The biology of external fertilization indeep-sea echinoderms

External fertilization by free spawning is much more common than
internal fertilization among bathyal and abyssal echinoderms. A few
species of echinoids may pseudocopulate by exchanging gametes in
masses of mucus, which are retained on the adult spines. In deep-sea
species of Aspidodiadema, elongate sperm nuclei may be an adaptation
for swimming through the thick mucus that holds the eggs. Denny's
(1988) model of fertilization success predicts that, all else being equal,
external fertilization should be a more viable strategy on smooth sedimentary
bottoms in the deep sea than in the more turbulent flow conditions
found in shallow water. This is contrary to Thorson's Rule, which
predicts greater incidence of brooding (and, by inference, internal fertilization)
with depth. Reproduction at low population density is sometimes
achieved by echinoids, holothuroids, and ophiuroids by aggregation
behaviors. Some species aggregate year round and others remain
with spawning partners only during the reproductive season. Gametes of
echinothuriids contain lipid reserves that apparently allow sperm to
remain motile more than three times as long as typical shallow-water
echinoid sperm. Functional significance of some gamete modifications,
including biflagellate, dimorphic, and pigmented sperm, remains unknown.