Life in the Open Ocean. Joseph J. Torres
Читать онлайн книгу.
3.15 Nematocyst structure. (a) Before discharge; (b) after discharge.
Source: Schultze (1922).
Venoms
Venoms are injected when nematocysts discharge. The virulence of the injected toxins varies considerably from species to species, depending partly on the venom itself but also on how much is delivered. For example, the number of tentacles contacted by an unfortunate swimmer brushing up against a Portuguese Man O’War (Physalia physalis) will likely be much larger than that in a similar encounter with Chrysaora, the sea nettle. The cubomedusae are rightfully most famous for the potency of their venom, particularly the sea wasp, Chyronex fleckeri, which can kill a grown man. Within the hydrozoa, the siphonophores are the most potent stingers, with the widely distributed Portuguese Man O’War the best known. It deserves every bit of its nasty reputation as a world‐class stinger.
All venoms have a few properties in common, and cnidarian venoms are no exception (Hessinger 1988). First, most venoms act on cell membranes. Membranes are the most accessible part of any cell, always play an important role in cell integrity and function, and can be disrupted in a variety of ways. Second, most venoms are proteinaceous. Of the three major types of biomolecules (proteins, lipids, and carbohydrates), proteins are the most plastic in structure and function. They range from enzymes, which can literally change shape, to blood pigments, to inert structural molecules such as the keratins. They are the most amenable to biological design. Also, most proteins are readily digested, so the predator is not poisoned by its own venom! Third, venoms act rapidly. To be effective, toxins must kill, stun, or paralyze the prey quickly to prevent escape.
When considered on a dose‐specific basis (μg kg−1 body mass), cnidarian venoms are among the most deadly known to science, on a par with those of the kraits and the mambas. Purified venoms of the sea wasp and Man O’War show a median lethal dose in mice of less than 50 μg kg−1 body mass. Symptoms in mice of poisoning with purified cnidarian venom include severe respiratory distress, convulsions, and loss of motor control (Hessinger 1988). Because mice are not within the normal prey spectrum of jellies, fiddler crabs have been used as test subjects for effects of cnidarian venom. Their symptoms include violent motor activity, paralysis, and in some cases, autotomy (spontaneous loss) of the walking legs (Hessinger 1988).
Most of the toxic effects associated with cnidarian venoms can be traced to compromised cell membranes. Excitable tissues such as nerve and muscle are particularly susceptible to membrane disruption because they require an intact membrane to function. Cardiovascular distress, loss of motor control, and violent convulsions are all indicative of compromised cell membranes in the neuromuscular system and will nearly always be the most obvious symptoms obtained when an experimental subject is injected with a purified venom.
That said, a world of difference exists between a jellyfish sting and the impacts of purified jellyfish venom, or our beaches would be empty much of the time. Only a very few people die each year from jellyfish stings, and many of those fatalities are due to an allergic response like that seen with bee stings. Most people only feel a minor bit of irritation that goes away in a few hours, even with a venom that is as potent as that of a rattlesnake. Why? The amount delivered by a jellyfish sting is minute compared with the amount delivered by a snake. A large rattler can inject volumes of a milliliter or more of pure venom when it strikes, which makes it very deadly. The larger jellies such as the Man O’ War and the Australian sea wasp can deliver large enough quantities of venom to cause severe distress or worse. For small creatures such as fish larvae and many species of zooplankton though, the amount of venom introduced by small jellyfish stings is enough to disable them. The many small “harpoons” of the nematocysts trap them on the tentacle, and they can then be conveyed to the mouth.
Interaction with Prey
As noted above, medusae are radially symmetrical tentaculate predators that rely on contact to entangle their prey. They are not “sight‐hunters” or raptors (e.g. Greene 1985) capable of actively seeking a prey item and chasing it down. Yet despite their seeming inability to select prey, virtually all gelatinous predators for which electivity indices have been calculated show some degree of selectivity in their diet (Purcell 1997).
Most pelagic cnidarians feed mainly on copepods, the dominant metazoan zooplankton group in the vast majority of the world ocean (Purcell 1997). This is not surprising; a nonliving bit of “marine flypaper” cut into the shape of a medusa and floating with the ocean current would mainly snag copepods. It is the exceptions to the rule that are intriguing and lead one to wonder about how the selectivity is achieved. For example, Purcell (1997) observed that the hydromedusae Bougainvillia principis and Proboscidactyla fed mainly on barnacle larvae and molluscan veliger larvae, respectively. Are cnidarian dietary preferences the result of a limited prey field or actual selectivity? We will investigate cnidarian hunting from a theoretical perspective. Interactions with prey have two basic elements: the encounter and the capture (Purcell 1997). Factors influencing the encounter phase have been treated in a number of studies and may be divided into four subcategories: direct interception, encounter zone, water flow and swimming, and attraction between predator and prey.
Direct Interception
Cnidarian tentacles may be considered as a large, loosely configured filter, and the concept of direct interception derives from filtration theory (Rubenstein and Koehl 1977; Purcell 1997). Because the spacing of tentacles in any cnidarian predator is usually much greater than the prey diameter, particularly for small prey, the direct interception of a prey item on a tentacle depends only on the diameter and swimming speed of the prey and the diameter of the tentacle. Further, the theory predicts that larger, faster prey would be selected for by tentaculate predators generally. It applies most directly to ambush predators such as the siphonophores and some of the medusae. Figure 3.16 shows how the ubiquitous medusan predator Pelagia noctiluca captures prey on one of its tentacles while swimming. Prey is trapped by nematocyst discharge and conveyed to the mouth with the cooperation of the oral arms as the Pelagia continues to swim.
Encounter Zone
Madin (1988) used field observations, videography, and measurements to produce a very useful conceptual model of feeding behavior in medusae, ctenophores, and siphonophores. In his model, the shape of the medusa’s bell, its arrangement of tentacles, and its swimming behavior create an encounter zone where probability of prey capture is maximized. Within the encounter zone, the likelihood that various prey types will be caught depends on the interaction of tentacle density, tentacle spacing, prey size, type, and the behavior and properties of nematocysts (Table 3.1).
Figure 3.17 illustrates modes of tentacle deployment for a variety of different medusae and siphonophores. Most of the medusae illustrated are hydromedusae, underscoring the diversity in their morphology. “Type” species represented in Figure 3.17 are listed in Table 3.1 and described briefly below.
Anthomedusae
Calycopsis typa (Figure 3.17a) has a globular bell and thick tentacles held out radially when fishing, creating a discoid volume about three times the diameter of the bell.
Stomotoca pterophylla (Figure 3.17b) is a species with two tentacles that may be 50 times the bell diameter when fully extended. The tentacles occupy only a tiny fraction of the spherical volume accessible to them. Prey must swim into the tentacles for successful capture.