On emergence from the capsule a hatchling whelk seeks
out prey items to drill. However, at this time the prey may be
more than 1000-fold larger in mass than the hatchling  

Hatchling¹ whelks Nucella ostrina begin feeding on barnacles and other prey from the moment they emerge from their egg capsules. Best attacking success by the hatchlings on a barnacle comes from drilling into the opercular-plate sutures, rather than into lateral plates.  Do the hatchlings learn this from experience as they get older?  The author of a study at the Bamfield Marine Sciences Centre suggests that if this were so, it would not be distinguishable from a genetically programmed behavioral change² with size.  Apparently, larger whelks have a tendency to crawl higher up on a barnacle, thus, increasing the opportunity for more successful opercular attacks.  It seems that this is not exploited until later in life, when the hatchlings reach a size large enough to resist dislodgement or attack by predators in such an exposed position. In experiments different sizes of Nucella ostrina hatchlings (1.5-5mm shell length) presented with a selection of sizes of barnacles³ show graded attack success depending upon their relative sizes (see graph upper Left).  Although even the smallest hatchlings are able to attack and eat a few of the largest barnacles, the data show that a given size of hatchling cannot eat all sizes of barnacles with equal success.  At some point a barnacle reaches a size at which it enters refuge from predation by a certain size of whelk.  This is not cut and dry, and is defined here by the author as the size where attack success is 50%, termed the median vulnerable size or SV₅₀.  This is the size at which a prey has a 50% chance of surviving an attack from, in this case, a whelk hatchling, and is the size at which the prey species can be considered to be entering refuge.  For these data, the SV₅₀ for a barnacle is 2mm opercular diameter when attacked by a 1.5mm hatchling predator, and about 5.5 mm opercular diameter when attacked by a 5mm one.  Palmer 1990 Ecology 71: 759.

NOTE¹  this term is a bit of a misnomer, because actual hatching occurs when the young snail breaks free of the egg membrane. This takes place within the egg capsule. Later, the juvenile snail emerges or escapes from the egg capsule via an operculum at the top.  While the little snail is really a “hatchling” from the moment it breaks free of the egg membrane, the term is commonly used for the young snail during some indeterminate time following its emergence from the egg capsule

NOTE²  this is referred to as an ontogenetic shift.  An example of an ontogenetic shift in mode of feeding in a gastropod is the moon snail Polinices lewisii, which eats algae for the first 5-6 months of life, then switches to clams

NOTE³  mixed species Balanus glandula and Chthamalus dalli, of 1-6mm opercular diameter.  Presence of drill holes are used to estimate “success”

So, larger barnacles enjoy greater survival, and larger whelks have greater attacking success.  Who wins the size race?  This can be estimated by plotting barnacle SV₅₀ against hatchling size, as shown in the graphs below.  The question posed by the author is what scaling relationship exists between size of prey and size of predator; in other words, what is the scaling of vulnerability for a barnacle?  There are 3 possible answers, as shown below.  To be sure that you understand what each signifies, think for a moment about the possibities, then CLICK on the scaling relationship that you think indicates that the hatchling Nucella will win the size race.  Data and ideas from Palmer 1990 Ecology 71: 759.

Now CLICK HERE to see the actual data. 

Other studies on feeding behaviour of newly emerged Nucella ostrina at the Bamfield Marine Sciences Centre, British Columbia indicate a slightly different ontogenetic pattern of feeding than what is described above.  The difference is that, rather than the hatchlings starting to feed from the moment they emerge from their capsules, they actually wait for about 3d from emergence before their first attack.

In other experiments the hatchlings are observed to attack and eat 3 species of barnacles and 3 species of bivalves.  However, several other possible prey species in the habitat, including limpets and littorines, are not eaten by the hatchlings. Additionally, 2 genera of bivalves eaten by the hatchlings, Lasaea and Musculus, are not eaten by adult N. ostrina.  So, although there is no ontogenetic shift in mode of feeding by the whelks, there is an ontogenetic change in type of prey eaten.  There seems to be no urgent physiological need for the hatchlings to feed, as they can survive for up to 120d after emerging without food.The authors note that their study is the first to document the feeding habits of whelks during the first few days after emerging from the egg capsule.  Gosselin & Chia 1994 J Exp Mar Biol Ecol 176: 1. Photographs courtesy Louis Gosselin, Thompson Rivers University, Kamloops, British Columbia.

NOTE  hatchling sizes used in the study range from 1-3mm shell length

NOTE  barnacles: Balanus glandula (1.2-12.6mm diameter), Chthamalus dalli (1.1-5.7mm), and Pollicipes polymerus; bivalves: Lasaea spp. (<5mm shell length), Musculus taylori (<5mm), and Mytilus spp. (1-5mm). See Research Study 4 below for images of these species

Optimal-foraging theory tells us that a predator will select the prey that maximises its net energy return.  Of course, there are other factors involved in prey selection, including taste, accessibility, and the extent to which nutritional requirements other than energy are met, but energy, at least in the past, has been a handy and easy indicator of potential diet suitability.  In the case of hatchling whelks, the question arises as to whether, in the absence of any foraging experience, they maximise their energy intake on a first attack.  This is tested with hatchling Nucella ostrina at the Bamfield Marine Sciences Centre, British Columbia by caging single hatchlings with one individual of each of 5 possible prey species and recording which of the prey is attacked first. Results show that  80% of the hatchlings attack mussels Mytilus sp. first out of a selection of bivalves and barnacles (see histogram upper Right).

When single hatchlings are caged individually with 5 different sizes of individual Mytilus, 73% of first attacks are on the 1-2mm size classes of mussels (see histogram lower Right).  Note the orange-coloured bars in this histogram. These are the expected attack frequencies based on liklihood of encounter, and indicate attack frequencies expected if mussels were to be attacked as they are encountered. 

Without prior foraging experience, then, the hatchlings are actually more selective than the juveniles or adults. Growth studies show that there is no difference in size increase of the hatchlings over 25d feeding on mussels or barnacles, nor is any difference found in energy content between these 2 prey types.  If the strong preference for small mussels cannot be explained by consideration of energy gain, what then is its selective value?  The authors suggest that the preference tends to keep the hatchlings within mussel microhabitats where they are protected from desiccation, predators, and dislodgement by waves.  Gosselin & Chia 1996 J Exp Mar Biol Ecol 199: 45.

NOTE  body volumes of the mussels are used as an estimator of “liklihood of encounter”.  Thus, the bigger the mussel, the more likely is the chance of a hatchling encountering it

NOTE  other factors that may be important in prey selection by the hatchlings, but not included in the study, are thickness of drilling site and accessibity of the prey, relating specifically to the difficulty of a hatchling climbing up a prey that is much larger than itself