Predators & defenses
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  Burrowing
  Considered in this section is burrowing, while other topics on predators & defenses, including SEA-STAR & NUDIBRANCH PREDATORS, SWIMMING, BIOLUMINESCENCE, and TOXIC CHEMICALS/UNPALATABILITY, are found in other sections.
 
Research study 1
 

photograph of a sea pen Ptilosarcus gurneyi buried in the sedimenthistogram showing burrowing responses of sea pens Ptilosarcus gurneyi to the presence of 3 different sea star speciesAlthough commonly described as "burrowing", the hiding in the sediment of sea pens Ptilosarcus gurneyi in response to predators is mostly a withdrawal into burrow space1 already occupied within the sediment, enabled by massive loss of water via the siphonozooids.  About one-third of the total length of the colony is usually buried, and this must represent the length of the internal skeletal rod because it pulls down into the burrow along with the rest of the animal.

A study at the Bamfield Marine Sciences Centre, British Columbia assesses levels of predatory “threat” of 3 asteroid species to Ptilosarcus.  In both field and laboratory tests involving physical contact with a known “specialist” predator of sea pens, the leather star Dermasterias imbricata, histogram showing degree of responses by sea pens Ptilosarcus gurneyi to the presence of sea stars Dermasterias imbricatacauses 73% of sea pens to rapidly withdraw into the sediment (histogram on Right).  Contact2 with a “generalist” predator, the sunflower star Pycnopodia helianthoides, causes only 23% of the sea pens to withdraw, and contact with a known non-predator of sea pens, the ochre star Pisaster ochraceus, elicits no response at all from sea pens. Control touches with a clean glass rod elicit no responses. The authors conclude that sea pens are able to distinguish3 between relative threat levels and adjust their behaviours accordingly.

The authors note that withdrawal into burrows in the field is not an “all-or-none” response.  It may begin with localised contractions, followed by partial burrowing, and then complete burrowing as shown in the histogram4 on the Right. A noteworthy observation in this study is that exposure of sea pens to water-borne chemical cues from leather stars and sunflower stars Pycnopodia helianthoides held at a distance elicit no responses. Under these circumstances the sea pens must also be touched with a glass rod.  Then the sea pens quickly burrow (60sec in laboratory conditions).  The authors conclude that physical contact with predatory sea stars is required to trigger defensive behaviours in Ptilosarcus.  This can be construed as an energy-saving strategy, not just in the costs of deflating and inflating, but also in feeding time that is lost while buried.  Weightman & Arsenault 2002 Can J Zool 80: 185.

NOTE1  the force required for a Ptilosarcus to create its burrow has never been measured, but it must be considerable, especially in rough shell/gravel deposits as in the above photo

NOTE2  tests involve touching a sea-star's arm to a leaf of the sea pen located halfway along the rachis, and holding contact for 5sec. Control touches are done using a clean glass rod

NOTE3  while the authors remark that theirs is the first study to provide experimental evidence for the ability to distinguish  predators from non-predators in cnidarians, such differentiation is, in fact, well known in many related anthozoans (e.g., sea anemones and sea pansies).  It has been termed “predator recognition” or, more lately, “predator classification”, although the terms appear to mean the same thing.  A comment to this effect is published by Dalby & Elliott 2003 Can J Zool 81: 556 and a reply to their comment is published by Weightman & Arsenault 2003 Can J Zool 81: 559.  The dialogue is interesting in that it discusses in general terms the adequacy of literature review in science

NOTE4 the data presentation by the researchers is somewhat misleading, in that they only tested a total of 11 sea pens. Thus, of 11 sea pens tested in the field for response to being touched by a leather star, 1 shows partial burrowing and is shown on the histogram as 9%. Use of the statistic percentile in cases like this that employ small sample sizes is not recommended, lest it mislead the reader. Furthermore, because percentiles or ratios have 2 components (numerator and demoninator), each with its own variance, their use in statistical analyses as done in this study is less meaningful than using the original numbers

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Research study 2
 

photograph of a bed of sea pens Ptilosarcus gurneyi courtesy Russ Wyeth & Owen Woodward
A sea pen withrawn into the sediment does not necessarily mean that it has been attacked or otherwise stimulated.  Studies in Puget Sound, Washington show that sea pens Ptilosarcus gurneyi may inflate and deflate several times a day, and at any given time as few as one-quarter of all individuals are up and feeding. Birkeland 1974 Ecol Monogr 44: 211. Photograph courtesy Russ Wyeth & Owen Woodward 0.1X.

 

 

 

Of 5 individuals visible in the foreground in this photo,
one is withdrawn and one is partially contracted

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Research study 3
  The burrowing response of a sea pen Ptilosarcus gurneyi to an attack by a nudibranch Tritonia diomedea is graphically illustrated in a unique study that employs in situ time-lapse video cameras to record field behaviour.  The predator approaches from downstream with oral veil lifted and lips protruded. It lifts its head and, on light contact with its prey, pulls back the oral veil, protrudes its lips even further, opens its jaws, and suddenly strikes.  On contact with Ptilosarcus, the radula grasps a pinnule and pulls it into the mouth where it is severed by the jaws. The entire strike and severence sequence takes less than 10sec. Note the missing pinnule as the sea pen retracts into the sediment. The prey now looks to be vulnerable in its withdrawn state, but the predator seems not to bother with it and moves on. Wyeth & Willows 2006 Biol Bull 210: 81.
 
 
one of a series of photographs showing an attack by a nudibranch Tritonia deiomedea on a sea pen Ptilosarcus gurneyi courtesy Wyeth & Willows 2006 Biol Bull 210: 81: extension of oral veil
Tritonia senses its prey from a downstream location. Oral veil is extended
one of a series of photographs showing an attack by a nudibranch Tritonia deiomedea on a sea pen Ptilosarcus gurneyi courtesy Wyeth & Willows 2006 Biol Bull 210: 81: first contact
The predator lifts its head and makes ight contact with the prey
one of a series of photographs showing an attack by a nudibranch Tritonia deiomedea on a sea pen Ptilosarcus gurneyi courtesy Wyeth & Willows 2006 Biol Bull 210: 81: mouth protruded
The oral veil of the predator is pulled back, and mouth and lips are protruded
one of a series of photographs showing an attack by a nudibranch Tritonia deiomedea on a sea pen Ptilosarcus gurneyi courtesy Wyeth & Willows 2006 Biol Bull 210: 81: strike
With jaws open the strike is made one or more leaves
one of a series of photographs showing an attack by a nudibranch Tritonia deiomedea on a sea pen Ptilosarcus gurneyi courtesy Wyeth & Willows 2006 Biol Bull 210: 81: leaf of sea pen pulled into mouth
On contact the radula grasps a leaf and pulls it into the mouth
one of a series of photographs showing an attack by a nudibranch Tritonia deiomedea on a sea pen Ptilosarcus gurneyi courtesy Wyeth & Willows 2006 Biol Bull 210: 81: leaf severed by jaws
A leaf portion is severed by the jaws. Note the slow response by the prey
one of a series of photographs showing an attack by a nudibranch Tritonia deiomedea on a sea pen Ptilosarcus gurneyi courtesy Wyeth & Willows 2006 Biol Bull 210: 81: sea pen contracts into sediment
The sea pen now contracts into the sediment. The arrow shows missing portion

Within a minute following the first strike the sea pen is well withdrawn into burrow
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