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Saron marmoratus (Olivier, 1811)
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Madeleine Glacken 2016
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Summary | |
Saron marmoratus or marbled shrimp (Olivier, 1811) are a very cryptic, little-studied species. They belong to the family Hippolytidae and are widely distributed throughout the Indo-Pacific, with specimens having been found in the Red Sea, Africa, Australia, French Polynesia and Hawaii (De Grave & Fransen, 2011). S. marmoratus is commonly found living in pairs on coral reefs, in coral rubble or under rocks (Holthius, 1947). It is one of the most popular shrimp in the aquarium trading business due to their beautiful colouration (Figure 1). This study will characterise the species of Saron marmoratus and will particularly focus on sexual dimorphism in the species.
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| Figure 1 |
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Physical Description | |
S. marmoratus has two pairs of long, thin antennae and three body segments (Baby et al., 2015). It can reach a total body length of 5cm (Calado, 2008). The shrimps have five pairs of biramous limbs, the first three of which function as mouthparts and are called maxillipeds. The last two are pereopods, which are adapted for functional use (Baby et al., 2015). The shrimps have a rostrum that is usually longer and larger than its carapace, and is curved strongly upward (Rothman, 2013; Baby et al., 2015). The dorsal side of the abdomen on the shrimp has tufts of feathered appendages (cirri).
The eyes of the S. marmoratus are pear shaped and the stalk is approximately 2mm long. The two pairs of long, thin antennae alsoserve as a sensory tool.
The shrimp range in colouration from mottled brown to green. The carapace has darker brown spots whilst the tail has orange patches. This colouration mimics the coral rubble and algal habitat of S. marmoratus (Rothman et al., 2013).
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Ecology | |
Habitat
The shrimps are marine, and are most commonly found in reef areas on coral rubble, under rocks and on branches of Sargassum Stylophora (Davie, 2002). Their dependence on coral restricts their distribution to the photic zone where there is living coral.
Organism and environmental interactions
Although many shrimps from the family Decapoda have many organism and environmental interactions such as commensalism with sea anemones and bony vertebrates, there has been no evidence to suggest S.marmoratus does.
Due to the territorial and predatory behaviour of S. marmoratus it has not been classified ‘reef safe’ when kept in aquaria. Calado (2008) stated that the shrimps would injure tridacnid clams, and damage corals and many other invertebrates in aquaria.
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Life History and Behaviour | |
Feeding
S. marmoratus is a territorial predator that is nocturnal. It shelters in crevices in the reef during the day and hunts for prey at night (Rothman etal., 2013). Its feeding style is said to be mainly predation and scavenging, however it is still unstudied what the shrimp’s preferred prey is.
Molting
One of the defining features of the phylum Arthropoda is the process of ecdysis, which is defined as the molting of the outer cuticle in invertebrates in order to increase in size (Chang, 1985). As the process is very complex and energy demanding, many aspects of the shrimp’s life history (including reproduction and metabolism) are synchronized with the cycle of molting (Chang, 1995) (see below section on Reproduction and Development for more information). Ecdysis is regulated by the hormone ecdysone.
Reproduction and Development
Reproduction in the family Hippolytidae is relatively similar across species. S. marmoratus is a gonochoric species, as it displays separate sexes. The shrimps reproduce through sexual reproduction, although precopulatory behaviour has not been previously observed (Bauer, 1976). In the infraorder Caridea there are two reproductive periods that are restricted to particular molt stages (Hartnoll, 1985). The ovary matures completely by the time of ecdysis, when the newly molted female is attractive to male shrimps and receptive to copulation and fertilization (Bauer, 1976). Immediately after the molt, mating and spawning take place. During this time males deposit the spermatophore on the underside of the abdominal segment of the female (Bauer, 1976). This is followed by the brooding of eggs by the female.
The larva of Caridean shrimps hatch in the form of a zoea, a more advanced form that exhibits locomotion by thoracic swimming appendages (Calado, 2008). They display a more continuous and gradual pattern of development, rather than distinct metamorphic stages. Despite this gradual pattern of development, shrimps from the genus Saron have four zoel stages. Diagnostic morphological features include a stout body, and the rostrumpointing downwards (Gurney, 1937).
Anti-predator behaviour and camoflage
The colouration of S. marmoratus is integral in minimizing predation by fish. At night the colouration of the shrimp shifts from the mottled brown and green, to a solid red colour. This has been hypothesized as a defensive mechanism that the shrimp may use to hide from predators at night. This mechanism of changing colour through diurnal and nocturnal periods results from the appearance of chromatophores (Chassard-Bouchard & Couturier, 1968), which are cells containing pigment. The eyes of the shrimp play an important role in regulating these chromatic changes (Chassard-Bouchard, 1965). Some decapod species use eyespots as threatening visual cues to potential predators (Bedini et al., 2002). However, further research is needed to determine whether the ocelli of S.marmoratus play an antipredatory role.
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Anatomy and Physiology |
External Morphology | |
Cephalothorax
The cephalothorax of the shrimp is split into two sections; the head and the thorax. The head bears the antennules, antennae, mandibles, and maxillae (Figure 2). This is also where the stalked compound eyes of the shrimp are located. The thorax bears the maxillipeds, which are modified to function as mouthparts. The walking appendages (pereopods) are also present on this segment. The cephalothroax is encased by the carapace, which is a hard outer shell protecting the inner organs and gills. The carapace also projects forward in front of the eyes. This section is called the rostrum.
Abdomen
The ventral side of the abdomen bears the pleopods (swimmerets) (Figure 3), which are primarily swimming appendages. They are also used for catching food and in reproduction for brooding eggs.
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| Figure 2 |
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| Figure 3 |
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Internal Morphology | |
Digestive System
The digestive system of the subphylum crustacea is composed of three regions; a short oesophagum and foregut, midgut, and hindgut (Ceccaldi, 1989). Figure 4 displays the general layout of the typical internal anatomical features of decapoda. The foregut is located on the dorsal side of the cephalothorax and is surrounded by the hepatopancreas (a large digestive gland that helps with foodabsorption, transport, secretion of digestive enzymes and storage). The chitinous sac of the foregut has two chambers – the cardiac chamber for the sorting and mastication of food, and the smaller pyloric chamber that deals with the finer particles (Felgenhauer, 1992). The midgut is elongate in caridean shrimps, extending from the foregut through the hepatopancreas and abdominal somites before linking up with the hind gut. Similarly to the fore gut,the hind gut of decapods is lined with chitin. It has cuticular scales that are directed towards the posterior of the shrimp, and are hypothesized to aid in the movement of fecal matter towards the anus (Felgenhauer, 1992).
Circulatory System
The classification of the decapoda circulatory system is highly debated in scientific literature. Traditionally it is classed as an open system, however a study by McGaw (2005) found that instead it may be ‘partially closed’. In decapods the highly developed circulatory system (Figure 5) is located posterior in the cephalothroax, arounda large heart (Felgenhauer, 1992).
Excretory System
Excretory organs in decapoda include the antennal, urinary and green glands and are positioned at the base of the antennae (Felgenhauer, 1992). The antennal glands are composed of four sections. The coelomosac performs ultrafiltration. The labyrinth is a transport system aiding the movement and reabsorption of ions and proteins. The proximal and distal tubules, also known as the nephridial canal is the conduit between labyrinth and bladder. Finally, the bladder stores urine (Felgenhauer, 1992).
Nervous and Sensory System
The nervous system of the order Decapoda is highly developed and is composed of a dorsally located brain connected to a nerve cord (Felgenhauer, 1992). The brain is made up of three sections; the protocerebrum, deuterocerebrum, and tritocerebrum. The nerve cord is located ventrally and is a ‘ladder-like’ structure with fused, paired ganglia (Felgenhauer, 1992).
Respiratory System
Most members of the order Decapoda possess four gills (branchiae) that are attached to the thoracic somites (Felgenhauer, 1992). The four gills – pleurobranch, two arthrobranchs and the podobranch – are arranged on the thoracic somites, pereopods and mouthparts.
Reproductive System
Males
The reproductive system in male decapods is comprised of the testes, vas deferens, spermatozoa (Felgenhauer, 1992). The testes are located on the dorsal side of the thoracic cavity, and in caridean shrimps are simple tubes that are connected anteriorly. Spermatozoa develop in the testes before moving through the vas deferens and exiting through the gonopore (at the base of the fifth pair of pereopods). In addition to transporting the spermatocytes the vas deferens packages the spermatozoa into a simple, cordlike mass called a spermatophore.
Females
The paired ovaries in female decapods is located dorsally in the cephalothorax (Felgenhauer, 1992). The ova are excreted from the ovaries and passed down the oviducts and exit via gonopores on the pereopods. The eggs are carried on the pleopodal sac (Felgenhauer & Abele, 1983).
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| Figure 4 |
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| Figure 5 |
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Sexual Dimorphism: A Closer Look | |
Introduction
Tirmizi and Kazmi (1971) completed a study on sexual dimorphism in S. marmoratus, and found that the species exhibited marked differences between the genders. It is hypothesized that this study will produce similar results.
Kemp (1914) first studied the sexual dimorphism within the species, using the size of the third maxilloped and the first pair of peraeopods, finding that in males these structures were enormously developed. There are many features of S. marmoratus that exhibit differences between sexes. These include rostral length, length of third maxilloped, carapace length and length of first peraeopod (Tirmizi & Kazmi, 1971).
Tirmizi and Kazmi (1971) suggested that one way to differentiate between males and females is counting the tufts of setae found on the dorsal surface of the abdomen. The number of tufts is greater in females than in males, with females exhibiting six to eight tufts, and males two to five.
Materials and methods
The specimens were collected from the scientific collection zone on the South Western side of Heron Reef. The collection occurred during the evening hours at low tide time in the intertidal lagoon area. Five shrimp specimens were carefully transferred to the laboratory and preserved in 10% Foramlin solution.
The specimen’s gender was determined by counting the number of tufts of cirri evident on the dorsal surface of the abdomen (third segment). Specimens with 6 or more tufts were classed as females, whilst specimens with less than six tufts were classed as males. There were four female specimens and one male specimen present in this study. The specimens carapace length was measured from the posterior orbital margin to the posterior margin of the carapace. The length of third maxilloped on all the species was measured and recorded. Both of these measurements were done using digital calipers (CraftRIGHT).
The ratio ofthe size of the third maxillopeds compared to carapace length was calculated and a t-test was used to determine if the difference in ratios between male and female groups was significant.
Results and Discussion
Whilst a graph comparing the two means (Figure 6) showed a difference in the ratios, with males having larger third maxillopeds than females, the t-test returned a p-value of 0.2053. This is not statistically significant. Therefore it can be concluded that this study did not find evidence of sexual dimorphism.
Although this study did not find evidence of sexual dimorphism within the specimens collected, it is still believed that S. marmoratus exhibits strong sexual dimorphism. This study had the significant limitation of a reduced sample size due to lack of access to specimens. S. marmoratus is a very cryptic and difficult to catch species. Further studies may have more success showing sexual dimorphism with a greater sample size.
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| Figure 6 |
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Biogeographic Distribution | |
S. marmoratus has a widespread geographical distribution, with the shrimps being found in India, Eastern Australia, the Red Sea, Hawaii, Eastern Africa and Iran (Miyake and Hayashi, 1966; Sheibani-Tezerji and Sari, 2007; Poupin and Junkcker, 2010, Radhakrishnan et al., 2012). The distribution of S. marmoratus coincides with tropical reefs and an abundance of living corals.
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Evolution and Systematics | |
A full classification of S. marmoratus, with some defining features of the groups, is outlined below.
Phylum – Arthropoda
- Presence of exoskleteon
- Segmented body plan
- Jointed appendages
Subphylum – Crustacea
- Two pairs of antennae
- Compound eyes
Class – Malacostraca
- 5 segments on head, 8 in thorax, 6 in abdomen making 19 total segments
- Appendages used as maxillipeds for feeding
Order – Decapoda
- Compound eyes are stalked
- Segments fused to create cephalothroax
- 10 pereopods
Suborder – Pleocyemata
- Eggs incubated by females on pleopods
- Lamellar gill structure
Infraorder – Caridea
Superfamily – Alpheoidea
Family – Hippolytidae
Genus – Saron
Species – Saron marmoratus
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Conservation and Threats | |
S. marmoratus currently does not have a special conservation status, and protection of this species is not a priority. Although its beautiful colouration has made the shrimp a popular aquaria species it is not in need of conservation efforts, as its numbers are not threatened.
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References |
Acknowledgements | |
The author is extremely grateful to Professor Ian Tibbets for the loan of the specimens to study and the reference material he provided. The author would also like to thank Jeff Ikin for the help and direction he provided for this project.
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Reference List | |
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