Predators of whelks include crabs, isopods, and other snails during embryonic development, and crabs, sea stars, birds, and fishes during adulthood.  Defenses include shell strength and sculpturing, and burying.

The buccinid whelk Neptunea pribiloffensis has an unusual way of protecting its eggs and early developmental stages from predation by green sea-urchins Strongylocentrotus droebachiensis.  The eggs, cemented together in a helically coiled tube of “corn-cob” shape, take 1yr to develop; hence, are potentially exposed to predators for a comparatively long time.  The female preferentially deposits its eggs near to sea anemones Urticina crassicornis where they appear to gain protection from the host’s tentacle “shield”.  Of 72 egg masses of Neptunea observed by the author over a 2-yr study period at sites near Homer, Alaska, 59 (82%) are located within 10cm of an anemone.  Green urchins are implicated as predators because of 122 U. crassicornis examined in the field, 11 had recently eaten a single urchin.  Laboratory observations by the author document both that green urchins are consumed by the sea anemone, and that whelk eggs are eaten by urchins.  In the field, more eggs survive to juvenile stage in masses located within 10cm of an anemone than in masses located more than 10cm away.  Moreover, if the “host” Urticina are removed, the egg masses are mostly attacked and eaten (although whether urchins are involved in this is not known).  Shimek 1981 Veliger 24: 62. Photographs courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, WA.

NOTE  the author provides evidence that green urchins are the culprits, but the strategy could work just as well for defense against other soft-bodied predators

Potential predators of egg capsules abound in the intertidal areas inhabited by Nucella spp.  Laboratory tests at the Bamfield Marine Sciences Centre, British Columbia with a variety of predatory species, combined with field observations, identify several major predators, each with its own characteristic killing “signature”.  For example, shore crabs Hemigrapsus spp. open the top of the capsule leaving edges that are comparatively smooth, or they may sometimes pop out the capsular plug with a squeeze of their claws (see photos below). 

Measurements on capsules of Nucella ostrina from different habitats disclose marked differences in capsule-wall thickness, and thus in strength of the wall as measured by resistance to puncturing and in force required to rupture the capsular plug.  Regressions for the 3 sites featured differ significantly (see graph). All sites are wave-protected, but differ in number and type of predators. For example, thickest capsule walls are found in Grappler Inlet, an area characterised by fewer crabs, but more isopods and chitons. In comparison, thinnest capsule walls are found at Ross Islets, an area with many crabs, but no isopods or chitons. Interestingly, differences in capsule-wall thickness are maintained over at least a 5-mo period in the lab, suggesting a genetic basis. Rawlings 1990 Biol Bull 179: 312. Photographs courtesy Tim Rawlings, Cape Breton University, Nova Scotia.

What are the possible causes of these inter-population differences in capsule-wall thickness in whelks Nucella ostrina? Think about the answers provided, then CLICK HERE for explanations.

Different exposure to the sun. 

Different levels of potential predation. 

Different exposure to wave action. 

Different seawater salinities. 

Different sizes of females. 

Observations at the Bamfield Marine Sciences Centre, British Columbia on isopods Gnorimosphaeroma oregonense, which may reach densities of 2000 individuals ^. m^-2 in whelk habitats, reveal that it preferentially eats thin-walled capsules over thick-walled ones.  The graph shows the cumulative number of capsules of each thickness opened by single isopods in pairwise tests. 

Other experiments address the question of whether there are differences in nutritional yield for the predators. Results show no difference in palatability of the walls of thick- and thin-walled capsules, nor in their contents.  An interesting follow-up question on this subject might be to conduct reciprocal translocations of snails from one habitat to another and measure capsule wall-thicknesses over time. Rawlings 1994 Evolution 48: 1301.

Isopods Gnorimosphaeroma
oregonense crawling on sponge 4X

Downstream exposure to feeding crabs can affect the reproductive physiology of whelks Nucella ostrina in other related ways.  Experiments at the Bamfield Marine Sciences Centre, British Columbia test the presence of crab scent on spawning, tissue mass, and behaviour of whelks Nucella ostrina.  9-13 pairs of reproductively mature male and female whelks¹ are held in 3 sets of replicate cages for 3mo and fed ad libitum on barnacles. Capsule production is monitored for each set during this PRE-TREATMENT time. After 3mo the sets of whelks are subjected to TREATMENT² of: 1) FED with NO CRAB ODOUR (= control), 2) NOT FED with NO CRAB ODOUR, and 3) NOT FED with CRAB ODOUR. In the last treatment the caged whelks are situated downstream of a crab Cancer productus that is feeding on N. ostrina.  Prior to these treatments , tissue masses are estimated using a non-destructive method³ . 

The results are unambiguous.  First, control snails continue to produce egg capsules through the 6-mo period, albeit with a mid-way lull (an expected seasonal mid-summer decline; see top set of bars in histogram on Left). Whelks not fed and not exposed to crab odour produce egg capsules for the first month of experimental treatment, then production declines to zero (see middle set of bars in histogram on Left). Finally, whelks not fed and subject to crab odour produce significantly less egg capsules than in the other treatments (see lower set of bars in histogram on Left). Second, significantly more tissue mass is lost by unfed whelks when exposed to crab odour than when not exposed, with females not unexpectedly losing more than males (see histogram on Right).  Finally, significantly more snails crawl upwards in their cages when exposed to crab odour than ones not so exposed, perhaps in an attempt to escape.  The author suggests that the presence of  crab effluent may create stress in the whelks manifested in both escape behaviour and tissue loss.  Effect of crab effluent on metabolic rate is not measured in the study, but could be a good subject for future study.  Rawlings 1994 J Exp Mar Biol Ecol 181:  67.

NOTE¹   the whelks are raised from egg capsules deposited in the laboratory

NOTE²   a fourth treatment, namely FED with CRAB ODOUR is not done, although it would seem to have been an obvious, perhaps even necessary, addition to the study. Without it, we are not sure of the extent to which the significant decrease in capsule production in Treatment 3 owes to lack of food or to the odour of crabs

NOTE³  a description of this non-destructive method for determining shell-less live mass of a Nucella snail in growth studies, yielding less than 11% error, can be found in Palmer 1982 Malacologia 23: 63.