For the most part, there are no external morphological differences between males and females. Most echino-derms are sexual reproducers, spawning copious numbers of eggs. The crown-of-thorns starfish, Acan-thaster planci, is estimated to release up to 60 million eggs per year (Box 26.1). Many echinoderms from the GBR spawn in the spring and summer and can have a very long spawning period. Mass spawning of echin-oderms is common and many species spawn at the same time as the corals on the GBR. Release of gametes
Figure 26.1 Asteroidea. A, Acanthaster planci (x 0.07) (photo: J. Keesing); B, Linckia laevigata (x 0.25); C, Linckia guildin-gii (x 0.30); D, Linckia multifora (x 0.25); E, Nardoa novaecaledoniae (x 0.33); F, Culcita novaeguineae adult (x 0.18); G, Culcita novaeguineae juvenile (x 1.00) (photo: S. Walker); H, Echinaster luzonicus (x 0.40); /, Fromia milleporella. (x 0.80) (Photos: M. Byrne, unless noted.)
by erect sea cucumbers during summer evenings is a sight often seen by divers. Reproductive periodicity of some echinoderms along the GBR changes with latitude. Archaster typicus, a widespread and abundant sea star on muddy sand flats throughout the Indo-Pacific, is unusual in showing pairing of males and females during spawning. As the breeding season approaches the male climbs on the female's aboral surface. This pairing behaviour undoubtedly enhances fertilisation success.
Most echinoderms are free spawners and have dispersive larvae. These larvae are beautiful and distinct for each class. In contrast to adults, the larvae have bilateral symmetry. Some need to feed in the plankton (planktotrophic larvae) and others are sustained by egg nutrients (lecithotrophic larvae). Some echino-derms like the asteroid Aquilonastra byrneae (Figs 26.2C) have benthic development and consequently lack a dispersive stage. Brooding echinoderms such as the asteroid Cryptasterina hystera (Fig. 26.2A) and the ophiuroid Ophiopeza spinosa (Fig. 26.7C) care for their young and give rise to mobile crawling juveniles. These two species are unusual in having a pelagic-type larva in the brood chamber and so have potential to brood and broadcast their young, but it is not known if they do so.
Asexual reproduction is also common in the echin-oderms of the GBR and occurs in asteroids, ophiuroids and holothuroids. This involves the animals breaking in half or fragmenting a part of their body. As a result these echinoderms often have an unusual body profile (Figs 26.1D, 26.2D). This process is followed by regeneration of each portion to form a complete individual. On the GBR fission (splitting in half) is particularly common in brittle stars and sea cucumbers. Some small asterinid sea stars are fissiparous (Fig. 26.2D). Holothu-ria atra and Stichopus chloronotus are well known for asexual reproduction by fission. The asteroid Linckia multifora has an impressive capacity to propagate asex-ually and is often seen with arms at different stages of regeneration (Fig. 26.1D). Whole stars can regenerate from a single autotomised arm.
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