Rogue Hydroids: Predatory Polyps in the Midwater
Georges Bank is a shallow (45 m depth) hummock in the Gulf of Maine, USA, made famous by its former bountiful harvests of cod (Gadus morhua), sadly now depleted. The area was the subject of an intensive multidisciplinary 1990–2000 oceanographic study as part of the international GLOBEC (GLOBal Ocean ECosystem Dynamics) program, funded by the USA’s National Science Foundation. The mission of GLOBEC was to describe the interaction of physical and biological processes in the life history of important species. In the case of George’s Bank, the target species was cod.
A GLOBEC sampling cruise in May of 1994 revealed large numbers of suspended hydroid colonies in the zooplankton over the shallows of Georges Bank (Madin et al. 1996). The colonies were typically fragments of 2–5 polyps each, were widely distributed over the bank, and were the overwhelming dominant component of the net‐caught zooplankton, reaching densities of 10 000 m−3 in the water column and 25 000 m−3 nearer the bottom. Colonies were found to be the polypoid life stage of the hydromedusan genus Clytia, predominantly Clytia gracilis, which normally grows attached to rocks, seaweed, or other available benthic substrate. The polyps suspended in the water column instead of being attached to substrate were not only alive, they were actively hunting. Examination of gut contents and shipboard experiments revealed that the hydroids were catching and digesting larval cod as well as copepod eggs and larvae. Madin et al. (1996) estimated that the hydroids were capable of ingesting half the daily production of copepod eggs and a quarter of the standing stock of copepod larvae per day: an impressive figure for a benthic life stage.
Feeding in the Cubomedusae
The cubomedusae are well known as having a potent sting, particularly the Australian sea wasp Chyronex fleckeri, a large jelly (football‐sized bell and bigger) that can cause open welts in humans and, with severe stings, even respiratory distress and death. The virulence of cubomedusan venom allows the group to take large prey.
Figure 3.19 Prey capture and ingestion in Carybdea marsupialis. (a) Prey capture; (b) Prey transfer to mouth; (c) Prey ingestion.
Source: Adapted from Larson (1976).
The importance of a strong venom and nematocyst system in cubomedusae is underscored by a description of the feeding behavior of the Caribbean cubomedusa Carybdea marsupialis (Larson 1976). Carybdea has four robust tentacles that can reach 30 cm in length when extended or about 10 times the height of the bell. Prey are captured on the tentacles by annular nematocyst batteries that paralyze and trap the prey on the tentacle with considerable adhesive force. The “sticking power” of the nematocysts is so strong that fish too large for Carybdea to handle must break the tentacle to escape. Once the prey is subdued, it is conveyed into the bell cavity and onward to the digestive system by an inward flexion of the tentacle (Figure 3.19). The outer digestive region, or manubrium, is short, and its outer lips are prehensile, so that when a prey item is contacted, it is quickly engulfed.
Though data are limited on the diets of cubomedusae, the little data available suggest a varied menu including polychaetes and small fish as well as the more typical small crustaceans (Table 3.4). Observations reported in Larson (1976) suggested some selectivity for fish by Carybdea marsupialis.
Locomotion
Medusae are among the very few aquatic taxa that swim using jet propulsion. Though “jet propulsion” does not necessarily invoke the mental picture of a slowly swimming medusa, the medusae, the squids and octopi, and the salps and doliolids are major marine taxa that swim using jet propulsion. Scallops also use it to escape from starfish predators by rapidly closing their valves to expel a jet of water, allowing brief forays into the water column.
Table 3.4 Diets of cubomedusae.
Source: Larson (1976), table 2 (p. 242). Reprinted by permission from Springer Nature Customer Service Centre GmbH, Cubomedusae Feeding, author R. J. Larson, in Coelenterate Ecology and Behavior, G.O. Mackie editor, 1976.
| Species | Coelenteron contents | References |
|---|---|---|
| Carybdea alata | Polychaetes, mysids, crab megalopae | Larson (unpublished) |
| Carybdea marsupialis | Polychaetes, misc. crustaceans (copepods, isopods, amphipods, stomatopod larvae, mysids, caridean shrimp and larvae, crab zoeae), chaetognaths, fish | Berger (1900), Larson (unpublished) |
| Carybdea rastoni | Polychaetes, mysids, fish | Gladfelter (1973), Ishida (1936), Larson (unpublished), Uchida (1929) |
| Chiropsalmus quadrumanus | Misc. crustaceans (amphipods, cumaceans, stomatopod larvae, Lucifer spp., caridean shrimp, crab larvae), fish | Larson (unpublished), Phillips and Burke (1970), Phillips et al. (1969) |
| Chiropsalmus quadrigatus and Chironex fleckeri | Caridean shrimp (Acetes spp.), other small crustaceans, fish | Barnes (1966) |
| Tripedalia cystophora | Copepods (Oithona spp.) | Larson (unpublished) |
Medusae swim by ejecting the volume enclosed by the bell toward the rear with a forceful contraction, propelling the animal forward (