Why Are Filter-Feeding Sharks
Lawrence Taylor wrote: "A friend of mine who is a whale biologist offered this comment. For the high-end carnivores there is no point in being larger than a size where you have no enemies. For the planktoners, the scaling is different. I suspect much must have to do with food storage. Zooplankton is notoriously patchy and seasonal (at high latitudes). To survive the bad periods without starving one must be big. As long as the size of the animal does not get larger than the size of the patch, the bigger the better (if food intake scales with length as or faster than metabolic rate). What do you think of this and: . . ."
An interesting and entirely plausible notion. In general, I would suggest that large size confers upon planktivores the following main benefits:
- increased foraging range (greater swimming efficiency enables traveling farther in search of planktonic 'prey', greater energy [food] storage capacity enables traveling farther between patches of rich feeding)
- increased water processing capacity (larger mouth and increased surface area of plankton-capturing sieves permit greater volumes of water to be filtered)
- relative freedom from predation (too big for most would-be predators to mess with).
The Basking Shark (Cetorhinus maximus) is a gigantic filter-feeder in temperate waters. It has enormous gill slits which nearly encircle the head, making the shark look nearly decapitated. Its modus operandi is laid-back and far from the razor-toothed predatory image of most sharks. Swimming slowly close to the surface (hence its name) with its huge jaws agape, the basking shark's low-density liver may enable it to cruise at an average speed of about 3.7 kilometres per hour without sinking. When feeding, the Basking Shark's normally streamlined head changes dramatically: its jaws expand to resemble a circular-mouthed butterfly net and its gill pouches billow spinnaker-like. Every 30 to 60 seconds or so, each Basking Shark closes its mouth, flutters its gills briefly, and swallows the planktonic creatures that had accumulated on its filtering mechanism. Over 1 650 tonnes of water an hour pass over its bristle-like gill rakers (actually modified dermal denticles), which strain tiny planktonic organisms from the water. In areas with thick concentrations of plankton, the Basking Shark is often associated with another giant filter-feeder, the right whale (Eubalaena). Despite the apparent simplicity of its filter-feeding mechanism, the Basking Shark may be a highly selective feeder — studies in the North Atlantic show that its diet consists almost entirely of copepods of the genus Calanus. Calculations suggest that — during summer months, when plankton concentrations are at their highest — the Basking Shark barely manages to collect enough food to sustain its titanic bulk. In winter — when plankton concentrations fall to levels insufficient to sustain it — the Basking Shark apparently sheds its gill rakers and disappears (it has been speculated that it may sink to the bottom and shift to other food or hibernate), re-growing its gill rakers in the spring; this is the only known example of an annual molt in fishes.
Whale Shark (Rhincodon typus)
Preliminary results from the Whale Shark (Rhincodon typus) telemetry program off Western Australia support the idea that these planktivores are nomadic, apparently following plankton blooms over large areas of the eastern Indian Ocean. Thus, the predictable appearance of Whale Sharks off Ningaloo during March-April of each year may be only one example of a more generalized behavior. (It is interesting to speculate how Rhincodon and other planktivorous elasmobranchs locate rich patches of plankton. When copepods and other zooplankters graze on phytoplankton, the latter release into the water a compound known as dimethylsulphide, which is a product of normal metabolism. Recent studies have shown that some species of procellariform seabirds [tube-snouts, such as albatrosses and petrels] are able to detect dimethylsulphide and that certain species [such as storm petrels, Pterodoma spp.] are strongly attracted to this compound. Given the well-developed and highly acute olfactory system of many elasmobranchs, it seems plausible that filter-feeding forms might also be able to detect dimethylsulphide and use this chemical cue to locate rich patches of plankton. I have no idea how cetaceans, with their poor-to-non-existent sense of smell, manage to locate rich patches of plankton.) In any case, the far-ranging, plankton bloom-tracking behavior of Rhincodon has profound implications for the management of this species.
". . . Do the largest sharks retain "heat" from metabolism because of small body surface area relative to large body volume and/or have counter-current blood flow for metabolic "heat" retention? . . . "
Since heat can only be radiated from the surface of objects and organisms, the familiar Cube-Square Law — as you suggest — seems directly relevant here. Therefore, gigantothermy — or at least some manner of 'thermal lag' — seems likely to be an important heat-retaining mechanism in larger ectothermic sharks, especially in deep-sea and boreal species such as the Bluntnose Sixgill (Hexanchus griseus) and the larger species of sleeper shark (Somniosus). I have not yet tested this empirically on Hexanchus and know of no one who has tested Somniosus. However, Carey et al. '72 reported slightly elevated body temperatures in Carcharhinus limbatus, a species which lacks retia.
Retia mirablia occur in all lamnid sharks. Retia also occur in several non-lamnids. For example, retia have been described in the Bigeye (Alopias superciliosus) and Common (A. vulpinus) Threshers, and I have found well developed orbital retia in the former. Guido Dingerkus once told me that Cetorhinus maximus has a vestigial retial system, but in the (admittedly very few) specimens I have dissected, I have not found any evidence to support this.
Not all sharks that have retia mirablia actively thermoregulate. The Longfin Mako (Isurus paucus), although apparently possessing well-developed lateral retia, is unique among lamnids in that it does not seem to actively regulate its body temperature. My feeling is that, since paucus is generally larger and quite a bit heftier than the Shortfin Mako (I. oxyrinchus), it may rely on passive thermal lag to save energy in the oligotrophic lower epipelagic and upper mesopelagic waters inhabited by this species (it's high aspect ratio pectoral fins probably permit a relatively slow minimum cruising speed, which would not generate as much metabolic heat as a more active lifestyle, but may be another adaptation to facilitate energy conservation). In addition, the large eyes of paucus suggest it may be a visual hunter, possibly including intermittant bursts of active swimming with glide-falls, as demonstrated in the Blue Shark (Prionace glauca) by Carey et al. '97. This behavior may also be an adaptation to conserve energy while permitting searching a wide range of depth in the water column.
It is interesting to speculate whether retia-equipped sharks — such as Isurus oxyrinchus and (especially) the very large White Shark (Carcharodon carcharias) suffer debilitating consequences when they spend prolonged periods in warm waters. Since most enzymes are clunky, unstable, and very heat-sensitive molecules, it seems likely that dumping excess heat may be a problem for these sharks in warm environments. Encased in blubber and lacking sweat glands, cetaceans are highly prone to heat prostration (as is well documented from empathy-evoking but highly informative instances of stranded cetaceans), yet some large whales (such as the Killer Whale, Orcinus orca, False Killer Whale, Pseudorca crassidens, and Bryde's or 'Tropical' Whale, Balaenoptera edeni) seem to do just fine in tropical waters for extended periods. If cetaceans can tolerate such thermal stresses better than endothermic ['cold-blooded'] sharks, it may be because of the much greater genetic variability of the former. Sharks display remarkably little genetic variability from population to population (as is typical of generalist fishes, counter-intuitive as it may seem). Most hormones can function only within very narrow ranges of temperature. The genetic sluggishness of sharks seems very likely to be reflected in the narrow range of temperature over which hormones (especially of a class known as kinases, which are important in muscle contraction) can effectively operate. Thus large, endothermic sharks in warm water may have a much tougher time coping with the problems of overheating than do cetaceans.
". . . In mammals, the larger the animal, the lower the relative metabolic rate. Does the same apply to sharks?"
Published studies on metabolic rates of sharks are rather sparse and patchy. In general, though, smaller sharks — like smaller mammals — appear to have higher metabolic rates than larger ones (Kleiber strikes again!) — see Schmidt and Murru '94 (Zoo.Biol., 13: 177-185) for a recent review of shark energetics.
Unpublished data from Jill Scharold (a former student of the late and lamented Frank Carey) suggest that the metabolic rate of Hexanchus griseus is significantly slower than smaller Squalean (sensu Shirai '96) sharks such as the Spiny Dogfish (Squalus acanthias), again in keeping with the general pattern for mammals. In addition, limited comparative data on haemodynamics of certain deep-sea squaloids (such as the Portuguese Dogfish, Centroscymnus coelolepis, Sherburne '73) also suggest that the smaller deep-sea sharks have a greater activity scope than larger forms, suggesting a correspondingly higher metabolic rate.
I hope at least some of the foregoing is useful or of interest.
— R. Aidan Martin