Learn About Moon Snails: Feeding, diets, & growth

The 3 topics of feeding, diets, and growth are arranged in subsections below.


Research study 1: Feeding

photograph of a moon snail Euspira lewisii crawling on or near a sea cucumber Parastichopus californicus

Adult moon snails are strict predators and mostly eat bivalves. As many of their prey live at depths of up to 20cm or more, the snails have to burrow quite deeply to find them. Burrowing by moon snails is enabled by a large foot that is capable of inflating up to 4 times the shell volume through uptake of seawater. The inflation is quick, allowing fast penetration and displacement of sand. The moon snail catches hold of its prey and hauls it to the surface to begin drilling.

A moon snail Euspira lewisii appears to be attacking a sea cucumber Parastichopus californicus, but the encounter is likely just by chance 0.25X

Research study 2: Feeding

Moon snails manipulate their bivalve prey so that the umbo is closest to the mouth. Whether this provides easiest handling, or whether it is to place the drill-hole directly over the bulk of soft body tissues, is not known.Another special feature of drill holes of Euspira lewisii is that they are countersunk. This feature allows the predatory records of the snails to be monitored more closely than that of, say, shell of a littleneck clam Protothaca staminea with its umbo drilled by a moon snail Euspira lewisiiwhelks (whose drill-holes are less distinctive). After a hole is drilled, the snail extends its proboscis hydraulically (the radula is at the tip of the proboscis), and commences scraping and eating the soft internal tissues.

NOTE drilling is considered in detail elsewhere in the ODYSSEY: LEARN ABOUT WHELKS & RELATIVES: RADULAR DRILLING

A shell-valve of a littleneck clam Protothaca staminea shows the distinct umbo-location and countersunk-drill hole made by a moon snail 3X

Research study 3: Feeding

In a study of bivalve shells in Departure Bay, British Columbia, drilled by moon snails Euspira sp., scientists at the Pacific Biological Station, Nanaimo report that 92% of the drillholes are in the umbo region. Furthermore, of all shells drilled, 61% are drilled in the left valve, and 39% in the right valve. Whether this difference is statistically significant is not known nor, if it is, whether it has any biological significance. Bernard 1967 Fish Res Bd Can Tech Rept #42.

NOTE mostly native littlenecks Protothaca staminea and butter clams Saxidomus giganteus

Research study 3.1: Feeding

photograph of sagittal section of accessory boring organ of moon snail Euspira lewisiiA later study by the same research group at the Pacific Biological Station, British Columbia on drilling of prey by moon snails Euspira lewisii focuses on the fine structure of the accessory boring organ (ABO). First, the location of the ABO in moon snails differs from its location in muricid gastropods such as Nucella whelks. In the latter it is positioned on the anterior/ventral part of the foot, while in the former it is located on the ventral distal tip of the proboscis. When erect (i.e., tumescent) it is mushroom-shaped and joined to the proboscis by a stalk (peduncle). All surfaces are covered in tall, columnar epithelia cells fringed with a brush border of microvilli. Tumescence of the ABO is linked to extension of the proboscis, as both organs are connected via a sinus system that can be engorged with hemolymph under pressure (see photograph). Numerous goblet cells permeate the epithelium of the ABO and are thought by the author to secrete an acid mucopolysaccharide substance. Other mucus-producing cells form a ring around the central epithelium. The authors suggest that secretion from these cells may create a mucous seal when the ABO is applied to a prey's shell. Bernard & Bagshaw 1969 J Fish Res Bd Can 26: 1451.

NOTE the authors provide a fascinating (and too short) history of investigation into the mode of drilling by gastropods, including muricid whelks and other naticids. The idea of a combined radular/chemical means of penetration dates from 1711 and an observation that a secretion from the “papilla” that “reddens litmus paper” dates from 1891. Even that the ABO and resultant hole diameters are identical has been known from the early part of the last century. More information on ABO function in gastropods can be found at LEARNABOUT WHELKS/FEEDING/RADULAR DRILLING

Research study 4: Feeding

photograph of a clam Humilaria kennerleyi with a partially drilled hole in its shell plus a closeup of sameFavoured prey of moon snails Euspira lewisii in British Columbia includes most larger species of bivalves: butter clams Saxidomus giganteus, Humilaria kennerleyi, littleneck clams Protothaca staminea, softshell clams Mya arenaria, Macoma nasuta, gaper clams Tresus nuttallii, and even a few cockles Clinocardium nuttallii (a species that has strong defensive capability). In their study the authors note that 97% of 286 prey littleneck clams are drilled in the umbo region. Peitso et al. 1994 Can J Zool 72: 319.

NOTE when a moon snail Euspira lewisii comes upon a gaper clam Tresus spp., boring is usually unnecessary as there is sufficient gape in the shells to allow the attack to occur at the base of the siphons or at the ventral mantle edge. Reid & Friesen 1981 Veliger 23: 25.

Shell valve of Humilaria kennerleyi only partially drilled by a moon snail. The inset photo shows that the characteristic countersinking of the borehole comes later in the drilling 0.8X

Research study 5: Feeding

Scientists in British Columbia note that a bivalve prey is often limp after drilling by Euspira lewisii. The authors consider and reject the idea of an injectable toxin, and propose instead that the prey may be suffocated. Clams have great capacity for anaerobic respiration, however, and further investigation of this may be justified. It is not unusual to find prey bivalves that have been eaten by moon snails but not drilled (see photo in Research Study 4 above). Reid & Gustafson 1989 Veliger 32: 327.

Research study 6: Feeding

graph showing relationship between innner borehole diameter and outer borehole diameter in different species of moon snailsAre naticid boreholes always the same shape when produced by a certain species and are there differences in borehole characteristics between different naticid species? Boreholes drilled by west-coast naticid gastropods have different shapes, and a species-specific component to borehole geometry is present. Researchers in this subject recognise a feature that they call a “functional borehole”, that is, one with a ratio of inner to outer borehole diameter that is greater than 0.5. Boreholes with a ratio less than 0.5 are “non-functional” because the hole would be too small for the snail’s proboscis to enter and feed. Various authors have questioned the usefulness and general applicability of this statistic. Researchers at the Bamfield Marine Sciences Centre, British Columbia test it by measuring dimensions of boreholes made by moon snails Euspira lewisii in 4 common bivalve species Protothaca staminea, Tresus nuttallii, Macoma nasuta, and Saxidomus giganteus. The results show significant differences in slopes for regressions of inner vs. outer borehole diameters between Euspira heros and E. lewisii, and also between E. heros and Neverita duplicata, but not between E. lewisii and N. dulicata (see graph). The average ratio of inner borehole diameter (IBD) to outer (OBD) for the naticids ranges from 0.53-0.78. Based on a “non-functionality” index of 0.5 as discussed above, about 30% of boreholes from feeding experiments with E. lewisii and 10% from field feeding of N. duplicata would be considered non-functional. Interestingly, there is no significant difference in shape of borehole drilled photograph of borehole made by moon snail Euspira lewisiiby E. lewisii on 4 bivalve prey. Grey et al. 2005 J Moll Stud 71: 253.

NOTE the other species used in the study are Euspira heros and Neverita duplicata, collected on the Washington coast along with specimens of drilled surf clams Spisulasolidissima

Borehole made by moon snail Euspira lewisii into a butter clam Saxidomus giganteus. The borehole ratio is 0.64,making it a "functional borehole" 5X


Research study 1: Diets

A comparison of frequency of occurrence of bivalve species with frequency of occurrence of shells drilled by Euspira lewisii in Departure Bay, British Columbia shows: 1) a preference for butter clams Saxidomus giganteus, and 2) an apparent avoidance of horse clams Tresus nuttallii. Horse clams live at depths of 0.5-1m, deeper than moon snails burrow, and this likely explains why they are infrequent prey. Bernard 1967 Fish Res Bd Can Tech Rept #42.

NOTE in the 40yr since this study was done, it is likely that the Japanese littleneck Venerupis philippinarum has supplanted the native littleneck Protothaca staminea in the Departure Bay area

Clinocardium nuttallii7<1
Protothaca staminea4846
Saxidomus gigantea2748
Venerupis philippinarum1<1
Tresus nuttallii141

Research study 2: Diets

photograph of many olive shells

Not all prey of moon snails Euspira lewisii are bivalves, nor are small prey necessarily excluded from the diet. In the Barkley Sound region of British Columbia, many olive shells Callianax (Olivella) biplicata, living sympatrically with simulated photo view of size disparity between an olive shell Callianax biplicata and a moon snail Euspira lewisiimoon snails, are drilled and eaten. Note the uniformity of position of the drill holes on the shells of the prey snails.

The simulated image on the Right indicates the size disparity between prey and predator 0.3X


Take a moment to study the array of drilled snails in the above photo and consider the sense or nonsense of each of the statements below. CLICK HERE for explanations.

  1. Most snails are drilled in a common location.
  2. Some of the Olivella with incomplete drill-holes may have died of fright, or at least expired "spontaneously".
  3. Some Olivella may have been killed by toxin.
  4. The holes are large and countersunk, suggesting a moon-snail predator.

Research study 3: Diets

A follow-up article by the same research group as in Research Study 6 above at the Bamfield Marine Sciences Centre, British Columbia assesses preferences of Euspira lewisii for thick- and thin-shelled specimens of littleneck clams Protothaca staminea. Results show that a total of 6 moon snails photograph of littleneck clam Protothaca staminea will borehole in umbo regionselect 35 thin- versus 7 thick-shelled specimens in paired preference tests. The mechanism of selection is not known. The authors give several suggestions for future work on the subject. Grey et al. 2007 Veliger 48: 317.

NOTE the authors actually test 5 shell features, but only shell thickness has significant effect on survival

Littleneck clam Protothaca staminea drilled by moon snail 1.6X

Research study 4: Diets

feeding preferences of moon snails Euspira lewisii in British ColumbiaA study by researchers at Simon Fraser University, British Columbia on feeding rates and prey preferences of moon snails Euspira lewisii on 2 beaches on Denman and Hornby Islands, respectively, shows that the preferred prey (of the 3 commonest species) is the native littleneck clam Protothaca staminea, with lesser preference for varnish clams Nuttallia obscurata, and avoidance of Manila clams Venerupis philippinarum (see graph). The more abundant and more commercially important V. philippinarum is eaten, but only in the absence of other prey choices. Data from both feeding enclosures and drilled shells collected intertidally are in agreement. Feeding rates are surprisingly low, averaging only 1 clam per 2wk in summer. Rates are significantly faster, however, on the preferred species than on the less-preferred ones. Overall predation by moon snails on the bivalve community is estimated by the authors to be only about 3%. From a commercial standpoint, the authors conclude that E. lewisii has minimal impact generally on the bivalve community in British Columbia, and particularly on the manila-clam industry. Cook & Bendell-Young 2010 J Shellf Res 29: 223; see also Cook 2008 MSc Thesis, Simon Fraser University, 104pp.

NOTE Venerupis philippinarum is the principal clam species cultured in B.C., representing just less than half of the total, and its value to the overall industry is enormous

NOTE the cages are 1 x 1 x 0.3m in size, constructed of plastic pipes and netting, and are dug into the substratum to a depth of 0.2m. Both feeding preferences and feeding rates are conducted in these enclosures over 5mo periods of 2 successive summers


Research study 1: Growth

photograph of shell of a moon snail Euspira lewisii Although the title of this section ostensibly includes growth, there appears to be no growth studies done on the west-coast moon snail Euspira lewisii. Indivuals would have to be marked in some way, perhaps by numbers scratched onto the shell, then set out in mark-recapture experiments. A moon snail will (eventually) pull back into its shell, allowing shell dimensions to be recorded, but this takes a long time, and some considerable poking and prodding is required. Because of the volume of water contained in the tissues, measurements of live mass would be useless. The shell itself has readily discernible growth lines, but the relationship of these with age is not known.

Moon snail Euspira lewisii in its usual habitat 0.4X