Factors That Inﬂuence the Reproduction of Sea Cucumbers by Angela Mackey (Biol 515 at SDSU, Fall 2001) Copyright © 2002 by Angela Mackey and Brian T. Hentschel (firstname.lastname@example.org)
INTRODUCTION The echinoderm class Holothuroidea consists of sea cucumbers, which can be described as worm-like organisms with elongated, soft bodies. Like most echinoderms, holothuroids have 5 rows of tube feet along their bodies. These tube feet aid the animal in attachment, locomotion and respiration (Pearse et al. 1987). Within the class Holothuroidea, there are about 1250 species, and these are distributed among 200 genera (Smiley et al. 1991). There is much variety among the habitats in which sea cucumbers reside. Some species, like Cucumaria fallax, inhabit quiet, still waters, while others, such as C. miniata, are found in areas that are constantly swept by tidal currents (McEuen 1988). Some holothuroids are tropical reef dwellers (Uthicke 1997); others live in temperate or polar waters (Hamel and Mercier 1995). Even the depth at which these animals can be found varies greatly. Many species live within the intertidal zone, but a few sea cucumbers live deep in the ocean, some among hydrothermal vents (Smirnov et al. 2000). Regardless of the environment in which they are found, sea cucumbers have become highly adapted to life in that environment, especially in their reproductive cycles. Sea cucumbers can reproduce through sexual reproduction as well as asexual reproduction. Each mode of reproduction has several environmental factors that have been proposed to control its timing. This paper will ﬁrst introduce the two types of reproduction found in sea cucumbers. I will then discuss some of the factors that have been shown to regulate sexual reproduction, speciﬁcally the processes of gametogenesis and spawning, and the factors assumed to control asexual reproduction. Finally, I will make a few suggestions as to directions that should be taken during future studies on external cues for reproduction in holothuroids. TYPES OF REPRODUCTION IN HOLOTHUROIDS Sexual Reproduction For most sea cucumbers, the primary means of reproduction is sexual reproduction. Male and female sexes are usually separate, but there have been a few reports of hermaphroditism in some species (Smiley et al. 1991). Sexual reproduction consists of two main phases: gametogenesis and spawning. Gametogenesis is the formation of sperm and ova, and this process takes place in the sea cucumber’s single gonad (Pearse et al. 1987). The term spawning refers to the act of releasing mature gametes into the water. Smiley et al. (1991) report two ways by which the process of spawning can occur. One method is called broadcasting in which sea cucumbers scatter their gametes in the water. Although a majority of holothuroids broadcast their gametes, it is estimated that 41 species (about 3%) of holothuroids are brooders (Smiley et al 1991). Brooders collect fertilized eggs and hold them either internally or externally, rather than releasing their gametes and leaving them. By doing this, brooders offer protection to the developing embryos. Angela Mackey: Factors That Inﬂuence the Reproduction of Sea Cucumbers p. 1 of 8.
McEuen (1988) found that regardless of the method of spawning, a particular stance is taken by the mature sea cucumbers prior to spawning. Each individual releases its grip on the substrate and lifts its anterior end. Shortly after taking this stance, males will release their gametes into the sea. Females then follow male spawning, typically within a timeframe of minutes to hours. McEuen (1988) also noted that males rarely shed all of their gametes at once, where as females do not spawn repeatedly. Asexual reproduction Asexual reproduction is another method by which sea cucumbers propagate. Dalyell (1851, in Crozier 1917) ﬁrst presented the idea of asexual reproduction, through the process of transverse ﬁssion, among holothuroids. This was based largely on...
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