produce a long-lasting glow; only the siphonozooids emit waves of light. In comparison, in winter the colonies do not retract the autozooids and light is emitted from both types of polyps.  Different polyp clusters may respond with different levels of brilliance.  Buck 1973 Biol Bull 144: 19.

NOTE  a large rachis may contain up to 500 autozooids and 2,000 siphonozooid clusters comrising perhaps 15,000 individual polyps

NOTE  this is confirmed in earlier studies in La Jolla, California on Renilla amethystina (koellikeri). Nicol 1955 J Exp Biol 32: 299; Nicol 1955 J Exp Biol 32: 619

The depictions below illustrate different wave patterns and "frenzies" manifested by mechanical stimulation of Renilla:

diagram of one type of luminescent waves moving across the rachis of a sea pansy Renilla amethystina
Depolarisation wave moves from L to R in the rachis (Left-hand image), and is reflected from the Right-hand side of the rachis. The wave is self-propagating, and may bounce back and forth in several cycles diagram of one type of luminescent waves moving across the rachis of a sea pansy Renilla amethystina Volley of waves induced by continual mechanical stimulation at X
diagram of one type of luminescent waves moving across the rachis of a sea pansy Renilla amethystina
3 stages in collision and cancellation of 2 luminous waves induced by mechanical stimulation at X diagram of one type of luminescent waves moving across the rachis of a sea pansy Renilla amethystina Simple "frenzy" in which a single wave bounces back and forth across the rachis
diagram of one type of luminescent waves moving across the rachis of a sea pansy Renilla amethystina A type of "frenzy" wave employing both convex- and concave-forward segments, which reverse after reflecting off the other side of the rachis diagram of one type of luminescent waves moving across the rachis of a sea pansy Renilla amethystina A luminescent "propeller" rotates around the surface of the rachis
Successive stages in a "frenzy" with 3 excitation centres, giving the impression of boiling luminescence    
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