Predators & defenses

photograph of a sea-pen bed Ptilosarcus gurneyiBeds of sea pens Ptilosarcus gurneyi are common along the coast and densities within the beds may reach 20 individuals .  m-2.  Sea-pen flesh is soft and highly palatable to certain specialised predators.  Neither the presence of spicules (2 types: 75 and 550um in size, composed of calcium carbonate/ protein) nor a purported toxin ptilosarcone appear to dissuade these predators from attacking.

Considered in this section are sea-star & nudibranch predators, while defenses, including BURROWING, SWIMMING, BIOLUMINESCENCE, and TOXIC CHEMICALS/UNPALATABILITY, can be found in other sections.

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Sea-star & nudibranch predators

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Research study 1

diagram of a food web based on sea pens Ptilosarcus gurneyiIn Puget Sound, Washington sea pens Ptilosarcus gurneyi are eaten mainly by sea stars Mediaster aequalis, Hippasteria spinosa, Crossaster papposus, and Dermasterias imbricata, and by nudibranchs Armina caifornica, Tritonia diomedea,and Hermissenda (Phidiana) crassicornis.  The food web based on Ptilosarcus, including percentage of diet represented by sea pens for each of the predators, is shown schematically.  The author considers Ptilosarcus gurneyi to be a “key-industry species”, defined as a species abundant enough to support a large guild of predators, "topped" here by the asteroid-eating sea star Solaster dawsoni. 

Note that a favoured prey of Solaster dawsoni is the sea star Mediaster aequalis, itself a major predator of sea pens.  Removal of of the top predator Solaster would lead to greater numbers of Mediaster and likely increased predation pressure on Ptilosarcus. Mediaster could also come to dominate the other predators, thus reducing their numbers.  The study is valuable in indicating the potential fragility of the food web to perturbation in numbers of a single component species.  Birkeland 1974 Ecol Monogr 44: 211; ideas from Paine 1980 J Anim Ecol 49: 667.

NOTE  a phrase  attributed to Charles Elton in his 1927 book: Animal Ecology Sidgwick & Jackson Ltd., London

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Research study 2

photograph of a nudibranch Tritonia diomedea about to make a bite strike on a sea pen Ptilosarcus gurneyi courtesy Owen Woodward & Russ Wyeth
Diets of Tritonia spp. in Puget Sound, Washington are made up principally of sea pens Ptilosarcus gurneyii and other cnidarians including the sea whip Virgularia sp. and the burrowing anemone Pachycerianthus fimbriatus.  The predators can sense their prey from some distance away using scent carried in water currents to their chemosensory rhinophores.  Wyeth & Willows 2006 J Exp Biol 209: 1441. Photo courtesy Owen Woodward & Russ Wyeth.

A nudibranch Tritonia diomedea prepares to
lunge at its prey. The buccal mass with jaws
and radula will be extended even further before
the strike is made, and the strike happens fast.
If successful, the predator will bite a chunk
from the sea pen and swallow it whole 0.33X

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Research study 3

photograph of a sea pen Ptilosarcus gurneyi and nudibranch Tochuina tetraquetraAnother predator of sea pens Ptilosarcus gurneyi is the nudibranch Tochuina tetraquetra. The photo shows what might be old predator bites on the base leaves of the sea pen.





Tochuina tetraquetra is reputed to be the world's
largest nudibranch, attaining over 30cm body length

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Research study 4

photograph of cleanly healed bite damage to a sea pen Ptilosarcus gurneyiClose inspection by researchers of possible bite scars on sea pens Ptilosarcus gurneyi in San Juan Islands, Washington reveals adventitious settlement of spores of the red alga Desmarestia sp. in the wounds. The seaweed is in its gametophytic stage and, on removal from the wounds, is healthy and able to be cultured through to its more familiar sporophyte stage in the laboratory.  Although the life cycle of Desmarestia is known from laboratory culture, the authors think that theirs is the first record of its gametophytic stage being collected in the field.  Dube & Ball 1971 J Phycol 7: 218.

Close view of what appears to be cleanly
healed bite damage to a gravid P. gurneyi 6X

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Research study 5

histograms showing distances travelled by colonies of sea pansies Renilla amethystina after uprooting in response to an attack by the nudibranch Armina californica and by a SCUBA-diver (control)photograph of a nudibranch Armina californicaIn the Los Angeles area of California, sea pansies Renilla amethystina (koellikeri) are preyed upon mostly by nudibranchs Armina californicaRenilla’s response to the predator is unusual in that, rather than pulling its body down into the sand, it retracts its peduncle up into the rachis, flattens the rachis, and waits until water currents catch an edge to lift it up.  The water currents lift the colony and float it away.  From first contact to being lifted free from the substratum takes about 1-2min.  Re-anchoring involves retraction of the polyps, pulling in of the peduncle, expulsion of water from the colony, settling onto the substratum, re-expanding the peduncle and inserting it into the sand, re-expanding the rachis, and extending the polyps.  Time required to commence re-anchoring is about 3min depending upon wave surge, but actual anchoring may take an additional 15min to several hours. 

A colony responding to an Armina attack drifts almost 3 times further than one experimentally uprooted by a SCUBA-diver (graph on Right). Renilla’s second major predator, the sea star Astropecten armatus, may be driven off by nematocyst discharge from the polyps or, if large enough, the sea pansy may exist in size-refuge from the predator.  The largest Renilla that Astropecten can ingest is about 40mm diameter.  Kastendiek 1976 Biol Bull 151: 518.

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Research study 6

The soft coral Cryptophyton goddardi is preyed upon by nudibranchs Tritonia festiva.  Studies at Cape Arago, Oregon show that the nudibranch uses its photograph of aeolid nudibranch Tritonia festiva courtesy Jeff Goddard, UC Santa Barbara, Californiachemosensitive frontal veil to position itself over an expanded polyp.  The attack, involving a lunge and a bite, needs to be swift and to be executed before the polyp contracts into the protection of the spiculate colony mass.  Once one polyp contracts, all nearby polyps contract - an alarm response photograph of a colonly of soft coral Cryptophyton goddardi with a juvenile predator, the nudibrach Tritonia festiva courtesy Jeff Goddard, UC Santa Barbarapossibly mediated by nerve impulses. The author remarks that this is a subject needful of further research study. Goddard 2008 Sea Slug Forum. Photos courtesy Jeff Goddard, UC Santa Barbara, California.

NOTE the rhinophore, featured in the inset photo on the Right, is a chemosensory organ used in long-distance perception

Colony of the alcyonium Cryptophyton goddardi
with a juvenile Tritonia festiva at the 7 o'clock
position. Tritonia reaches sizes of up to 10cm

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Research study 7

A study at Auke Bay Laboratories, Alaska on damage caused to sea whips Halipteris willemoesi by bottom-trawl fishing gear shows that dislodgement, fracturing of the axial rod, and soft-tissue damage may not cause immediate death, but most damaged colonies nonetheless photograph of a bed of sea whips Halipteris willemoesi in Alaska, courtesay Malecha & Stone 2009 Mar Ecol Progr Ser 388: 197generally succumb within a year.  In situ experiments to simulate typical trawler-induced woundings reveals an almost complete inability of Halipteris to repair the axial rod even during a year-long recovery period.  During the experiment selected individuals are tethered to small pegs in the mud to prevent them from being moved away by water currents after being experimentally uprooted by SCUBA divers. Interestingly, in the field these damaged colonies attract the presence of predatory nudibranchs Tritonia diomedea, who take bites of the sea whips lying on the seafloor.  Such predation may cause up to 90% tissue loss of the Halipteris willemoesi being preyed upon by a nudibranch Tritonia diomedea, courtesy Malecha & Stone 2009 Mar Ecol Progr Ser 388- 197already damaged sea whips and lead to certain death of the prey.  Malecha & Stone 2009 Mar Ecol Progr Ser 388: 197.

NOTE  after their uprooting about half of the colonies are able to rebury their peduncles and right themselves, a feat not previously known for this species.  In almost all cases, though, these re-righted individuals fall over again and perish.  The authors discuss this and acknowledge that the tethering arrangement (see photograph on Left) may have caused increased drag on the sea whips and/or interfered with their ability to maintain normal functioning after re-righting

A nudibranch Tritonia diomedea feeds on a sea whip Halipteris willemoesi
knocked over by SCUBA-divers during simulated trawler-damage experiments

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Research study 8

photograph of aeolid nudibranch Flabellina eating soft coral AlcyoniumSoft corals Alcyonium spp. appear to have no special defenses other than their nematocysts, and are likely preyed upon by various species of aeolid nudibranchs that may even use these nematocysts in their own defenses.





An aeolid nudibranch, possibly Flabellina fusca, appears
to be eating polyps of the soft coral Alcyonium sp.
The nudibranch's mouth is directly under the soft
structure at the 8 o'clock position in the photo 1.5X

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