West-coast researchers are more familiar with species of amphipods that inhabit high intertidal and supratidal regions of the shore, than with the many species that inhabit low intertidal and subtidal regions.  In these high intertidal and supratidal areas the major potential predators of amphipods are birds and beetles, and perhaps mammals such as rodents and raccoons.  Intertidal and subtidal species are likely common foods of fishes.  Active defenses include burrowing, nocturnal activity, and jumping, while passive defenses include mimicry and possible chemical deterrence

Burrows provide protection from surface predators. Strong jumping helps evade capture. This section is divided into nemerteans and insects.

Along the central California coast the semi-terrestrial amphipod Traskorchestia traskiana is uniquely preyed upon by a high intertidal-inhabiting nemertean Pantinonemertes californiensis.  At sites studied the maximal tidal height is 2m, with the distribution of the worm being 1.3-1.7m.  Because Traskorchestia is essentially semiterrestrial, the nemertean is catching Traskorchestia at the lower limits of its vertical distribution.  Roe 1993 Hydrobiologia 266: 29. Photograph courtesy Mary Jo Adams and BEACH WATCHERS.

Talitrid amphipods Traskorchestia
traskiana 2X

On beaches in central California rove beetles Thinopinus pictus, both larvae and the wingless adults, attack and eat amphipods.  The beetles are nocturnally active and ambush the amphipods as they crawl or sometimes jump by from about 5mm distance.  Large, sickle-shaped mandibles enable the attack.  The beetles move seawards during neap tides and landwards during spring tides, apparently following moisture gradients in the sand.  The distribution of the beetle overlaps its 2 major prey species of amphipods Megalorchestia californiana and M. corniculata.  The author notes that several other beetle species may also prey on amphipods.  Craig 1970 Ecology 51: 1012.

Beetle tracks criss-crossing the upper part
of Pachena Beach, Vancouver Island 0.3X

Rove beetles Thinopinus pictus are major predators of talitrid amphipods Megalorchestia californiana on beaches around the Bamfield Marine Sciences Centre, British Columbia.  Observation of activity patterns of the two species in the field indicate that whereas the beetles tend to be most active at dusk or just after, activity of the amphipods varies with the tidal cycle. Note in the data set shown here for mid-July that high tide is in early morning and so beetle foraging coincides with maximum beach activity of amphipods (see graph on Right). However, when high tides occur at night the amphipods tend to remain in their burrows, and so foraging by the beetles is more likely to be unsuccessful.  The beetles alternate their attacks between active and ambush modes, with an overall 9% successful capture of individuals attacked.  Their "hit-and-miss" success rates and “dry” periods of foraging are not a problem for the beetles.  Laboratory feeding experiments show that periodic feeding with up to 4-d intervals is physiologically accommodated by Thinopinus and good health is maintained.  Richards 1983 J Exp Mar Biol Ecol 73: 213; Richards 1984 Ecological Entomology 9: 189. Photograph of beetle courtesy Lovell & Libby Langstroth, California and CALPHOTOS. Photograph of isopod courtesy Jonathan Wright, Pomona College, California. SoCalOniscidea.

NOTE  the author remarks that about 40% of the diet of male and female beetles is made up of amphipods, with about 8% comprising terrestrial isopods Alloniscus perconvexus (see photograph on Right)

NOTE  the author uses pitfall traps and direct observation

Adult rove beetle Thinopinus pictus 2X

Beach isopod Alloniscus perconvexus 2X

On sandy beaches in Barkley Sound, British Columbia rove beetles Thinopinus pictus live in temporary burrows well above the reach of the tide, and emerge at night to feed on amphipods Megalorchestia californiana at the high-tide zone.  The amphipods themselves live in burrows just above high water level and come out at night to feed on stranded algae.  As the amphipods move to and from their food, the beetles lunge at them from hiding places and grasp them in their mandibles.  A digestive enzyme is secreted and the liquified insides are sucked out by the beetle.  The beetle can sense its prey from a distance, and the distance it can do this increases with size of prey (see graph upper Left).  The means of sensing the prey may be via vibrations in air or sand, via scent, or possibly a combination of the two.  Sight appears not to be involved, as blinded¹ beetles are still capable of capturing amphipods. 

Investigation of optimal foraging in this predator/prey relationship, that is, to test the prediction that prey will be selected by a predator in accordance with overall profitability², begins in the laboratory with a determination of the number of amphipods of different size classes required to satiate an adult beetle.  This ranges from 2-4 for smaller prey to around 1 for larger prey. Total consumption leading to satiation for any size class of prey is about 45-50 live mg.  Handling time or time between capture³ of prey and completion of feeding, however, is significantly greater for the smaller-sized prey, and this means that the net return on effort is significantly greater for larger-sized prey. Do the beetles, then, forage optimally in the field?  No, because they fail to discriminate between small and large amphipods in the field, and all sizes are included in their diets.  Part of the reason for this, as explained by the author, is that beetles tend to attack rapidly amphipods that land near to them after jumping.  This leaves no time for size discrimination.  Also, small amphipods tend to clump together and a beetle is able to capture several in one area in a short period of time.  Richards 1982 Oecologia 55: 325; for a review of traditional optimal-foraging models and a new slant on them see Richards 1983 The Am Nat 122: 326.

NOTE¹  eyes are covered with laquer paint

NOTE²   measured here as amount of amphipod eaten per unit handling time, where “handling time” is the time between capture of prey and completion of eating

NOTE³   larger amphipods (>18mm in length, not including antennae) are actually harder for the beetles to capture because they put up more of a fight than smaller ones.  Not only are attempts to jump more powerful, but the gnathopods  and antennae are brought into play, often preventing the beetle from grasping the body of its intended prey.  Once captured, though, the return on effort is obviously much greater for large prey than for small ones