Kyne & Simpfendorfer.. - Shark Specialist Group

SECTION II.
LIFE HISTORY OF DEEPWATER CHONDRICHTHYANS
Introduction to the life history characteristics of chondrichthyans
The chondrichthyans are generally considered to be K-selected species, displaying
conservative life history parameters such as relatively slow growth, late age at maturity, low
fecundity and low natural mortality (Hoenig and Gruber 1990, Musick 1999). This results in a
limited reproductive output, putting their energy into a few well-developed young which
resemble miniature adults at birth. As such, the productivity of many chondrichthyans falls
within the range of other long-lived animals, such as many large mammals (Cahmi et al.
1998), and is in contrast to r-selected species whose life history strategy is to produce large
numbers of offspring with a limited lifespan (Hoenig and Gruber 1990). The restricted life
history characteristics of the chondrichthyans place them at risk of overexploitation and
population depletion, with an inability to recover from reduced population levels once
depleted. This vulnerability is generally heightened in the deepsea, a relatively stable, cold
water environment where food resources are more limited. Subsequently growth rates are
slower and recruitment to the population is reduced.
An understanding of several biological parameters is important to accurately assess the
productivity of an individual species and thus make inferences concerning its vulnerability to
fisheries. These parameters include size and age at maturity, fecundity, longevity and
reproductive periodicity. For the vast majority of deepwater species, this data is lacking,
highlighting a significant gap in our knowledge base. Section II of this report collates
available (published and unpublished) information on the life history of deepwater
chondrichthyans and provides an assessment of the productivity of the group.
Provided here are brief introductions to the reproductive biology and age and growth of
chondrichthyan fishes, including basic methodological backgrounds to the information
provided in the life history accounts which follow. Methods for undertaking the assessment of
the productivity of deepwater chondrichthyans are provided at the beginning of that
assessment.
In all chondrichthyans, fertilization is internal although they display a diversity of
reproductive modes. The terminology of reproductive modes and the descriptions following
are taken from Musick and Ellis (2005). In oviparous species, egg-laying can be either single
oviparity (a single egg case deposited from each oviduct) or multiple oviparity (multiple egg
cases retained in the oviduct for an extended period of embryonic development before
laying). In viviparous chondrichthyans, the most basic form is yolk-sac viviparity where
fertilized ova develop in utero, and the embryo obtains nutrition from a single yolk-sac. This
form of viviparity together with both forms of oviparity are considered lecithotrophic modes
of reproduction, whereby there is no additional maternal nutritional input other than that
provided in the ovulated egg. The vast majority of the deepwater fauna is lecithotrophic
(oviparity or viviparous forms). The remaining reproductive modes are matrotrophic,
whereby embryos rely on additional maternal nutritional input for at least part of their
development. The matrotrophic modes are histotrophy (embryos obtain nutrition from a
histotroph or 'uterine milk' secreted by trophonemata in the maternal uteri), oophagy
(embryos subsist on unfertilised ova ovulated by the mother; this includes the advanced form
known as adelphophagy whereby one embryo in each uterus feeds on its sibling embryos
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