Little is known about the predators of flatworms and their possible defenses.  Most or all species of polyclads have ight-sensitive eyespots that may serve a protective function in finding under-rock hideaways. The eyespots, or ocelli, are present in the Müller’s larva as a single pair, and in the adult as hundreds of ocelli. The ocelli are simple in structure and are non-image-resolving.  Interestingly, the eyes in the larva are asymmetrical in structure, although the difference is minor.  Studies on the turbellarian Pseudoceros canadensis at Friday Harbor Laboratories, Washington show that each larval ocellus is comprised of a cup-shaped pigmented cell and 3 sensory cells.  In the adult worm the many ocelli are in two clusters, located above the cerebral ganglia in the location shown in the photograph. The ocellus shown in close view above is actually that of the larval worm, but its shape is similar to that of an adult worm.  The authors suggest that the pigmented cell in an ocellus of a Pseudoceros larva or adult may function as a shadow-casting structure.  The way this functions is described as follows. A sensory cell or its photoreceptive cilia may be illuminated by incident light, or shaded if the light first strikes the pigmented cup, and this could allow the direction of light to be detected by the worm.  The authors do not speculate whether this mechanism could lead to a protective “shadow response” or escape response in the adult.  Eakin & Brandenburger 1981 Zoomorphology 98: 1. Photograph of Pseudoceros courtesy Kevin Lee, Fullerton, California diverkevin.

NOTE  the authors term these “pericerebral eyes”. Flatworm eyespots are variously termed “cerebral ocelli”, “epidermal ocelli”, or “tentacular ocelli”, depending upon their location

In a large study lasting over almost 2 decades, examination of prey in stomachs of 85,454 fishes belonging to 27 species and 14 families reveals the presence of nemerteans in only 276 individuals, or 0.3% of the total.  The author notes that this is likely to be a conservative estimate owing to bleaching of colours from stomach acids, lack of identifiable hard parts, and difficulty associated with identifications in the field.  As an example, nemertean remains are found in 223 of 26,642 (0.8%) fishes examined in microscopic detail in the laboratory, but in only 53 of 58,812 (0.1%) of fishes examined in the field.  Thus, almost 10 times more nemerteans are identified from careful and more controlled scrutiny in the laboratory.  From these data it seems that fishes are not major predators of nemerteans, yet, some large species of nemerteans, for example, Cerebratulus spp., have a history of being used by fisherfolk as bait in areas of North America and elsewhere.

As for birds, worldwide only 3 species of birds are known to feed on nemerteans, the black-bellied plover Pluvialis squatarola, semipalmated plover Charadrius semipalmatus, and the herring gull Larus argentatus.  All 3 are fairly common west-coast species.

A review of the world literature on invertebrate predators of nemerteans indicate only spotty consumption. For example, there are 4 positive records from examination of 2183 gut contents of short-fin squid Illex illecebrosus and 1 in 50 guts of green crabs Carcinus maenus.  Several species of nemerteans are known to ingest other nemerteans.  A review of laboratory feeding experiments in which 7 species of nemerteans belonging to 3 orders are presented to a variety of potential invertebrate predators, shows almost wholesale rejection by polychaetes, crabs, lobsters, and hermit crabs.  McDermott 2001 Hydrobiologia 456: 7. Photograph courtesy Pamela Roe, California State University Stanislaus, California.

The large heteronemertean Cerebratulus montgomeryi is
capable of enormous extension, to several meters or more

The nudibranch Aglaja diomedea is reported to eat Paranemertes peregrina, but in tests with adult specimens the predator seems only able to bite off a portion of the nemertean's tail.  Several luminescent nemerteans are described, but it is not known if the luminescence functions in some way in defense.  Roe 1976 Biol Bull 150: 80.

NOTE  at least one luminescent species Empletonema kandai is described for Japan, but there is no record of any bioluminescent species of Emplectonema on the west coast. Kanda 1937 Rigakukai 35: 5.