tubular extension of the body, a stolon. Many of these sections (10–50) remain
attached to each other side to side, and develop into chains (or a closed circle in
Cyclosalpa) of new, fully formed, filtering cylinders – new salps. This aggregate
stage reproduces sexually. The gonad, which is wrapped into the posterior mass with
the gut, operates first as an ovary, producing a single egg (thus, the sexual phase
reproduces but does not multiply the stock), which is fertilized by sperm pumped in
from the surrounding water. The egg is retained in a chamber in the test wall and
nurtured through an umbilical connection to the aggregate individual. The gonad then
proceeds to produce and release sperm. Eventually, the embryo breaks out through the
body wall, becoming the solitary phase, and the cycle repeats. The cycle is rapid, a
few days, with the production of many chains of many aggregate individuals leading
to very strong population outbursts – salp blooms. Towing a plankton net through an
unsuspected salp bloom can produce a mass of watery tissue too heavy to swing
aboard. We do not have clear understanding of the conditions stimulating, or
conversely preventing, these blooms.
Doliolidae
(^) These are very like salps, but differ in specific points of anatomy and details of the
life cycle. In doliolids, the test muscles completely circle the body, whereas in salps
there is a space along the ventral surface with no muscles. The doliolid solitary
produces young by budding, which migrate along and then attach to a tube of tissue
(stolon) trailing behind their “mother”. The solitary stage tows this column of its
young aggregates as they feed and grow. Eventually, the solitary loses most of its
structure, becoming simply a muscular towing engine for the train of aggregates that
feed it through the stolon. Some of the individuals in the aggregate train separate and
mature sexually, with gonads that produce both eggs and sperm. Their embryos hatch
as “tadpoles” (tailed larvae) that grow into new solitaries. The typical genus is
Doliolum.
Pyrosomida
(^) Pyrosomes are colonial in a different format from salps or doliolids, with the barrel-
shaped pumping and filtering individuals embedded across the wall of a gelatinous
tube that is closed at one end. Their incurrent openings are on the outside of the tube,
with the excurrent ones on the inside. Thus, the colonial tube is propelled by the water
jet from its open end. Typical colonies are 2 cm diameter by 8–20 cm long, but giants
occasionally develop that are >50 cm diameter and several meters in length. The
individuals are dramatically bioluminescent, hence the name meaning “fire body”.
Individuals in the tube wall respond with prolonged flashes to rubbing or poking on
the colony sides. The individuals in the colony reproduce sexually, as hermaphrodites,