H. scabra are a part of the Holothurian class within the phyla Echinodermata. Holothurians we given the name sea cucumbers as they have a cylindrical body shape and are tapered at the end. They are more often than not slimy and have a pliable body. They share with their sister classes the pentaradial body symmetry and a water vascular system. Unlike the other classes of Echinoderms, Holothurians express their pentaradial symmetry horizontally rather than vertically due to the lengthening of the body across the oral-aboral axis (Cannon & Silver, 1987).
The lifestyle and ecology of these animals drove the evolution of these animals' body plans. H. scabra are deposit feeders and have elongated bodies with a blunt end. Their mouth parts are directed downwards towards the surface of the substrate while their anus is located more dorsally giving it a convex appearance.

H. scabra are grey or black to olive in coloration often with black or green transverse lines across the dorsal portion of their body (Figure 1). H. scabra are darker dorsally than they are ventrally which tends to be grey or white in colour and is characterized by having many "speckles" out of which the tube feet extend (Figure 2). These ventral tube feet are used for movement.

  H. scabra appear to be speckled as well dorsally as they have blunt projections or papillae extending vertically from the surface of their epidermis that should not be confused with the ventral tube feet (Cannon and Silver 1986)(Figure 3). 

Figure 3.  Up close view Holothuria scabra exhibiting the dorsal papillae and the ventral tube feet. The animal's mouth is oriented to the right most portion of the photo.

The coloration of H. scabra makes them quite conspicuous within coral rubble or debris or within sand in sea grass beds (Figure 4). 

           
  As adults H. scabra measure between 150 and 400 mm and may weigh up to between 500 and 2000 g but this varies dramatically between geographical ranges (Conand, 1989). Their distribution within a region varies between life stages (see settlement and distribution)
.

The most important aspect of where H. scabra is found, is the place of settlement of the species’ larvae. There is strong evolutionary pressure to settle in a location that may increase chance of growth and survival (Giese et al., 1991). Hypothesises have been stated that H. scabra larvae are highly selective when it comes to settling on a substrate. Mercier et al. (2000) found that H. scabra selectively settle upon sea grasses found within the adult habitat and larval settlement should be correlated with levels of sea grass within a habitat. As well, their data showed that larval settle more upon sands that are higher in organic material and contain coral rubble. In conclusion, sea grass beds are important to H. scabra settlement and early life activity.

H. scabra individuals are found in different locations within a region correlated to its size. Research done by Mercier (2000) showed that distribution of individuals is correlated with habitat characteristics such as depth, granulometry, richness of substrate, and presence of sea grass beds. This studies results indicated that larger individuals are found in deeper waters and in less dense sea grass beds due to the increased food availability in these areas; dense sea grass beds have root systems that could interfere with the larger individuals burrowing cycle (Mercier 2000). As well, deeper water tends to be more stable in temperature and other variable factors and larger individuals tend to be more sensitive to change than smaller individuals. In addition to these factors, H. scabra compromise their depth in relation to their preferred substrate. These animals prefer fine sediment without crushed shells and under 30% organic material within the sediment (Mercier 2000). There is a compromise that has to be made as finer sediment may be found in deeper waters but more organic material may be found in the shallower areas. The results of this study helps explore the reasons as to why different sizes of H. scabra may be found at different depths.

H. scabra are often closely associated with sea grass beds as they need them for larval settlement, early life stages, and food production. It is through this connection, many studies have assessed the importance of the presence of H. scabra in relation to sea grass bed and ecosystem maintenance. They play a large role in the nutrient availability within an ecosystem.

Through their burrowing cycle and deposit feeding H. scabra are considered important contributors to bioturbation or the moving and disturbance of sand and other particles (Wolkenhauer, 2008). These deposit feeders may have a direct impact on the productivity of an area due to nutrient recycling within the sediment and water column (Moriarty et al., 1985). Hughes et al. (2004) found that nutrient availability is a limiting factor to growth and success of sea grass beds. Wolkenhauer (2004) looked to determine the relationship between sea grass productivity and H. scabra presence. Sea grass productivity was found to be lower without the sea cucumbers present. The researchers hypothesized that there was stress to the sea grass through increased shading as organic material built up and decreased nutrient availability. This relationship between sea grass productivity and sea cucumber presence is thought to be tied to the bioturbation that sea cucumbers perform. The bioturbation causes nutrients to become more available and accessible to seagrasses (Wolkenhauer, 2004). Through these results it can be inferred that sea cucumbers play an important ecological role within their environment.

H. scabra have separate sexes that with no visual sexual dimorphism. It is almost impossible to identify the sex without internal or microscopic investigation (baskar, 1994). In both sexes the gonads are yellow, branching, and hang freely. They have a gonoducts that opens externally near the anterior end that is used for spawning. In females the gonadial tubules are short and wide while in males they are longer (Baskar, 1994). At sexual maturity males will be shorter than females (Shelley, 1981).

Spawning events happen periodically. However some research has shown that spawning may occur all year around (e.g., Krishnan, 1968). However, there is also extensive research that shows there is at least two major spawning peaks within the year (MPEDA, 1989). During spawning, H. scabra will lift the anterior end of its body and make a sweeping motion. During this time, the gonad papilla will deflate (Conand, 1993). The gametes will be released through the gonoporeand into the water column (Mary Bai, 1980). Research suggests that temperature and salinity may be the triggers for gamete release (Moiyadeen, 1994). Variation in these factors may trigger the spawning events depending on their timing within the year.

Once the adult H. scabra spawn, the eggs and sperm meet. From there division occurs where within three days a feeding larvae or auricularia is formed. After up to eight days of feeding, the auricularia undergoes metamorphosis into a non-feeding dollolaria. At this point, the non-feeding larvae spends up to ten days finding the best location to settle. At settlement the dollolaria undergoes metamorphosis into the benthic form of the pentactula. From here tube feet and tentacles become more apparent and the tables in the body wall begin to form (D. B. James et aL, 1988). From settlement the juvenile sea cucumber begins to grow and migrates further from the sea grass bed in correlation to its growth rate (See Settlement and Distribution).

H. scabra are deposit feeders consuming sediment and organic material from the substrate. Their mouth parts are faced downwards and they feed while moving slowly over the substrate or sometimes while they are buried. They use their mucus covered peltate tentacles to extend and stick to sand particles (Figure 5).

Once the tentacle has food particle, the animal retracts their tentacle back into their mouth and removes the particles from their tentacles (Baskar, 1994). Through analysis of their gut contents, it was observed that H. scabra consume mud, sand, shell debris, molluscan shells, and some biotic material. Through this analysis, it was also determined that H. scabra prefers to feed on muddy substrate over sandy substrate (Baskar, 1994). 

Timelapse (30 frames per second) of H. scabra feeding in captivity  See: https://youtu.be/TQdUuO-Tx60

When limited to compact substrate or substrate void of sandy materials, H. scabra have been observed feeding on algae and bacteria materials from hard substrates and tanks (P. S. B. R. James, 1996; Battaglene et al., 1999). This behaviour was also observed in the aquarium at University of Queensland. A single individual was placed in a small tank without sandy substrate, the tank however, was full of bothy leafy and crust algae. The animal had free roam within the tank for 24 hours. Upon observation, a large amount of algae was seen to be removed from the tank and four distinct faecal piles were counted as well (Figure 6). This observation is congruent with previous research that shows H. scabra will consume algae and bacterial materials outside of the normal sandy substrate.

Figure 6.  View of tank after algae consumption.

Feeding rates and times have been highly discussed among researchers. Some researchers believe that H. scabra are continuous feeders (e.g., P. S. B. R. James, 1996) while other research shows that the animals only eat while they are completely un-burrowed and burrowing halts the feeding process (e.g., Mercier et al., 1999). Some research showed that since absorption of organic material takes place in the stomach and when dissected most animals have empty esophagus and stomachs but full intestines, there must be a stoppage point in these animals (Baskar, 1994).

Most congruence comes from the notion that feeding is correlated to burrowing cycles and feeding occurs while exposed on the surface of the substrate and when movement is occurring. Mercier et al. (1999) showed that juveniles will spend up to half of their day buried within the sand and the other half on the surface. Burrowing cycles were disrupted in individuals that experienced completely 24h darkness. It was hypothesized due to this research that this burrowing regimen was due to higher predation risk of the juveniles during the day and they therefore spend more time buried during daytime hours.  It was also hypothesized that burrowing cycles and therefore feeding cycles are affected by light changes.

Night time feeding was observed within the aquarium at University of Queensland. A single individual was placed in a small tank with roughly 25 % of the tank in the darkness while the other 75% was lit by artificial light. The animal had free roam within the tank for 24 hours. The lights of the aquarium are turned off for roughly half of a day. Observations showed that the animal fed on up to 100% of the substrate in the dark portion of the tank as characterized by the absence of any algae on the tank surface post trial (figure 7). As well, it can been seen that although there was movement within the lit portion of the tank as seen by the fecal piles in the tank, it does not seem that any significant portion of the algae within the lit portion was removed (Figure 8). This shows that the animal spent most of its time feeding within the dark area rather than the lit area. These findings are in line with the hypothesis proposed in previous research that feeding must be in part influenced by light change. A hypothetical ‘before’ picture shows what the tank may have looked like prior to the trail (Figure 9).

More research done by Wolkenhauer (2008) showed that adults also experience diel burrowing cycles but is not only contingent on light but also on temperature. It was shown in this study that animals spend more time burrowed as temperatures decrease and therefore are less active. However, these animals did not change their affinity to daytime burrowing but instead that they remained burrowed longer. In addition to this research it has been shown that burrowing cycles may be affected by stress, tide and current changes, predation, and changes in salinity and desiccation (Dance et al., 2003; Mercier et al., 2000; Purcell et al., 2006; Skewes et al., 2000).

H. scabra is a seemingly defenseless animal that is easy for the taking by predators. however, it has multiple defense mechanisms that keep sit safe from consumption. H. scabra house toxins within their body wall tissues that will inflict distress and death in fish and other predators (Rao et al., 1985). Other holothurians have Cuvierian tubules that when excreted attach and hinder predators and therefore save the animal. however, H. scabra has been noted to not posses these tubules. However, some studies have noted that auto-evisceration or the ejection of internal organs, although rare in nature, may occur when the animal experiences either physical or chemical stress (Conand, 1989).

Holothuria scabra

Phyla: Echinodermata

Class: Holothuroidea

Animals in this family are large animals and often cryptic. They have sausage shaped bodies with thick body walls. They are deposit feeders, ingesting sand and organic material within the substrate. These animals have tube feet present outside of modified feeding tentacles (Cannon& silver 1986).

Family: Holothuriidae

These animals range in size from small to large (<100mm to >300mm). their body is generally circular. They are often concealed within rocks or rubble and sometimes half buried in the sand. Most species are most active at night while some animals experience diel activity. There is a large range in colours and external morphology allowing the identification between live animals, however more detailed assessmentshould be done through analysis of their internal anatomy. Animals in this family have singular gonads, separating them from the paired gonad family, Stichopodidea (Cannon & silver 1986; Rowe, 1969).

Genus Holothuria Linnaeus, 1767

These animals are characterized by having tables present amongst the spicules (however very rarely these tables are absent) as well they have a robust, instead of sinuous, calcareous ring. Usually their body is sausage shaped with variation in body wall thickness. Various life styles, preferred depth, and sediment partiality are exhibited within the genus (Cannon& silver 1986).

Subgenera Holothuria (Metriatyla) Rowe, 1969

The subdivision of the Holothuria genus has been widely argued (James, 1995; Pearson, 1913-1914) through the discussion of plates, buttons, rods, rosettes (primitive or not), the importance of anal teeth etc. but the subdivision proposed by Rowe is most widely accepted throughout the field.  

The size of these animals ranges from less than 100mm to 200mm. Twenty tentacles surround the mouth which is downward facing towards the surface of the substrate. The body is generally arched in shape with a flat ventral surface. The tables are robust without cruciform holes and the buttons simple and knobbed within the spicules (Cannon& silver 1986; Rowe, 1969).

Species Holothuria(Metriatyla) scabra Jaeger, 1833

Spicule tables are well developed with spires and knobbed buttons within this species. Buttons are highly numerous and often very large. These animals are found within reefs and coastal habitats; generally exposed completely or partially buried (Cannon& silver 1986; Massin, 1999; Rowe, 1969).

By understanding the evolutionary history of an animal, you can understand where its body plan was derived from and its connecting to its sister species. A few hypothesis exist with the dating of Holothurians within fossil records. Some research proposed a phylogeny of Holothuroids through the use of the morphology of the ossicles and calcareous ring elements left behind after decomposition (Kerr and Kim, 2001). This research proposes that Holothurians arose within the Devonian and Aspidochirotida arose within the Triassic with the class Holothuriidae showing no presence until the Jurassic. 
However, more recent research looked at the use of micro- and macropalaeontological methods to identify phylogenetic relationships within the Holothurian family (Reich, 2010). It is believed that the first unequivocal Holothurians are dated to the early Darriwilian but the exact time location of Aspidochirotida is hard to identify as the ossicles are not completely representative and there has been no interpretation of the calcareous ring by palaeontologists (Reich, 2000; Reich, 2010). However, it is estimated that Aspidochirotida arose sometime within the upper Ordovician. There is still much debate over the exact date arrival of the ancestral animal of Holothuria scabra but the relationship of the Holothuroid orders is widely agreed upon (Figure 5).

Figure 5.  The commonly agreed upon phylogeny of the orders of Holothuroidea. See Kerr (2000) and Reich (2010) for argued time history of these orders.