Brittle stars have long, thin, distinctly separated arms extending from a central disc. O. savignyi is a small bodied species of brittle star that commonly has 6 arms and hexamerous symmetry (McGovern 2002a; Mladenov & Emson 1990). Due to its ability to asexually reproduce and regenerate, this species is commonly asymmetrical with 3 long arms and 3 shorter arms or alternating long and short arms (Figure 3) (asexual process explained in life history and behaviour section).

Due to the species extremely wide distribution and diverse ecosystem interactions, features of its physical appearance can slightly vary between locations. (The following describes O.savignyi located in North-eastern Brazil) (Paim et al. 2015). O.savignyi’s central disc is covered in small, strong, irregular, imbricated (overlapping) scales with some spines on the dorsal side. The disc diameter ranges from 0.5-11.5 mm (McGovern 2002b; Paim et al. 2015). Its radial shields, which are 2 paired scales located on the dorsal surface of the central disc at the base of the arms, are large and triangular (image Gin Figure 2). Its mouth is located at the centre of the ventral side of the central disc (image I of Figure 2). Its oral shield, located at the distal end of the jaw, is slightly wider than it is longer. On each side of the oral shield are the adoral shields. It has most commonly two oral papillae located at the edge of the mouth on each side of the jaw. It has large bursal slits located on the ventral side of the central disc on each side of the arm. Its Dorsal arm plates are sub-rectangular (image J in Figure 2) and ventral arm plates are octagonal (image L in Figure 2). Small spines protrude for the entire length of the arm on both sides. The arms progressively narrow to form a tip. 

Specimens sourced from Moreton Bay, QLD, Australia, have a darker central disc and lighter arms. The dorsal side of their central disc is a grey brown colour patterned with large dark brown scales positioned at the base of its six arms (radial shields) and dark scales at the centre of the disk (Figure 1). Their arms have a stripped appearance, alternating between dark brown and light brown or cream (image B in Figure 3). Their ventral surface is all white or lightly coloured (image A in Figure 3).

O. savignyi is found in all reef zones, seagrass beds, mangroves and tidepools (Chao & Tsai 1995; Hendler et al. 1995, as cited in Gondim etal. 2013).Populations are commonly found within small crevasses between the branches of coral, among algal fronds, and in sponges (Mladenov & Emson 1990). 

O. savignyi is frequently found living in association with marine sponges (McGovern 2002a). By inhabiting the exhalant passages of the sponges, O. savignyi can feed on excess food particles that are pumped out of the sponge, creating an epizoic interaction (Chao & Tsai1995; Mladenov & Emson 1990). These sponge dwelling populations live in very high densities and are usually genetically identical due to asexual reproduction (Mladenov & Emson 1990). Boffi (1972) conducted a study on O. savignyi at the northern coasts of Brazil where a maximum density of 1,892 individuals/ 100g of dry sponge was found. The Sponge species Damiriana hawaiiana, Laubenfels 1950, at the Hawaiian Islands has been found with around 20 individuals living within its canal system. The small brittle stars were immobilized by slimy secretions of the sponge, unable to raise their arms. It was unclear how these individuals feed or reproduced (Casper 1985, as cited in Byrne &O’Hara 2017).

Brittle star predators are usually unspecialized and local predators tend to attack most local species. O. savignyi predators include many decapod species. The crab species Hexapanopeus paulensis, Rathbun1930, and Pilumnus dasypodus, Kingsley 1879,  have been observed eating O. savignyi in an aquarium environment (Boffi 1972). The lobster species Panulirus echinatus, Smith 1869,  has been observed predating upon O. savignyi off the coasts of Brazil (Goes & Lins – Oliveira 2009). O. savignyi’s predator recognition is influenced by microhabitat. Algal populations show broad recognition contrasting to sponge populations which show less recognition due to sponges being efficient physical refuges and restricting the brittle stars exposure to predators (Majer etal. 2009). Specimens of O. savignyi have also been found with endoparasitic decapods, belonging to the family Thespesiopsyllidae, living internally (Boffi 1972).

Asexual Reproduction:

Overview- O. savignyi is a fissiparous brittle star meaning it can asexually reproduce using fission (Mladenov et al. 1983). Fissiparous brittle stars account for at least 34 out of roughly 2000 extant species of Ophiuroids and 15 extant species within the genius Ophiactis (Emson & Wilkie 1980, as cited in Byrne & O’Hara 2017). This fission process involves the central disc splitting, dividing one individual with 6 arms into two individuals with 3 arms. The other half of each new individual is regenerated (3 new arms and half of the central disc) (recently divided individual shown in Figure 4) (Emson & Wilkie 1980,as cited in McGovern 2002a). Mladenov et al. (1983) describes fission in another species of brittle star Ophiocomella ophiactoides, Clark 1900, stating division starts with one furrow at the edge of the disc and continues unilaterally, progressing across the disc to separate the individual. This form of asexual reproduction also causes some individuals to have greater or fewer than 6 arms when fully regenerated (Mladenov et al. 1983).

Advantages- Fissiparity in these brittle star species is thought to have evolved in association with small body sizes. These small body sizes are associated with low fecundity and planktotrophic larval development. By asexually reproducing, an individual brittle star can increase its genetically identical biomass while skipping the small, vulnerable larval stage, thus increasing fecundity and alleviating the disadvantages of small body sizes (Mladenov et al. 1983; Mladenov & Emson 1990). This increase in biomass may also increase sexual reproduction of the clone by having a larger spatial dispersion and therefore higher chance of finding a mate, as well as an increased overall clone longevity which increases the number of reproductive seasons the clones experience (McGovern 2002b).

Multiple Division Planes- Fissiparous brittle stars including O. savignyi have unique adaptations to optimise the number of divisions one individual can complete while also retaining a high level of fitness. These brittle stars perform second fissions before full regeneration (Mladenov et al.1983). This results in minimising the time between successive fissions and maximises the total number of offspring that has been asexually reproduced (Chao & Tsai 1995). One specimen alone has shown to give rise to 15 offspring through asexual fission (Mladenov et al. 1983). In addition to this, fissiparous brittle stars also divide across multiple planes. This results in the second or third division creating two individuals with both fully grown (long) and still regenerating (short) arms. Brittle stars use their arms for a wide range of functions (feeding, locomotion etc) and therefore arm length restricts these capabilities and influences the fitness of the individual (Mladenov et al. 1983). By alternating arm length through multiple division planes (B in Figure 5), the overall probability of survival of both new individuals is increased rather than one having high fitness with long arms and another having a lower fitness with short arms when one division plane is used repeatedly (A in Figure 5) (Mladenov et al. 1983) .

Timing- O. savignyi asexually reproduce at different times of the year, determined by their geographical location.Populations at Brazil, Taiwan and Jamaican waters have shown division through out the entire year, however, Taiwan populations have shown higher frequencies in warmer months (Boffi 1972; Chao & Tsai 1995). Populations in Florida have shown fission to be highest from mid to late fall through to early spring (McGovern2002a). Fission seems to occur after sexual spawning, reducing the chance of wasting the development of gonads due to energy demands of regeneration (Chao& Tsai 1995). On top of this, O. savignyi is more likely to divide before maturation compared to after (McGovern 2002a). This is thought to be due to non-mature brittle stars not having an energetic trade-off between sexual and asexual reproduction (described in Reproductive Plasticity section)(Emson & Wilkie 1980, as cited in McGovern 2002a).

Unequal Sex Ratio- Fissiparous has caused an unequal sex ratio in O. savignyi with populations around the world being male dominant. Females O. savignyi have a significantly higher chance of not maintaining gonads after division which is thought to be caused by the high energetic demand of their gonad production. (Boffi 1972; Chao & Tsai1995). This results in males being significantly more likely to divide than females due to a greater cost of division for females. It is thought that these division rate differences may lead to ecological and evolutionary changes for the species into the future (McGovern 2002b).

Controlling Division- It is still unknown how O. savignyi completes fission. Scientists have considered whether it is achieved by force, with the individual pulling two halves in opposite directions, or if tissue strength is internally mediated and weakened, resulting in separation. Evidence has been shown in another fissiparous brittle star (Ophiocomella ophiactoides) that fission begins with softening the side of the disc rather than tearing it apart. This showed that brittle stars capable of mediating the tensile strength of their connective tissue using internal processes which may be the first step in the asexual fission process of O. savignyi (Mladenov et al. 1983).

Division Induction-It is also still unknown what induces sasexual reproduction in O. savignyi. Many theories suggest that environmental stresses can induce fission which has been shown in other Echinoderms. Chao & Tsai (1995) concluded that environmental temperature was not a critical factor inducing fission in populations of O. savignyi located at Taiwan. Another theory suggest that fission is under internal control as seen in species of sea stars (McGovern 2002a). Alternatively, O. savignyi fission my not be induced by a specific factor, but instead, multiple external and internal factors may have to be combined for fission to take place which may be specific to locations and populations around the globe (Chao & Tsai 1995).

Sexual Reproduction:

Sexual Maturity: O. savignyi is dioecious with both male and female individuals. They are gonochoristic and remain either male or female through their entire life after maturity. O. savignyi must reach a size threshold to become sexually mature (contain mature gonads). McGovern (2002a) found that O. savignyi collected at Florida could not reach sexual maturity until the individuals were larger than 3 mm in disc diameter.  Males mature faster than females, resulting in an average smaller sexual maturity body size for males. The ability of males to reach sexual maturity at a smaller size could result in a greater proportion of males than females becoming sexually mature which may be contributing to the male sex ratio as well as asexual reproduction (McGovern 2002b).

Spawning- Female O. savignyi have oocytes measuring between 70 to 90pm and a female with a 5.5mm disc diameter can contain approximately 10,000 eggs (Emson et al. 1985). O. savignyi is thought to use broadcast spawning however, little is known about their spawning behaviour (McGovern 2002a; McGovern 2003). Male sperm broadcasting has been observed and even though no female broadcasting has been observed, there has also been no observation of brooded embryos and therefore it is assumed both sexes are broadcast spawners (McGovern 2002a, McGovern 2003).

Timing- Timing of sexual reproduction in this species varies geographically with several studies suggesting it occurs during most of the year (Chao & Tsai 1995; Emson & Wilkie 1984; Mladenov & Emson1984). Studies at Florida and Taiwan show high sexual reproductive rates during warm months of spring and summer (Chao & Tsai 1995; McGovern 2002a). Although this occurs for much of the year, successful sexual recruitment rates are low and asexual fission dominates when increasing population numbers (Emson & Wilkie 1984; Mladenov et al. 1983; Mladenov & Emson 1984; Mladenov & Emson 1990).

Reproductive Plasticity:

Asexual and sexual reproduction can occur simultaneously in individuals of this species (Boffi 1972).However, there seems to be trade-off between sexual and asexual reproduction in some cases. Either an individual produce’s gametes, reducing resources to complete division or an individual completes division, creating two half individuals that may not have the capabilities to satisfy energy demands of sexual reproduction, potentially absorbing gonads for energy usage (Chao & Tsai 1995; McGovern 2002a). Because of this trade-off, O. savignyi shows plasticity in its reproductive strategies depending on its environment. A study conducted on populations located in Florida concluded that O. savignyi reduces its resource allocations for asexual reproduction when mates are present or when offspring production will be affected by division due to loss of fecundity. This behaviour maximises sexual reproduction when mates are available but also maximises survival of the clone population when mates are unavailable through asexual fission (McGovern 2003).

O. savignyi can feed on both detritus (detritivore) and suspended material. Individuals graze on detritus by keeping their oral surface on the substratum that they are feeding on, while they move the tips of their arms horizontally (Boffi 1972). Large brittle stars can extend their arms and collect prey and suspended material from the water column (McGovern 2002). This species has no mucus on their arms that aids them in feeding, instead they collect food material with their podia (tube feet) and transport them to the mouth (Boffi 1972).  Detritus, silica grains, bryozoans, foraminifera’s, and remains of small gastropods have all been found in the stomach content of O. savignyi (Boffi 1972).

No research has studied the movement in O. savignyi however, Ophiuroid species share the same mechanisms for locomotion. Ophiuroids use coordinated movements of their muscular arms to move across a substrate, contrasting to the tube feet and water vascular system method of locomotion in other echinoderms (Clark et al. 2019). The evolution of this alternative locomotion method allows ophiuroids the ability to move at a much faster speed (Clark et al. 2019). The radial symmetry of ophiuroids and other echinoderms allow them to move omnidirectional or in all directions. This arm structure allows both back- and- forth and up- and -down arm movements (Watanabe et al.2012).

​Quotation marks indicate the abbreviations of each structure used in Figure 7 by Ezhova et al. (2015) . The names of these structures may vary between articles.

Haemocoel Structures:

Ophiuroids contain haemocoel structures that are involved in the transport of blood around the body (diagram b in Figure 7). This system allows for gas and nutrient exchange between organs, coeloms and the external environment. The main haemocoel structures within ophiuroids include the oral haemal ring “orb”, gastric haemal ring “grb”, genital haemal ring “gnb” and the axial organ (Ezhovaet al. 2015). Haemocoel associations are described in the following sections.

Water Vascular System and Associated Structures:

Echinoderms are famous for their water vascular systems which consist of fluid filled coelomic passages and associated parts (Ferguson 1995). The fluid within these coeloms is pumped to different locations within the system in order to extend appendages including podia (tube feet) (Ferguson 1995). Ophiuroid podia extend out from the ventral side of the arm and emerge from the openings between the ventral and lateral arm plates (Smith 1937).