847
Comparative feeding behavior of planktonic ctenophores
Steven H. D. Haddock1
Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA
Synopsis The phylum Ctenophora (known as comb jellies) consists of gelatinous marine carnivores found from the
surface to several thousand meters depth. Their morphology can be simple or complex, ranging from a sac-like shape
with no tentacles to large lobed forms with sinuous ‘‘auricles,’’ papillae, and two different kinds of tentacles. This
diversity appears to reflect adaptations to many different diets. For example, some species can continuously ‘‘graze’’ on
small crustaceans or larvae, others engulf larger jellies, and some are able to snare individual larger prey through a variety
of strategies. Thus feeding behavior can help explain the high morphological diversity in this relatively small phylum.
Because of their fragility, comb jellies are difficult to study alive and the natural histories of many types, especially those
found in the deep sea, have not been examined. This account categorizes ctenophore feeding methods using published
reports as well as new observations using submersibles and blue-water scuba diving.
Introduction
Ctenophores (Fig. 1) are unique among animals in
their possession of large macrocilia and a particular
type of adhesive cells known as colloblasts. They use
these in a variety of ways, including for locomotion
and feeding. Because ctenophores are exclusively
carnivorous, their principal feeding task is the
capture of prey; there are no herbivorous ctenophores, and only one genus that can sometimes be
parasitic. As has been noted elsewhere (Greene et al.
1986), apt analogies may be drawn between ctenophore feeding strategies and those of terrestrial
spiders. For example, there are ctenophore counterparts to sit-and-wait orb weavers, ambush predators
like Salticid jumping spiders, and the Bola spiders
that dangle a sticky droplet at the end of a fine
thread.
A few coastal species of ctenophores, particularly
the ‘‘cydippid’’ Pleurobrachia pileus, the lobate
Mnemiopsis leidyi, and the sac-like Beroe cucumis,
have been studied for many years (Mayer 1912; Main
1928, and many others). Their ready accessibility and
relative robustness make it possible to study their
feeding in laboratory settings and, therefore, these
species have formed the basis for our textbook view
of the phylum.
Blue-water diving and submersibles have provided opportunities for in situ observation and have
provided new information on many fragile and
deep-sea species. As compared with other reviews
(Purcell 1991), here the focus is on variation in
behavior across the phylum, as also true of the
studies by Harbison et al. (1978) and Matsumoto &
Harbison (1993), and not on physiology or energetics, which have been well covered in particular
experiments (Kremer et al. 1986; Purcell et al. 1991;
Scolardi et al. 2006).
Feeding modes
One impediment to discussing and comparing
ctenophores is that the standard taxonomy and
nomenclature is now known to be a poor reflection
of the relationships among them. Class Tentaculata
refers to all genera except Beroe (including Lobates
and Thalassocalycida), and so because of its breadth,
is not useful in this context. Even the Order
Cydippida, encompassing a range of genera that
primarily use two long, opposed tentacles, has been
clearly shown to be polyphyletic (Podar et al. 2001),
as was suggested some time ago (Harbison 1985).
Therefore, a comparative analysis has to be carried
out almost genus-by-genus or a family at a time.
In addition, for this article, the term ‘‘cydippid’’ is
highlighted since it is used in the classical morphological sense but without its standard taxonomic
implications. Below are listed the known feeding
modes, sorted according to ctenophore morphology.
They are separated into groups that (1) use tentacles
From the symposium ‘‘Integrative Biology of Pelagic Invertebrates’’ presented at the annual meeting of the Society for Integrative and
Comparative Biology, January 3–7, 2007, at Phoenix, Arizona.
1
Email: [email protected]
Integrative and Comparative Biology, volume 47, number 6, pp. 847–853
doi:10.1093/icb/icm088
Advanced Access publication September 18, 2007
ß The Author 2007. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved.
For permissions please email: [email protected].
848
Fig. 1 The lobate ctenophore Deiopea kaloktenota. Structures
relevant to feeding are labeled.
for feeding, (2) use lobes for feeding, (3) feed by
engulfing, and (4) those that are trophic specialists.
Use of tentacles for feeding
Pleurobrachia has a sit-and-wait strategy as thoroughly described by Tamm and Moss, (1985) among
others. It swims in a horizontal semicircle, deploying
its two long tentacles gradually so that the tentilla
hang down evenly spaced (Fig. 2A). Once the
tentacles are fully extended, the ctenophore eases to
a stop with its mouth upright, and begins to wait.
When a prey item, in this case typically a copepod,
swims into the sticky tentillar net and becomes
ensnared, Pleurobrachia reacts by retracting the
tentacles and swimming ahead until the prey item
is brought close to the body. At that point, the
ctenophore starts rotating in place so that the prey is
brought around to the mouth and ingested. One
interesting aspect of this behavior is how it is
brought about by a few simple rules and associated
neural response patterns (Moss and Tamm 1986;
Moss 1991).
This feeding behavior has several implications: the
rate is dependent on motion of prey (Greene et al.
1986), and the efficiency depends on the ctenophore
being undisturbed enough to carry out the deployment of its net. As a result, researchers originally found clearance rates between 5 and 10 l/
day for Pleurobrachia, and about twice that for
S. H. D. Haddock
Mnemiopsis (Greene et al. 1986). However, using
larger containers, later studies showed that
Pleurobrachia was capable of clearing 10 times that
volume (Gibbons and Painting 1992). The closely
related genus Hormiphora operates in a similar
manner, except that it may deploy its tentilla
vertically in a U-shaped pattern, and species in
the temperate Pacific feed primarily on larger
euphausiid krill. The main morphological difference
is that the tentacular bulbs of Hormiphora are longer
than in Pleurobrachia, providing a stronger anchor
for retaining prey.
Other ctenophores use similar strategies, although
targeting different sizes of prey. The mertensiids
(Mertensia, Callianira, and many undescribed deepsea genera) are not particularly close to the
pleurobrachiids phylogenetically, but they also feed
by deploying two tentacles with a web of tentilla and
passively waiting for encounters from actively swimming prey. Like Hormiphora and Mertensia from the
Arctic, Callianira from the Antarctic feed on adult
and larval krill and copepods (Hamner and Hamner
2000; Scolardi et al. 2006). Some of the large
deep-sea mertensiids, which have not yet been
described (for example, Sp. 2 in Fig. 1 of Podar
et al. 2001), have extremely strong and sticky
tentacles that can span two meters, and these are
able to capture large shrimp.
Many other deep sea genera such as Bathyctena
(in its own family) are physically fragile, but wellequipped with fine tentilla used for immobilizing
small prey. Their capacious stomodæum is an
indicator of their ability to handle the large prey
found in the deep sea (Fig. 2C). A few undescribed
species have tentilla so fine that they appear to form
a hazy cloud. These species would have access to the
smallest of nauplii and copepods, which are known
to be very abundant (Hopcroft et al. 2001), although
perhaps not easily captured if they are associated
with marine ‘‘snow’’ particles (Alldredge and
Silver 1988). Charistephane fugiens is a rare, small
(1.5–2 cm), flattened species found between 100 and
300 m in the Pacific and Atlantic Oceans and known
from the Mediterranean Sea (Chun 1880). Its
tentacles are so fragile that it is difficult to consider
how they might function. The consistent presence of
colorless oil droplets in their gastrovascular canals
suggests a diet of small copepods. Euplokamis is
at the other extreme of tentillum development. Its
prehensile side-branches are widely spaced and
spirally coiled into a droplet-shape (Mackie et al.
1988). They mechanically snare copepods like a bola,
wrapping around them when triggered.
Comparative feeding behavior of planktonic ctenophores
849
Fig. 2 (A) Pleurobrachia bachei deploys a web of sticky tentacles to capture active prey. (B) Bathyctena chuni from the deep Pacific with
its mouth open showing the voluminous stomodæum. (C) Dryodora glandiformis, a polar species that specializes in larvaceans. The outer
‘‘vestibule’’ (ve) holds the larvacean and its house after initial capture. (D) The lobate Ocyropsis maculata immaculata has no functional
tentacles as an adult and uses its muscular lobes to capture prey directly. The white spots are parasitic amphipods. (E) The
‘‘showercap’’ ctenophore Thalassocalyce inconstans from the aboral end. In this species, the lobes are fused and there are no auricles.
(F) The lobate Leucothea pulchra viewed along the body axis from the oral end; it is swimming straight toward the viewer. Note the
sinuous auricles (au) between the lobe surfaces. (G) Beroe forskalii, having ingested a large portion of a Leucothea. The comb rows
(arrowheads) and orange papillae of the prey can be seen inside the mostly transparent gut of the Beroe.
850
The mertensiids, although they are a diverse group
and defy generalizations, are interesting to consider
because they are found near the base of ctenophore
phylogeny, and thus might be most indicative of the
ancestral ctenophore state. The mertensiid form is
also typical of the ‘‘cydippid’’ stage through which
lobates pass, so the cydippid form may constitute an
important part of their ecology, even though
they adopt a very different feeding style later in
development.
Use of lobes in feeding
Lobates are ctenophores with an apparently derived
morphology in which broad lobes grow out at, or
near, the oral end, and long trailing tentacles are
generally replaced by a row of tentacles running
along either side of the mouth (Fig. 1).
Of the lobates, the most commonly studied is
Mnemiopsis from the Western Atlantic (Main 1928;
Sullivan and Reeve 1982; Costello and Coverdale
1998). It is relatively passive, even for a ctenophore,
and if laboratory studies can be extrapolated to the
field, generates feeding currents with its auricles.
In the coastal and estuarine environments where it is
found, this is an effective feeding mode, and as a
result Mnemiopsis can have a major predatory
impact on crustaceans and on fish larvae, notably
in the Black Sea (Vinogradov 1989). Its filter-feeding
mode of accumulating prey does not seem typical
of other lobates such as Kiyohimea, Deiopea,
Eurhamphaea, and Leucothea. Bolinopsis falls somewhere between in activity levels. To understand
lobates, it is important to acknowledge that ctenophores are not oriented like medusae, with their
mouth tucked in the lee of an umbrella-like structure
(but see Thalassocalyce subsequently). Unlike medusae, which lead with their aboral end when swimming (Costello and Colin 1995), nearly all
ctenophores encounter the environment with the
oral end first. Leucothea (Harbison et al. 1978;
Matsumoto and Hamner 1988) and Kiyohimea ‘‘fly’’
through the water with their lobes spread like the
wings of a biplane (Fig. 2F). Auricles beat sinuously
between the two sticky surfaces of the lobes, while a
veil of oral tentacles trails from the line of the mouth
across the main body of the animal. Copepods or
other prey that pass between the lobes are disturbed
by the motion of the auricles, and in their
efforts to escape often leap into contact with the
lobe.
Bathocyroe is an exclusively deep-sea genus
(4400 m) with large lobes and a small, pigmented
stomodæum (Madin and Harbison 1978a). The oral
S. H. D. Haddock
tentacles extend along the length of the lobes. Its redpigmented gut serves to mask the bioluminescence of
its copepod prey. In addition to slow forward
swimming propelled by the comb plates, this genus
also has an escape response in which the muscular
lobes are clapped together, thereby sending it backwards through the water—a behavior also seen in the
genus Ocyropsis.
Ocyropsis (Fig. 2D) species are unique among
pelagic ‘‘Tentaculata’’ in having no functional
tentacles as adults (O. maculata has a rudimentary
tentacular apparatus and O. crystallina lacks
tentacles altogether). Instead, they use their muscular
lobes to capture food, grabbing items individually,
and bringing the mouth over to take the prey. They
have been found with ctenophores, euthecosome
pteropods, euphausiids, and fish larvae in their
guts (Matsumoto and Harbison 1993). Thus,
they also do not fit within the typical lobate
feeding mode. This genus is very successful and
abundant in oligotrophic tropical waters, perhaps
because of its adaptation to feeding on certain larger
and less frequent prey.
In Deiopea kaloktenota (Fig. 1), the lobes are much
reduced, although this species still has an oral veil of
tentacles. In captivity, at least, its mode of feeding
is between the mechanical methods of capture
of Ocyropsis and the more filter-like grazing of
Leucothea. Deiopea is seen to ingest siphonophores
and other noncrustacean prey, in addition to
copepods. Morphologically, Deiopea looks like an
earlier developmental stage of Kiyohimea or one of
the other large lobates.
The cestids Cestum and Velamen can be considered as morphological and evolutionary extremes of
the lobate body form (Ceccaldi 1972; Stretch 1982;
Harbison 1985; Podar et al. 2001). Their oral surface
has been extended laterally, while the lobes themselves have been reduced giving them a flying-wing
appearance. A veil of oral tentacles drapes across the
body’s surface, and the subtentacular comb rows are
reduced, while the substomodaeal canals extend
the full width of the body. These genera proceed
laterally through the water, periodically reversing
direction and moving up or down in the water
column, while accumulating small copepods as prey
(Stretch 1982; Matsumoto and Harbison 1993).
Feeding by engulfing
Beroe (Fig. 2G) are sack-shaped ctenophores that lack
tentacles for their entire lives. Nonetheless, they are
powerful predators on other gelatinous organisms
and employ a raptorial feeding mode (Swanberg
Comparative feeding behavior of planktonic ctenophores
1974). Depending on the body size and species, Beroe
engulf prey completely (B. cucumis feeding on
Pleurobrachia), or bite off pieces of prey (B. cucumis
feeding on Bolinopsis and B. forskalii feeding on
Leucothea). They are sensitized during hunting by
chemical and mechanical cues. Their mouths are
lined with tooth-like macrocilia that can advance
their lips rapidly over the surface of the prey, while
they are consuming it (Tamm 1983; Tamm and
Tamm 1991).
Specialists
There are several interesting specialists among the
two-tentacled ‘‘cydippid’’ ctenophores. Lampea looks
superficially like Hormiphora, but with tentacles that
exit laterally rather than aborally, and with an
extensible Beroe-like mouth. It feeds almost exclusively on salps. As juveniles, when they are much
smaller than the salps, they attach themselves by
their mouth, and form a parasite-like plaque
(Harbison et al. 1978). Eventually as they grow,
Lampea species are able to engulf individual or entire
chains of salps. They can be seen trailing behind an
actively pumping chain, hanging onto the end-most
units. This shift in feeding from parasite to predator
on the same prey allows them to consume salps at all
sizes of development and effectively respond to
massive blooms (Madin 1974).
Although Haeckelia is classified as a ‘‘cydippid’’ it
is the best example of why the classification of
ctenophores needs a revision. Like Lampea, this
small (0.6–2.5 cm) genus has an extensible Beroe-like
stomodæum, but also has two orally-exiting tentacles
that are simple, meaning they have no side branches.
Mills and Miller (1984) showed that H. rubra were
able to eat the tentacles of Aegina, a narcomedusa,
while incorporating its nematocysts into their own
tentacles. It is not clear whether the nematocysts
serve any protective or predatory function in the
genus; it is interesting that the narcomedusae from
which it obtains the cnidae use them to capture and
adhere to gelatinous prey. Haeckelia beehleri and
H. rubra have been observed to feed by deploying
their tentacles many body lengths above their mouth,
in two arcs that look like fine antennae. They passively wait for active predators of jellies that forage
with their tentacles held ahead of them. Commonly
for H. rubra, this is narcomedusae in the genus
Solmaris, which can occur in dense blooms. When
the ctenophore encounters prey, it rapidly retracts its
tentacles and attempts to engulf the medusa. The
speed with which this occurs is startling, considering
the standard view of the behavior of gelatinous
851
animals. Haeckelia species are also occasionally found
to ingest siphonophores.
Aulacoctena is a deep-sea genus containing one
described species from the Arctic (Mortensen 1912)
and likely several yet to be described. It is presently
classified in the same family as Haeckelia due to its
simple, orally-exiting tentacles. Internal morphology
and molecular evidence (unpublished results), however, show that this is an example of convergence,
and that simple tentacles, without side branches,
have arisen at least three times within the phylum
(including Dryodora; see subsequently). For those
who consider there to be strong selection for spacing
of the tentilla in the evolution of feeding,
Aulacoctena presents a perplexing case. Its prey and
mode of feeding is still unknown, although there is
evidence of a unique diet from the Arctic (Raskoff,
personal communication), as well as consumption of
large crustacean prey (personal observation). There is
no evidence yet that it feeds on deep-sea narcomedusae, as might be suspected by the morphology of
its tentacles and homology with Haeckelia species.
In addition to Lampea and Haeckelia, the genus
Dryodora (Fig. 2C) has one of the most specialized
diets known for ctenophores. Harbison et al. (1997)
found them to be adapted for feeding on larvaceans,
urochordates that filter-feed from inside mucus
houses. The ctenophore has two simple tentacles
with which it encounters prey. It has a unique
‘‘vestibule’’ outside its mouth, and in this antechamber it engulfs a larvacean together with its
house. The ctenophore holds its outer mouth closed
until the larvacean senses something wrong and
wriggles free. Unable to escape, however, the
larvacean eventually ends up getting snared by the
ctenophore’s inner mouth and trapped in its
gut, while the mucus house is eventually discarded.
Like Aulacoctena and Haeckelia, Dryodora has
simple tentacles, although preliminary molecular
data show that this is yet another independent
origin of this trait.
Related to the lobates is a recently described,
umbrella-shaped group including the genus
Thalassocalyce (Madin and Harbison 1978b). Their
distinguishing feature is a lack of auricles, and lobes
that are fused into a continuous dome (Fig. 2E).
Young individuals are found near the surface, but
adults are generally found deeper than 200 m. Other
auricle-free ctenophore species (with and without
fused lobes) are abundant in the deep-sea, but have
not yet been described because of their extreme
fragility. Even using remotely operated vehicles it is
difficult to collect and observe these alive. Despite
this fragility, species of this auricle-free morphology
852
are able to capture relatively large crustaceans and
subdue them. They capture prey by sitting with the
mouth open and then snapping it shut when an
organism ventures inside. In the capturing position,
they have only their outermost oral ring closed, so
that the prey is smothered. This passive strategy is an
effective way for fragile jellies to capture robust
crustaceans and other prey that they would be
otherwise unable to overpower.
Discussion
Despite their very low genetic divergence (Podar
et al. 2001), species in the phylum Ctenophora
represent a great deal of morphological differentiation. Much of this variability appears to be directly
related to specialization for feeding on prey of
different sizes, behaviors, and abundances. Because
they do not have stinging cells that immobilize prey,
they must use other means for its capture, most
often mucus and their special colloblasts (gluesecreting cells). In addition, there are many ways in
which their giant cilia are used for propulsion,
conveying food, and biting or engulfing certain types
of prey. Even for marine biologists, understanding
these adaptations, however, is difficult if the
ctenophores are not observed under natural conditions, because they are often depicted as though they
are aberrant medusae. Unless they are themselves
escaping, ctenophores encounter their environment
oral end first, whether actively hunting or deploying
their feeding webs.
Bioluminescence, a trait of nearly all ctenophores
(Haddock and Case 1995), is linked in many ways to
feeding ecology. Many ctenophores have red pigmentation that is thought to absorb light produced
by ingested prey. One example of how this might be
an advantage for the prey is in the nonlethal feeding
of Beroe on Bolinopsis. An attack in which the lobate
is not completely engulfed could prove dangerous for
the Beroe, as prey tissue persists for many minutes or
hours during digestion (Fig. 2G). This tissue can be
seen to glow steadily, creating a signal that would
attract predators. Bioluminescence may also be
dependent on ingestion of the luciferin coelenterazine via luminous prey. Although this dependence
has only been demonstrated to occur in hydromedusae so far (Haddock et al. 2001), it will be
interesting to test in a variety of ctenophore species.
This could possibly help explain why members of a
krill-eating ctenophore genus (Hormiphora) do not
luminesce, since euphausiids contain a different
luciferin. That potential link and how this might
S. H. D. Haddock
relate to the other nonluminous, but copepod-eating,
genus Pleurobrachia remains to be established.
Given the often unconvincing fossil record, it is
difficult to establish the sequence through which
ctenophore feeding might have evolved. Some
paleontologists suggest that atentaculate Beroe-like
species were first to evolve, but there is molecular
evidence (Podar et al. 2001) that tentacles were
present in the ancestor of extant species, suggesting
that a loss of tentacles is the derived condition.
It is clear that for this strictly carnivorous phylum,
tentacles and cilia have been the foundation for the
diversification of feeding.
A thorough interpretation of ctenophore behavior
will require a revision of their phylogeny and
systematics. The cydippids have been shown to be
polyphyletic (Harbison 1985; Podar et al. 2001) and
the tentaculate genus Haeckelia is more closely
related to the nontentaculate Beroe than it is to
other tentacle-bearing genera. Only after resolving
these relationships will the evolution of feeding be
interpretable. It has been suggested that the lobates
have a major advantage over the ‘‘cydippids,’’ given
their ability to exploit a small size-fraction of prey
and to feed nearly continuously. However, in many
habitats, there are also advantages to being able to
capture larger prey. The spacing between tentilla
does not seem to be the main factor determining
efficiency of tentaculate feeding (as suggested by
Costello and Coverdale 1998), and rates used in such
comparisons can be strongly affected by experimental
conditions (Gibbons and Painting 1992).
This small phylum continues to surprise with its
unpredictable variation. In addition to the few wellknown modes of feeding, there are many morphological adaptations that allow species to exploit a
variety of prey available in the planktonic
environment.
Acknowledgments
I would especially like to thank Claudia Mills and
Richard Harbison for educating me about ctenophores over many years. Erik Thuesen and Claudia
Mills provided valuable feedback on the article.
Research was enabled by many blue-water divers and
by the crews of the ROVs Ventana and Tiburon, and
supported by the David and Lucile Packard
Foundation.
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