Defenses
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  Ceratal autotomy
 

Defenses of nudibranchs and their relatives include ceratal autotomy, considered in this section, and
CAMOUFLAGE (CRYPSIS)
,
FAST CRAWLING & SWIMMING,
MUCOUS COATINGS,
NUTRITIONAL CONTENT,
SPICULES
,
NEMATOCYSTS
,
VACUOLATED SKIN WITH PROTECTIVE SPINDLES
,
ACID SECRETIONS
,
INK & OPALINE SECRETIONS,
SECONDARY METABOLITES
,
ALARM PHEROMONES,
APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

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Research study 1
  photograph of cerata of aeolid nudibranch Hermissenda crassicornis
The cerata of many species of aeolids are readily autotomised, such as after contact with rainwater in tidepools or with cnidarians such as cup corals.  For a while after their autotomy, the cerata may wriggle about, perhaps acting as an “attention-getter”, or decoy, for potential predators. If cerata are experimentally removed from the aeolid nudibranch Hermissenda crassicornis, they regenerate within a few weeks.  The cerata of Hermissenda are conspicuously coloured with white- or light-coloured tips surrounded by orange bands, and white lines trailing down the sides.  Inside each can be seen the brown diverticula of the digestive gland.  Zack 1975 Veliger 17: 271; Miller & Byrne 2000 Invert Biol 119: 167.
 
Research study 2
 

drawing of nervous system of nudibranch Melibe leonina with respect to autotomy of ceratadrawing of basal part of ceras of Melibe leonina showing autotomy planeA strong aversive stimulus, such as a firm pinch, to the ceras of Melibe leonina will cause autotomy.  Initiation of autotomy of a ceras is mediated by 2 rings of nerves originating from the ceratal nerve (see drawing on Right), the latter of which derives from the pleural nerve running the length of the body (see figure on Left).

Severance occurs at the cerata base in an area surrounded by two bands of sphincter muscles known as the autotomy plane. The actual autotomy appears to be a disruption of connective-tissue components at the autotomy plane that causes the tissues to separate. 

drawing of basal part of ceras of Melibe leonina to show autotomy plance and various nervesContraction of the sphincter muscles seals off both sides of the wound (see figure lower Left).  The entire process, from stimulus to autotomy, occurs within 10sec at temperatures of 9-12oC.  The author notes that autotomised cerata in Melibe show little spontaneous movement.  Pinching of a ceras or other aversive treatment may also initiate swimming.  The author observes “galloping” from contact with certain sea stars, but no ceratal loss, and swimming and ceratal loss from being pinched by crabs. Bickell-Page 1989 Phil Trans Roy Soc Lond 324B: 149.

 

 
Research study 3
 

photo/drawing compsite of cerata of aeolid nudibranch Hermissenda crassicornisDetailed studies on ceratal autotomy and regeneration in Hermissenda crassicornis at the Bamfield Marine Sciences Centre, British Columbia show that a ceras detaches through severence along an autotomy plane located at its basal constriction (see Research Study 2 above). The cross-sectional drawing on the Left shows these constrictions, as well as the presence of mature cerata along the mid-sagittal axis and smaller, immature cerata along the outside margins of the body. 

About 4d after autotomy regenerating cerata appear as small protuberances. By 24d their structural organisation is complete and by about 40d they are indistinguishable from surrounding cerata (see drawings below).

In laboratory experiments, kelp crabs Pugettia producta induce autotomy in H. crassicornis by grasping cerata in their chelipeds. Later, the crabs are observed to eat the "decoy" cerata while the previous owner crawls rapidly away.  Although this is highly suggestive of a defensive function for autotomy, other tests with hermit crabs and sculpins fail to induce similar behaviours in Hermissenda.  However, the authors suggest that ceratal autotomy may be an important means of escape from Pugettia, which is common in their habitat.  The study is the first to detail ceratal regeneration in an aeolid nudibranch.  Miller & Byrne 2000 Invert Biol 119: 167.

 

drawing 1 in a series showing post-autotomy regeneration in a ceras of the aeolid nudibranch Hermissenda crassicornis

autotomised cerata leave behind a relatively flat surface

drawing 2 in a series showing post-autotomy regeneration in a ceras of the aeolid nudibranch Hermissenda crassicornis
ceratal and digestive gland rudiments appear

drawing 13 in a series showing post-autotomy regeneration in a ceras of the aeolid nudibranch Hermissenda crassicornis
digestive glands grow into cerata

drawing 4 in a series showing post-autotomy regeneration in a ceras of the aeolid nudibranch Hermissenda crassicornis
cnidosac rudiments appear
drawing 5 in a series showing post-autotomy regeneration in a ceras of the aeolid nudibranch Hermissenda crassicornis
orange & white pigments are visible
drawing 6 in a series showing post-autotomy regeneration in a ceras of the aeolid nudibranch Hermissenda crassicornis
By 40d (Left) the cerata are mature
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