oil globule, about 1/25th of its volume, enough to increase buoyancy slightly. In any
case, the eggs and early larvae are buoyant, so they rise to become part of the near-
surface plankton. Fish seem to be the only group of nekton in which eggs of most
species rise. Their fecundity can be very high. A 3 cm female of Cyclothone, probably
the most numerous vertebrate genus in the sea, can produce 300 eggs, and a deep-sea
gulper of modest size that spawns once and dies (Eurypharynx pelecanoides, mature
at 60 cm – mostly jaw and whip-like tail) can produce 33,000 or more 1.3 mm eggs
(Nielsen et al. 1989). Most eggs are eaten as they ascend, and only a few survive to
hatch. Larval mortality is also high. Of course, high fecundity to compensate for low
egg and larval survivorship is a common strategy in all marine groups. On the whole,
the early larvae are very simple, worm-like creatures 1–3 mm long, usually with a
persistent, ventral yolk sac. Early feeding, usually on microplanktonic protozoa and
crustacean nauplii, is important to survival in many species, and often begins before
the yolk sac is fully absorbed. Later larvae can be very unusual-looking fish indeed. A
favorite example among planktologists is the larva of the black dragonfish,
Idiacanthus, a deep-sea predator with a prey-attracting chin barbel. The larva carries
its eyeballs on slender, mobile stalks up to half as long as the body. Processing
information about the location of prey from these distant, variably positioned eyes
must involve complex proprioceptive as well as visual sensations. Development
usually includes a fairly distinct metamorphosis to a juvenile form resembling the
adult more closely, accompanied by a downward ontogenetic migration.
(^) “Life in extreme environments” has become a catchphrase in oceanographic circles,
a selling point for the funding of research programs. Meso- and bathypelagic habitats
are extreme environments, and the animals making homes there have marvelous and
mostly interpretable adaptations. We should class them, along with hot-vent archaea,
as extremophiles, and give them the attention their interest deserves.