The life cycle of
echinoids is simple. Reproduction is strictly confined to adult stages that
generally involve the spawning of eggs and sperm freely into the seawater
followed by the external fertilization (Kelly, 2000). Most echinoids are free
spawners that produce a vast number of small, yolk-poor eggs that develop into
planktonic, feeding larvae known as echinoplutei in response to environmental
factors that signal the availability of suitable benthic habitat (Mc Edward et
the gonads release large quantities of eggs and sperm into the water for the
increased chance of survival in the marine environment (Bolton et al., 2001).
When sperm are added to the water, the outer gelatinous layer around the egg
dissolves and the sperm become attached to the jelly coat (Tamaru et al.,
2010). A fertilization membrane lifts from the surface of the egg after about 3
minutes, stretching the jelly layer and leaving a perivitelline space. Zygotes
were classified with the formation of the fertilization membrane around the
circumference of the egg (Kelly, 2000). Said membrane started to form within 8
minutes after insemination. With the penetration of the sperm, it was pushed
forward by its microtubules towards the center of the egg (Rahman et al.,
2009). When the egg’s cortical layer is reached by the sperm, exchange of
genetic material happened and cytoplasmic movements are increased. Before the
first cleavage to happen, the membrane stopped the vibration, the cell surface
becomes regular and hyaline layer thickened (Sarifudin et al., 2016).
In most species,
the hatched egg turns into a planktonic larva (Poulin & Feral, 1996).
Development biologists investigated the early embryonic development through the
cleavage stages, blastula formation, gastrulation and larval morphogenesis (Mc
Edward et al., 2007). Blastula stage emerged as the end of the cleavage stages.
This stage forms an epithelial monolayer surrounding with a central, spherical
blastocoel whose cells in between one another during the further development
(Spiegel and Howard, 1983). Numerous microvilli are present in the exterior
apical ends of the apical ends which are tightly rooted in the apical lamina
(Hall and Vacquier, 1982). As the development continues, the blastula cells
become resistant to small sugar molecules from the external environment (Moore,
1940). At this point, it breaks the envelope allowing the embryo to hatch and
become a free-swimming blastula. Several yolk vesicles are enclosed in the
peripheral layer and deposited in the ectoderm during the later development of
the embryo (Hardin, 1987). Followed by gastrula wherein gastrulation triggers a
different pattern of motility to the developing embryo and dramatic decrease of
the overall rate of mitotic divisions are noted (Gustafson and Wolpert, 1967;
1963). With about 1000 cells, the epithelium of the vegetal pole of the embryo
flattens and thickens to form the vegetal plate in the animal pole (Spiegel and
In the larval
stage, the echinoid is developing into a juvenile. The developmental patterns
are determined by three factors: mode of nutrition, habitat, and type of
morphogenesis (Mc Edward et al., 2007). Planktotrophic larvae develop from
their eggs which uses a portion of energy to metamorphose. Larvae with higher
feeding concentration tend to develop and metamorphose quickly into bigger
juveniles compared to their siblings that are less fed (Reitzel et al., 2005).
cells were not present in the ventral (oral) region of the embryo at prism
stage. During the course of the complete development of prism larva, the
surface of the embryo was covered by cilia with an apical tuft on the anterior
pole and a ciliated ring around the anus (Rahman et al., 2012). After this
stage, the two-armed larvae will develop as lecithotrophic larvae that depend
on the nutrition coming from the yolk.
is complex, the pelagic larva that generally possesses eight anterior-directed
arms that bear the ciliated feeding structure that is supported by calcareous
skeletal rods (Okazaki, 1975). It develops a complete, functional gut and
mostly feeds on suspended particulate matter (Burke 1987). Nerves are
distributed along the ciliated band and in the esophagus for swimming and
feeding coordination (Burke, 1978; Strathmann, 1971; Mackie et al., 1969) and
possess a neuropil, a dense network of interwoven nerve fibers, which controls
the metamorphosis that is located at the anterior end of the oral hood (Burke,
The internal cavity
of the larvae is mostly filled with gel-like blastocoelic space (Strathmann,
1971). Its body form is very diverse due to variation in the number and
relative sizes of the larval arms (Pearse & Cameron, 1991). However, all
echinoplutei share the same larval body plan that is defined by body symmetry,
morphogenetic axes, coelomic organization, nervous system, vestibule, and
metamorphic fates of larval structures (Mc Edward & Miner, 2007).
stage takes one (1) month but this varies to the species (Emlet et al, 1987).
Its development includes growth and elaboration of the larval body to the
formation of the rudiment of the juvenile echinoid in preparation for the
induction of settlement and metamorphosis to juvenile adult. Its
post-metamorphic development includes growth and sexual maturation to become a
reproductive adult (Mc Edward et al., 2007).