Why Are Filter-Feeding Sharks

So Big?

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 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.

Like other filter-feeding elasmobranchs, the
Whale Shark (Rhincodon typus) evolved large body
size, a broad terminal mouth, reduced dentition, and large gill slits.

". . . 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.

Cheers,

— R. Aidan Martin

[Posted to SHARK-L December 5, 1997]

NOTE: Several of the assumptions behind these
calculations were recently shown by David Sims to be faulty, resulting in an
overestimation (by about three times) of the energy Basking Sharks require to
swim. [Return to text from footnote]