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Salmacis sphaeroides

Dale Mullin 2016


Salmacis sphaeroides, commonly known as the short-spined white urchin (Sarifudin et al. 2014) or the green sea urchin (Chen et al. 2010) is a species of echinoid found in the warm tropics of North-Eastern Australia and beyond (Rahman et al. 2012). It is a regular sea urchin of the order Camarodonta (WoRMS 2016), defined by its spherical shape, in contrast to irregular urchins that have flattened bodies (Ruppert et al. 2004). The individual specimens examined were collected from Amity Point on Stradbroke Island, Queensland, Australia by staff of the University of Queensland, and are kept in an aquarium system on the universities' St. Lucia campus. The goal of this web-page is to provide an in depth description of the echinoid Salmacis sphaeroides, as well as to provide further detail on regular echinoids where that on S.sphaeroides is lacking. Under the Anatomy & Physiology section can be found a short investigation, the aim of which was to examine the tube feet (podia) of this urchin species and thus determine whether a difference in morphology exists between those podia on the urchins' oral verses aboral surfaces.
Figure 1

Physical Description

External Physiology

The test of Salmacis sphaeroides can appear in a range of hues from green to cloudy white, and is comprised of an amorphous calcite crystal precursor known as magnesium calcite (Politi et al. 2004), while its spines appear in a variation of colours, most commonly white with red banding (Rahman et al. 2012). Individuals of the species range in size from 5-8cm in diameter with their spines reaching a maximum length of 1-1.5cm (Rahman et al. 2012). The pentaradial adult echinoderm body plan  is divided into five sectors known as rays, arranged around an axis of radial symmetry refereed to as the oral-aboral axis (Minsuk et al. 2009). The following description (Figure 2, Ruppert et al. 2004), summarised from Ruppert (2004) is shared amongst all echinoids, S.sphaeroides included. The mouth of the urchin is present on the oral pole, while the anus is situated on the opposite end of the body at the aboral pole. From the five rays, the body of urchins is further divided into ten sections comprised of five ambulacral plates which contain podia, and five sections devoid of tube feet known as the interambulacral area with each section of both of these areas again divided into two separate rows. A detailed description of the podia may be found under the Anatomy & Physiology section. Alongside spines for defence, the test of sea urchins contain pedicellariae; small appendages supported via a stalk topped by jaws, which are used for both defence and self-cleaning. On the oral end of the urchin the mouth is surrounded by the peristomial membrane, whilst on the aboral end is situated the periproct; a small circular membrane that surrounds the anus. Also present on the aboral end are five large genital plates, each in alignment with the interambulacral areas, and five smaller ocular plates that align with the ambulacral areas. 
Figure 2


All echinoids posses primary and secondary spines arranged more or less symmetrically in the ambulacral and interambulacral areas (Ruppert et al. 2004), used as both mobile appendages and for protection to provide defence against large predators (Coppard et al. 2010). The spines of S.sphaeroides are spade like (Rahman et al. 2012) and range in size, with noticeably larger spines observed to be present toward the oral end of the oral-aboral axis. The following, summarised from Ruppert (2004), describes the basic morphology and function of sea urchin spines. At the base of each spine is a ball-and-socket joint, with the ball (known as a tubercle) present on the surface of the test, and the socket at the base of the spine. Between the ball and socket are two sheaths of fibres, the contraction of the outer sheath allowing for the directional movement of the spine, and the shift to rigidity in the inner sheath allowing for the spine to become locked in place. Most sea urchins are known to have an evenly distributed number of longer primary and shorter secondary spines, although the size and shape of spines is known to differ extremely between species. Some species are known to have hollow spines which are externally barbed and loaded with an irritant to inflict serious wounding upon contact.  


The pedicellariae are jawed appendages developed by sea urchins to deter both pests and predators, as well as for cleaning their own test and spines (Coppard et al. 2010). They are attached by muscles to a freely rotating joint on the surface of the test and consist of a stalk, a neck, and valves which form jaws (Nichols 1962). All are comprised of a head, between 2 and 5 valves that open and are manipulated using abductor and flexor muscles and (in most types) close using large adductor muscles, as well as a stalk that is at least partially supported by a skeletal rod (Coppard et al. 2010). The opening and closing of their jaws occurs after direct stimulation through an inbuilt reflex arc, or through stimulation of the test via nerves under the skin (Campbell 1983). Pedicellariae are functionally and structurally highly evolved and come in an array of different shapes, with a large diversity having already arisen by the Mesozoic approximately 252ma, and all four major types having become established by 50 million years later (Coppard et al. 2010).

The four major types are more often than not present on a single individual (Coppard et al. 2010), with pedicellariae having been observed on S.sphaeroides (Figures 3&4), and although unclear which type was seen, it is likely each of the other types do in fact occur on this echinoid species. Coppard (2010) describes the four types of pedicellariae as follows. Tridentate are the largest and most common type used to remove larger particulate matter and deter smaller pests such as polychaetes. Triphyllous are the smallest of the four and function to remove bacteria and small particulate matter from the test. Ophicephalus pedicellariae are large and inward projecting, referred to as handles and reported to provide greater grasping pressure, while globiferous pedicellariae are venom glands that terminate in fangs or teeth designed to pierce and deliver venom. Secondary loss of defensive pedicellariae in epifaunal echinoids is known to occur in a few groups and can be explained by the greater development of defensive spines and the migration of species to deep water where fewer fouling organisms are present and contact with large predators is less common (Coppard et al. 2010).
Figure 3
Figure 4


Salmacis sphaeroides occurs at depths ranging from 0 to 90 metres but is generally found in the shallows amongst seagrass meadows (Figure 5) and in muddy sublittoral zones, as well as in coral reef areas (Rahman et al. 2012). Sea urchins, and in particular regular echinoids (Kroh et al. 2010) often play an important ecological role as keystone ecosystem engineers and bioturbators, also acting as an important part of the food chain as prey for carnivorous fish and crustaceans (Dupont et al. 2010). Herbivores and omnivores present in the marine environment profoundly influence the structure and function of primary producers in their relative ecosystems by exerting a top-down control on macrophyte communities as well as influencing fundamental processes such as bioerosion and habitat creation (Ng et al. 2014). S.sphaeroides is an omnivorous, generalist echinoid (Tsuchiya et al. 2009) and is one of the dominant grazers in shallow marine communities (Kroh et al. 2010).

Urchin species can be particularly important in the removal of macroalgae in coral reef ecosystems, with evidence of ecosystem collapse when their populations are diminished, and yet contrasting with this is evidence of urchins overgrazing on seagrass communities leading to severe impacts on that ecosystem (Carpenter 1985). Thus depending on the urchin population density, generalized grazing can either enhance or lower the algal species diversity present in a given ecosystem (Carpenter 1985). For example, grazing impacts have been known to vary anywhere from less than 5% of the food present up to 100% in seagrass communities (Klumpp et al. 1992). Sea urchins also interact with other algae grazing animals present in coral reef and seagrass environments, as studies suggest that exploitative competition for algal resources may be occurring between herbivorous fishes and sea urchins (Carpenter 1985). Even the algal species themselves have shown evidence of a response to urchin grazing, as Carpenter (1985) observed that the absence of macroalgae under urchin grazed conditions indicates that either the algal spores do not settle in grazed areas, or that they settle but do not survive (Klumpp et al. 1992).
Figure 5

Life History and Behaviour

Feeding & Locomotion

The diet of Salmacis sphaeroides is extremely generalist, with evidence of the species consuming fronds, seagrass debris, red algae, algae coated sediment, rubble, other urchin species and even other S.sphaeroides individuals (Klumpp et al. 1992, Tsuchiya et al. 2009). Sea urchins feed using a highly developed protrusible jaw apparatus known as the Aristotle's lantern (Ruppert et al. 2004). Movement of sea urchins is closely related to feeding activity and to achieve locomotion echinoids do so largely through the use of their podia, but are also somewhat assisted by their spines which are largely utilised for pushing and raising the oral surface off the substratum (Ruppert et al. 2004). Ruppert (2004) reports that due to the podia present on the aboral surface, sea urchins are easily able to right themselves if displaced and are also able to move in any direction using any one of the ambulacral areas as the leading section. S.sphaeroides has been reported to display no evidence of periodicity and a high degree of location variation in its feeding habits which coincides with its generalist diet (Klumpp et al. 1992). Macroalgae is an important part of the diet of S.sphaeroides which it scrapes from the surface of rubble using its lantern, as is seagrass, likely due to the nutritional benefits (Klumpp et al. 1992). Sea urchins have a general ability to to efficiently digest seagrass and absorb its nutrients with Klumpp (1992) showing that S.sphaeroides absorbed the organic matter from dead seagrass debris at twice the efficiency of Tripneustes gratilla, another sea urchin species sharing the same habitat. 


Most sea urchins have five pairs of peristomial gills on the peristomial membrane, each of which is a highly branched out-pocket of the peripharyngeal coelom and lined with ciliated peritoneum (Ruppert et al. 2004). The peristomial gills are likely the gas-exchange surface for the extensive lantern musculature, as coelomic fluid is pumped to and from the gills by a system of muscles and ossicles associated with the Aristotle's lantern (Ruppert et al. 2004). An alternative means to respiration occurs in echinoids through the use of their tube feet, in which aboral podia can lack a sucking disc and thus be used for gas exchange instead of substratum attachment (Smith 1978). Although the peristomial gills provide some oxygen to the muscles associated with the Aristotle's lantern they appear to function primarily to accommodate pressure changes in the pharyngeal coelom during feeding movements of the lantern complex, and thus the function of gas exchange is performed principally by podia (Hickman et al. 2014). 


In echinoids the sexes are separate and both eggs and sperm are shed into the sea for external fertilization (Hickman et al. 2014). Species such as S.sphaeroides have echinopluteus larvae which live a planktonic lifestyle and then metamorphose into adults (further described under the Development sub-section) but some species of echinoid are known to brood their young in depressions between their spines (Hickman et al. 2014). Regular echinoids are known to have five gonads suspended along the interambulacra on the inside of the test, with a short gonoduct extending aborally from each gonad and opening at a gonopore located on one of the five genital plates to facilitate the release of gametes (Ruppert et al. 2004). 


Sarifudin (2014) describes that Salmacis sphaeroides larvae, like many other echinoid species larvae are only able to tolerate a small range of salinity fluctuations and are thus stenohaline. Sea urchins undergo indirect development producing planktotrophic larvae (Ruppert et al. 2004) and are known to pass through five stages of development before reaching juvenile stage, comprised of a prism stage followed by a 2, 4, 6 and finally 8-arm pluteus stage (Metaxas 1998). Prior to the prism stage is gastrula stage which follows the process of embryogenesis, characterized by the immediate cleavage of a fertilized egg into a greater number of small cell formations (Lepage et al. 1992). This is followed by the fertilization envelope thinning and finally vanishing as the organism secretes hatching enzymes to digest it, releasing the larvae (Lepage et al. 1992). As larvae, echinoderms are bilaterally symmetrical and adult development begins with the establishment of left-right asymmetry, which in most sea urchins is followed by the specification of the coelomic mesoderm and vestibular ectoderm on the left side of the larval body which meet and form the two layered adult ruminant (Minsuk et al. 2009). Rahman (2012) describes the stages of larval development of S.sphaeroides under laboratory conditions as follows.

The prism stage is a simple, cone shaped larvae (Ruppert et al. 2004) covered by cilia (Figure 6). The 2 arm stage that follows sees the formation of a mouth and non-functioning gut, and 48 hours after fertilization the 4-arm stage occurs, providing the capability to feed on unicellular algae. The 6-arm stage follows with the formation of skeletal rods to support the larval arms, and 16 days after fertilization the 8-arm stage occurs (Figure 7). 12 days after this follows the premature larval stage where increased tissue differentiation occurs and developed tube feet and spines become active inside the larval body. A continued degeneration of larval tissue follows, and after 35 days the larvae firmly attach to the substrate with their tube feet and metamorphosis begins, during which the re-absorption of larval tissue and the development of the complete juvenile including adult spines, tube feet and well-developed pedicellariae occurs (Figure 8). The planktotrophic larvae, also known as the echinopluteus is known in some species to swim for up to several months before settling (Ruppert et al. 2004). From the juvenile stage onward the urchin continues to grow until it reaches sexual maturity and eventually dies.
Figure 6
Figure 7
Figure 8

Anatomy and Physiology

Internal Transport & Excretion

The following information is summarised from Ruppert et al. 2004.
The echinoid hemal system is well developed with a complex network of vessels in the wall of the gut mesenteries and coelomic fluid acting as the principle circulatory medium containing an abundance of coelomytes . Some coelomytes are known to form clots and are important in wound healing while others remove particulate wastes and carry these accumulations to the gills, tube feet and axial organ for disposal or storage, whereas larger particulate matter that passes through the digestive system is excreted via the rectum and anus. The axial organ system containing the heart and the kidney is similar to that of class Asteroidea, though it seems that this system has a non-excretory function and instead has a role in nutrient transport, with these echinoderms also known to excrete nitrogen in the form of ammonia via diffusion across thin areas of the body wall such as the podia. 

Aristotle's Lantern

The following information is summarised from Ruppert et al. 2004.
The Aristotle's lantern is a defining feature of class Echinoidea. This feeding apparatus is comprised of a complex set of circumpharyngeal ossicles and muscles, with the lantern structured as a pentamerous apparatus of five large, calcareous plates known as pyramids, each shaped like an arrowhead with the point directed toward the mouth. Each pyramid is joined to adjacent ones by transverse muscles, and along the vertical midline that runs the inner side of each pyramid is a long, calcareous tooth band. The oral end of this tooth band projects beyond the tip of the pyramid as an extremely hard, pointed tooth, resulting in five teeth projecting from the oral end of the lantern; one for each pyramid. The region of tooth growth occurs in the curled upper end of the tooth band, which is enclosed in a dental sac. Specialized muscles allow for the protrusion, retraction and rocking of the lantern as well as the opening and closing of the teeth (Figure 9).
Figure 9

Digestive System

The following information is summarised from Ruppert et al. 2004.
Chewing movements of the Aristotle's lantern removes bits of food from the substratum and compacts them in the buccal cavity where mucus is secreted to bind the particles into a pellet. These pellets are then swallowed via radial muscles that allow the pharynx to dilate. The lantern encloses the buccal cavity and the pharynx which itself joins the oesophagus, that then descends along the outer side of the lantern and enters the stomach. As food moves through the oesophagus by peristalsis, water is removed by the siphon before it enters the stomach as to not dilute digestive enzymes. The siphon is a slender, tubular bypass originating from the oesophagus that parallels the inner edge of the stomach for some or all of its length. Its junction is near the stomach and it rejoins the gut at the union of the stomach and intestine. At the junction of the stomach and oesophagus is usually a blind pouch known as the cecum. The stomach circles the body cavity in the equatorial plane and is suspended by mesentery, secreting enzymes to allow for extra- and intracellular digestion, endocystosis and nutrient storage. Following this the stomach turns 180 degrees and joins the intestine, which circles the body cavity clockwise in aboral view and functions chiefly in endocystosis, intracellular digestion and nutrient storage. The intestine lastly joins a vertical rectum which empties through the anus (Figure 10).
Figure 10

Water Vascular System & Ampullae

The following information is summarised from Ruppert et al. 2004.
The water vascular system begins on the aboral side of echinoids, in which one of the genital plates around the periproct is modified to become the madreporite. From this, a slender calcified canal descends orally to join the ring canal, which itself encircles the oesophagus and lies in the aboral membrane of the jaw apparatus in jawed echinoids such as S.sphaeroides. Radial canals extend from the ring canal and run along the inside of the ambulacral areas of the test, each of which terminates aborally in a small terminal tentacle that penetrates the apical-most ambulacral plate. The lateral canals of one side of the radial canal alternate with those of the other side and terminate in ampullae and tube feet (Figure 10). These canals are doubled as from each ampullae, two canals pass through a pore pair, piercing the ambulacral plate to join the tube foot. These tube feet are internally partitioned to length-wise by a septum to separate the bidirectional flow of coelomic fluid into outgoing and incoming streams (Figure 11).
Figure 11

Nervous System

Ruppert (2004) describes the echinoid nervous system as follows. The major part of the echinoid sensory system is comprised of the spines, pedicellariae and podia, with the buccal podia surrounding the mouth of regular echinoids particularly important in sensory perception. The nervous system is made up of a circumoral nerve ring that encircles the pharynx inside of the lantern, with radial nerves passing between the pyramids of the lantern and running along the underside of the test. Nearly all urchin species, S.sphaeroides included, have statocysts located in spherical bodies called spheridia that function in gravitational orientation, which are stalked and located at various places along the ambulacra. Echinoids in general are negatively phototatic, tending to seek shade in crevices in rocks or under shells and although it is unclear whether S.sphaeroides responds to light in a similar way, it is well known for covering itself with dead coral and detritus (Sarifudin et al. 2014). These five buccal podia are also important in the settlement of echinoid larvae as tactile stimulation of the podial nervous system when combined with other cues associated with suitable adult habitat may serve as the initial steps in the settlement and metamorphosis of echinoid larvae (Burke et al. 1980).


Podia, commonly known as tube feet, are a defining feature of the phylum Echinodermata. Tube feet present on the base of regular echinoids are defined as being in the oral position, and are known as the buccal podia, comprised of five pairs of short, stocky tube feet which are attached to the peristomial membrane; a flexible membrane surrounding the mouth (Ruppert et al. 2004). The tube feet present along the oral-aboral axis of the body are known as the coronal or aboral podia (Santos et al. 2004). They appear within five ambulacral grooves, each presented in an upside-down v-shape (Figure 12) extending from the edge of the apical system on the aboral surface to the edge of the peristomial membrane. (Santos et al. 2004). In regular echinoids, suckered aboral tube feet are only found in species with more than one row of tube feet per ambulacral column (Smith 1978) which is a feature shared by S.sphaeroides. These podia are connected internally via the water vascular system, in which ampullae are fed via radial canals that then pass through a pore pair to pierce the ambulacral plate and terminate at the tube foot (Smith 1978). 

The basic podial structure is common across all echinoid species (Santos et al. 2006). An individual tube foot consists of a proximal extensible cylinder known as the stem, topped apically by an enlarged, flattened disc (Flammang 1996). It is hypothesised that species inhabiting hard substrata in areas with high hydrodynamic forces possess more robust discs than those typical of soft substrata in less exposed zones, and thus substratum surface characteristics significantly influence tube foot critical attachment force and tenacity (Santos et al. 2006). Smith (1978) has categorised coronal podia into four categories, each described by the maximum environmental energy the feet can withstand relative to their degree of development and thus the size of their disc.  There is some evidence that the structure of the tube feet in echinoid species living in different environments will be somewhat differently structured (Santos et al. 2004), though Santos (2006) found that no relationship between the tube foot tenacity, the adhesive secretory granule ultrastructure, and species habitat could be established.

The structure of the podia consists of four layers of tissue stratification: an inner myomesothelium surrounding the water vascular lumen, a connective tissue layer, a nerve plexus and lastly an outer epidermis covered externally by cuticle (Santos et al. 2006). The internal structure of the disc is supported by a calcified skeleton composed of two superimposed structures; a distal rosette comprised of 4-5 ossicles, and a proximal frame comprised of numerous arc shaped spicules (Santos et al. 2006). This make-up likely allows for structural support of the flattened disc. Attached to the podial disc is the stem (Figure 13). The stem contains both muscle and connective tissue, and when force is exerted on the tube feet the connective tissue layer is the only tissue layer bearing the load, and is seen to have an extensibility similar to that of rubber elastin (Santos et al. 2005). Under short pulses of wave generated forces attached discs most likely behave elastically, distributing the stress along the entire contact area in order to avoid crack generation (Santos et al. 2005). 

Echinoids use these tube feet to attach themselves to various substrata, as well as to move objects around, for locomotion and even to bury themselves. The discs present on the end of the podia secrete substances that both allow the individual foot to attach to a surface as well as remove itself. The epidermis of the disc contains a duo-glandular adhesive system which comprises two types of cells that release separately adhesive and de-adhesive secretions (Santos et al. 2006). The adhesive secretions are delivered through the disc cuticle onto the surface where they form a thin filament that binds the tube foot disc to the substrate (Flammang et al. 2005). To remove itself from a surface, de-adhesive secretions are released from within the cuticle of the podial disc, causing the discarding of its outermost layer, thus leaving behind a footprint comprised of the adhesive material (Santos et al. 2006). A somewhat similar process can occur in waste excretion, where the end of the podia is pinched off along with an accumulation of waste-laden coelomytes (Ruppert et al. 2014). Due to adhesion of echinoderm feet appearing to be stronger on rougher substrata than smoother ones, it is hypothesised that the disc itself behaves viscously to adapt to substratum roughness whilst under slow self-imposed forces, while the adhesive only fills out very small surface irregularities, on the scale of nanometres (Santos et al. 2005). 

Short Investigation:

Besides locomotion, podia perform a variety of functions including adhesion, feeding, shading, sensing and respiration (Leddy et al. 2000). The buccal podia are seen to have a thicker stem wall and wider disc compared to the coronal podia present on the aboral surface (Smith 1978). These morphological differences are accompanied by mechanical differences, with the buccal tube feet being significantly more extensible and robust than coronal tube feet, also potentially producing more adhesive power to allow for a stronger attachment to the substratum (Santos et al. 2005, 2006). Coronal tube feet have been known to be either respiratory in function, or thin-walled and terminate in a small disc (Smith 1978). The coronal podia are also known to play a role in locomotion as well as the capture of drift material, while the buccal podia also function in wrapping together food particles, thus facilitating their capture by the Aristotle's lantern (Flammang et al. 1993).  It is this variation in function between tube feet that has led to a short investigation involving the podia of S.sphaeroides. The aim of this short investigation is to determine whether there can be seen a clear difference between the coronal podia at the extreme ends of the oral-aboral axis of S.sphaeroides. It is hypothesised that there will be a clear morphological difference observable.

Methods - 
Tube feet were cut from the extreme top and bottom of the ambulacral grooves of 5 individual S.sphaeroides, as well as from the widest point of the body, and then suspended in marine water within separate Petri dishes for examination. Individual podia were examined under a light microscope using cavity slides, and then photographed for further visual comparison. The tube feet from either end of the coronal surface were then compared based on morphological traits. 

Results & Discussion - 
Based on the morphological traits observed, it was found that at 10x magnification there was a clear visual difference between the size and robustness of the aboral, middle and oral coronal podia. The differences observed occurred largely in the disc, which could be seen to appear noticeably larger as we move from the podia on aboral surface to those on the oral surface (Figure 14). There was no clear difference observed in stem size between the three podia. This thus confirms the hypothesis as a difference between the three podia was observed. The smaller discs present toward the aboral hemisphere is supported by Flammang (1992). The podia of S.sphaeroides were compared only between an individual organisms own self, as it was noted that podia differed in size between those individuals whose body size differed, presumably the result of an age difference. The larger urchins' podia were noticeably larger, with a slightly thicker stem. It is intuitive that with multiple processes being undertaken by different tube feet that there may be observed a morphological difference between podia along the length of the body, which was found to be the case. 

Future Studies - 
To gain a more accurate measurement of the morphological differences between the compared tube feet, the use of an electron microscope would have given a much more detailed view of the podial structure, though access to one was unavailable. The buccal podia and coronal podia were not compared due to literature making clear the difference in function between the two (Ruppert et al. 2004). Some difficulty was experienced in removing the tube feet accurately due to the individual echinoids reacting to the presence of the small scissors being used, and thus retracting their podia. Some form of sedation, allowing for all podia to remain extended would be optimal for any future studies of this form. Interestingly observed are the presence of a species of worm, completely speculatively, Phylum Nematoda, within the stem of each of these tube feet, all centred in a clump on a specific side (Figure 15). The description of these worms and the reason for their presence may also be of interest in future studies. 
Figure 12
Figure 13
Figure 14
Figure 15

Biogeographic Distribution

The known distribution of Salmacis sphaeroides extends from its area of greatest abundance in the Indo-west pacific where it can be found from China to the Solomon Islands and Australia, as well as the warm temperate regions between Malaysia and Singapore (Rahman et al. 2012). The distribution is known to extend south along the Eastern coast of Australia, at least as far as Southern Queensland (Figure 16). 
Figure 16

Evolution and Systematics

Echinoid fossils date back to the Cambrian approximately 541ma (Zamora et al. 2014), and as a result of having a multiplated skeleton and test composed of high-magnesium calcite they have left a rich fossil record (Kroh et al. 2010). Salmacis sphaeroides is classified under phylum Echinodermata, class Echinoidia, order Camarodonta and family Temnopleuridae (WoRMS 2016). Classified by Linnaeus in 1758 (WoRMS 2016), S.sphaeroides is a regular echinoid within the order Camarodonta; a natural group sharing a distinctive and derived lantern structure, and in all but a few primitive taxa, echinoid-style ambulacral plate compounding (Kroh et al. 2010). As an echinoderm, S.sphaeroides is defined as being a dueterostome with radial cleavage, regulative development and enterocoelous origin of the coelomic cavities (Ruppert et al. 2004). One of the defining features of class Echinoidea is the lack of arms and resulting abmulacra and oral surfaces having expanded aborally to cover most of the body, excluding the aboral anus and periproct (Ruppert et al. 2004).

Order Camarodonta is the largest clade of regular extant echinoids, placed within the family Temnopleuridae, defined by a pitted or granular test ornament, dicyclical apical discs and uniserial pore zones (Kroh et al. 2010). The pedicellariae of echinoids are also a feature commonly used in their taxonomy to define at the species and genus levels (Coppard et al. 2010). It is believed that one of the defining features of class Echinoidia; the test, has resulted from the secondary fusing of the arms present in most other echinoderms, with many fossil echinoids displaying the former configuration of the arms remaining visible in the arrangement of the ambulacral folds (Gudo 2005). Below is displayed the current understanding of the echinoid phylogenetic tree, based on morphological and genetic data as composed by Kroh (2010) (Figures 17&18). 
Figure 17
Figure 18

Conservation and Threats

No current IUCN listing exists for Salmacis sphaeroides, most likely due to a lack of data. Many sea urchin species are directly threatened by the impact of overfishing as their gonads in particular are used for food in Asian markets, leading to populations becoming depleted (Rahman et al. 2012). It is unclear the impact of fishing practices on the populations of S.sphaeroides but it may be one of the less desirable urchin species for human consumption, with no market for the species in Malaysia existing as of 2012 (Rahman et al. 2012), and the species only being found in Hong Kong restaurants in recent years (Chen et al. 2010). The other greatest threat to this species, and to a large number of other urchin species, is ocean acidification caused by man-made pollutants entering the atmosphere, alongside sea temperature rise. Ocean acidification occurs due to the continued uptake of carbon dioxide by the oceans, increasing the concentration of hydrogen ions and thereby reducing the oceans' pH (Dupont et al. 2010).

The bodies of echinoderms, and echinoids in particular, are highly calcified and with this carbon dioxide driven climate change comes the risk of calcium carbonate present in their bodies becoming particularly susceptible to dissolution in acidic waters (Dupont et al. 2010). The larval skeletal rods, adult test, teeth and spines are known to be formed from an an amorphous calcite crystal precursor known as magnesium calcite, which is 30 times more soluble than normal calcite (Politi et al. 2004). In sea urchins the impact of ocean acidification is seen to be very species-specific, as certain species that experience more natural variation in seawater pH during their natural time of spawning may be pre-adapted to an ever changing environment (Dupont et al. 2010). There is little data behind the effects of ocean acidification on adult S.sphaeroides, but in equal if not greater danger than the adults of this species are its larvae. 

Evidence has shown that when raised in near future ocean acidification conditions, sea urchin larvae develop more slowly (Dupont et al. 2010). This is detrimental due to the fact that during larval development, rapid development is presumably advantageous, and delayed settlement can impact local populations by reducing larvae numbers through an increased chance of loss via predation and/or by delay in the opportunity to settle in high quality habitat (Dupont et al. 2010). Whether or not this affects Salmacis sphaeroides directly is unclear, but a change in ocean acidification levels will likely have a broad range of effects on many species of echinoid. Overall, Salmacis sphaeroides may be deemed to be under a low level of threat, but this will likely increase in the near future. 


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Cari Rivers, 2016. Representative image.

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