Limpets & relatives
Predators & Defenses: Defenses Of Keyhole Limpets

Keyhole limpets Diodora aspera are often richly adorned with growths of colonial tunicates, sponges, and bryozoans.  Whether this is for visual or chemical camouflage is not known. The individual in the photo on the Right seems visually camouflaged (to our eyes), but not the one on the Left.

At the bottom are three more examples of Diodora aspera with possibly camouflaging growths. All photos are of specimens living on current-swept promontories on islands in Barkley Sound, British Columbia. Diodora grows to sizes in excess of 6cm shell length.

With growths of colonial tunicates, mostly Distaplia occidentalis
With tunicates, sponges (at the anterior end; Right), and sea anemones
With miscellaneous growths, but mostly bryozoans, possibly Hippodiplosia insculpta
   
Defenses of keyhole limpets: mantle response

Research Study 1: Defenses of keyhole limpets: mantle response

A Diodora aspera has initiated its mantle response without apparent contact with a sea star Orthasterias koehleri. Note the extension of the posterior mantle tentacles by the snail
Mantle response of Diodora aspera after being stimulated by contact with a sea star

A unique defensive response is employed by keyhole limpets Diodora aspera against attack by sea stars.  Within moments after contact with a predatory sea star the limpet rises to twice its normal height and extends part of its mantle up and over the shell and another part downwards to curtain the side of the foot.  Additionally, the limpet extends its siphon up and out of the hole.  Probing tube feet of the sea stars appear to be unable to gain a hold on the mantle tissue and may even be chemically repelled by it.  About 20min after being stimulated the limpet recovers and the mantle is withdrawn. Contact is not required to initiate the behaviour, suggesting the involvement of some kind of water-soluble substance.

NOTE as seen in the accompanying images, the rising-up response is not always so obvious as described

Some views of the mantle response of Diodora aspera:

Mantle response beginning. Note the siphon protruding out of the hole at the top
Mantle response nearing completion. The anterior end of the snail is at the Left.
Close view of the extended mantle showing how it is drawn out into sensory "fingers".
A keyhole limpet Diodora aspera responding to touch by a sea star Orthasterias koehleri.
Margolin   1964   Anim Behav 12: 187

Research Study 2: Defenses of keyhole limpets: mantle response

Studies at Friday Harbor Laboratories, Washington show that 9 of 17 species of sea stars tested induce a mantle response in Diodora aspera.  Is there a “commonality” to these species, perhaps phylogenetic, or dietary, or habitat? The 17 species are arranged in the first two columns below in colour-coded taxonomic categories of ORDERS, separated as to whether they induce a response (POSITIVE) or not (NEGATIVE):

Positive Negative
Pisaster ocraceus Mediaster aequalis
Pisaster brevispinus Dermasterias imbricata
Pisaster giganteus Luidia foliata
Leptasterias aequalis Solaster stimpsoni
Evasterias troschelii Solaster dawsoni
Orthasterias koehleri Pteraster tesselatus
Pycnopodia helianthoides Crossaster papposus
Hippasteria spinosa Henricia leviuscula
Asterina miniata  
Orders
Order Forcipulatida
Order Valvatida
Order Velatida
Order Spinulosida
Morphological Phylogeny
Courtesy Blake 1987 J Nat Hist 21:481
   

Morphological/Molecular Phylogeny
Courtesy Lafay et al. System Biol 44: 190

Molecular Phylogeny
Courtesy Wade et al. Molec Phylog Evol 6: 97

How does this “Diodora-derived” classification accord with asteroid evolutionary relationships?  Well, in two phylogenies derived from morphological and combined morphological/molecular characters, respectively, the forcipulatids and velatids/spinulosids) are placed in distant relationship while the valvatids are placed closer to the velatids/spinulosids, which accords to some extent with the Diodora-escape data (see schematic on upper Right). The splitting of the valvatids into both "positive" and "negative" response-columns, however, is awkward to resolve. Margolin 1964 Anim Behav 12: 187.

NOTE  of the 7 commonly recognised Orders of asteroids, the Forcipulatida is one of the largest and includes many common west-coast representatives. Approximate numbers of world species in each of the four main Orders are as follows:

Forcipulatida: about 300 species in 68 genera, distinguished by their forcipulate (i.e., forceps-like) pedicellariae

Valvatida: about 400 species in 165 genera, often distinguishable by the presence of marginal ossicles, but the Order is quite diverse morphologically

Velatida: about 200 species in 25 genera, often characterised by thick bodies and large central discs

Spinulosida:  about 120 species in 9 genera

A final molecular phylogeny adds little, if anything, to what we have seen from the previous depictions (see schematic on lower Right):

What about DIETARY AFFINITY? This seems promising, as all of the spinulosids in the "POSITIVE" column (with the exception of Pisaster brevispinus) would be expected to prey on keyhole limpets. However, of the two valvatid "POSITIVES", Hippasteria spinosa eats sea pens and Asterina miniata is an omnivorous scavenger of plant and animal matter (although it is recorded as preying on sea urchins Lytechinus anamesus and bryozoans in south- and mid-California, respectively). Of the species in the "NEGATIVE" column above, only Crossaster papposus is recorded as consuming snails, and then only a minor dietary component of populations in Alaska. The other velatids are fairly specialised on holothurians (Solaster stimpsoni), sea stars (Solaster dawsoni), and sponges/anemones/ascidians (Pteraster tesselatus). The spinulosid Henricia leviuscula is a ciliary plankton feeder that may also eat sponges/bryozoans. So, other than the odd-ball valvatids in the "POSITIVE" column, dietary affinity seems promising and may be deserving of closer scrutiny.

   

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