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Katelysia scalarina (Lamarck, 1818)
The Stepped Venus


Alexander Hamish Bezzina 2017

Summary

The Stepped Venus, Katelysia scalarina (Lamarck, 1818), is a common venerid found in intertidal sand flats and estuaries around much of southern coasts of Australia. K. scalarina are commercially fished for both bait and food as a marine bivalve.

 

Ecology, life history, anatomy, distribution, evolution, and conservation of this species are all discussed within this web page using a number of sources and observations made in class. A particular focus will be place upon Eullemibranch gills, a type of Lamellibranch gill.



Physical Description

K. scalarina is one of the three members of the Katelysia genus, which are difficult to distinguish morphologically (Maguire et al, 1998) which can be seen in the figures below. Like most bivalves the Stepped Venus is comprised of two laterally compressed calcium carbonate shells that are joined at a single hinge. The length of shell varies between sources with some suggesting a range of 30-50mm, but most suggest that they can reach 45mm (Wilson, 2002). It can be identified as a member of the Veneridae family by the presence of three cardinal teeth on near the hinge (Wilson, 2002). The shells are elongate-oval, with the umbos of centre and re-curved concentric ribs. The outside of the shell is white-gray in colour, marked with light brown zigzags, and the interioir is primarily yellow, white, and often some purple at the muscle scars (Jansen, 2000). In calm water two siphons can be seen extruded from a partially opened clam.

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Figure 1

Ecology

The Stepped Venus, like most members of the bivalve class, is a strictly marine. It is a temperate species and one of the most plentiful shallow water bivalves throughout southern Australia (Cropp et al, 1998). The main environment for K. scalarina is within estuaries and bays where the species acts as key faunal sections of shallow sub tidal and soft-bodied intertidal zones. In the intertidal zone individuals are found gathered in groups of 6 or more making their distribution scattered (Gribben et al, 2013). Members of this species bury themselves 2-4cm below the surface, inbetween tidemarks in sand that is either fine or medium grain (Cropp et al, 1998).

 

K.scalarina, similar to most clams and cockles, is a suspension feeder. They filter feed by passing water in and out through the exposed siphons and over past there gills which picks up suspended food within the water column. This food is mostly made up of zooplankton and other particles (Gribben et al, 2013).

Life History and Behaviour

The majority of life, in the ocean, including K. scalarina, shares a common factor, a pelage-benthic lifecycle. This means that as larvae they have a pelagic lifestyle, floating in water currents until they settle on a substrate and metamorphose into their benthic adult form (Biotech, 2014). Information on development for stepped venerids is limited as few studies have been conducted so this section will include information on closely related member of the same genus Katelysia rhytiphora to fill in the gaps.

 

Reproduction:

Most members of the veneridae family are bi-sexual, that reproduces through broadcast fertilisation, which maximises the dispersal range (Barnes et al, 2004). With this type of fertilisation however there must be synchronous release of gametes to ensure the highest fertilisation rate and survival of the species. For K. scalarina populations in Australia this synchronous spawning occurs in spring from September to November (Goard et al, 1994).

 

Larvae:

Once fertilisation is complete the zygote begins to develop into a trochophore larvae, a defining characteristic of the lophotrocozoans. After 24 hours the first trocophore larvae of the generation will have developed into a D-veliger larva and by 48 hours all of the surviving larvae will have developed into this state (Goard et al, 1994). By 14 days all individuals will reach the pediveliger stage of development and will search for living and empty shells on which to attach to as spat. There they will begin metamorphosis (Goard et al, 1994).

 

Adult:

Through the process of metamorphosis the larvae reconstructs itself into it’s a juvenile version of it’s adult form. This is the same form as the adult, only smaller and the juvenile will reach maturity through growth as all major changes to the body plan are completed. The adult form is the benthic stage of the life cycle in which reproduction and filter feeding will occur until death. The figure below is a visual representation of a typical bivalve life cycle, which K. scalarina follows.

 

Feeding:

Trochophore Larvae- In this stage of the life cycle the stepped venerid larvae feeds on phytoplankton using cilia to beat move these organisms into place for feeding (Barnes et al, 2004).

Benthic Adult- As adults this species filter feeds on suspended particles and zooplankton by inhaling and exhaling water through its siphons which it exposes by opening its hinge using abductor muscles (Barnes et al, 2004).

 

Burrowing and Seagrass Cover:

K. scalarina individuals are predominantly found at shallow depths beneath the sand, however there are many can still be found on the surface of the sand. These populations can also be found in varying degrees of cover by seagrass (Gribben et al 2013). A study conducted in 2013 examined the shallow burrowing behaviour of stepped venus clams and the effects of sea grass cover. The results show that this species is using burrowing behaviour as a form of defence from predation, as predation was 4x higher for unburied clams. It also suggested that stepped venerids prefer the cover of sea grass as highest densities were found in high cover, even though this had little impact on the rate of predation on unburied clams (Gribben et al 2013). Further study into the impact of seagrass cover is still needed to understand how this behaviour arose.

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Figure 2

Anatomy and Physiology

Unfortunately information on the biology of the K. scalarina species is lacking (Bellchambers, 1998) and requires much more extensive study and thus much of this information will be generalised for the molluscs and Veneridae.

 

Shell:

As with all members of the bivalve taxon the shell is compressed of two valeves, held together by a hinge ligament, which is comprised of elastic proteins that open the shell. The abductor muscles negate this opening force, forcing the shell shut as a form of protection (Biotech, 2014). The shell is comprised of the layers, with the outermost being the periostracum, which protects the other calcium carbonate layers from dissolving in acidic conditions and creates a tight seal when the hinge is closed. There are usually between 2-4 calcareous layers beneath the periostracum, all composed of layered ‘bricks’ of aragonite that form an incredibly strong structure. These layers are secreted in development from different sections of the mantle, propagating from the umbo, which is the oldest section of shell (biotech, 2014). A detailed visual description of the K. scalarina shell can be found in the physical description section above.

 

Mantle:

The mantle is a layer of tissue that lines the inside of both valves encasing all the visceral mass completely when the shells are closed. The gills lie in the mantle cavity, which is the space between the mantle and the visceral mass (Barnes et al, 2004). As previously stated the mantle is responsible for secreting the shell and does so by producing aragonite and other calcium carbonate products through it’s epithelium lining. This process also results in the production of pearls when particles fall into the mantle cavity. In many species the mantle also houses sensory structures (Barnes et al).

 

Muscular Foot:

The muscular foot of bivalves went through a major evolutionary change from a wide, and flat, creeping appendage, to a modified burrowing anteriorly directed appendage which is blade like and laterally compressed. The foot is controlled through a combination of blood pressure and the pedal retractor muscles causing movement that is used to burrow into the sand (Biotech, 2014).

 

Visceral mass:

The visceral mass is the main body of the animal containing major systems and organs.

 

Respiratory system:

Oxygen is needed in order to complete many metabolic functions, and is transported within the blood, attached to hemoglobin, to the organs via the open-circulatory system. This majority of this oxygen is obtained through gas exchange that occurs across the surface of the gills, however a small portion is obtained through exchange across parts of the foot and mantle (Biotech, 2014). The gills of bivalves have are relatively inefficient and is only passable due to the large surface area of the gills, especially in Lamellibranchs. Water is drawn through the incurrent siphon into the mantle cavity, by cilia on the gills, and passes across the gill surface area where oxygen is absorbed and carbon dioxide diffused out. The water is then forced out through the excurrent siphon (Barnes et al, 2004).

 

Digestion:

The digestive system of lamellibranchs follows a set pathway, with food particles trapped in the gills being passed to the mouth by the labial palps, then through the esophagus, stomach, intestines, rectum and finally out through the anus. Extracellular digestion occurs within the stomach, and intracellular digestion occurs in the digestive ceca (Barnes et al, 2004).

 

Internal transport system:

Bivalves like most molluscs have an open circulatory or hemal system. This system transports molecules around the body in hymolymph (blood), which contains hemocytes. The hymolymph is transported throughout the body via heart, aortae, hemocoelic blood sinuses and veins. Within the heart a pair of lateral atria pass oxygenated blood from the gills into a central ventricle, which pumps this blood around the visceral mass (Barnes et al, 2004 & Biotech, 2014).

 

Excretory System:

All animals produce metabolic waste that must be excreted to avoid damaging cells. This waste is excreted in the form of urine in bivalves and is made up of nitrogenous waste and excess molecules and ions. The formation and excretion of this urine in bivalves follows a generalised path, beginning with the transportation of metabolic by products to the pericardial gland via the circulatory system in blood. An ultrafiltration barrier, made up of the pedicels, underlying basal lamina and protoplasmic extensions, filters out the urine into the kidney (Barnes et al, 2004). Within the kidney any useful molecules are reabsorbed and toxic and non-useful and the modified urine is excreted through the kidney opening into the mantel cavity, which is then flushed out through the out current siphon.

 

 

Nervous System:

Compared with the rest of the mollusca phyla, bivalves have a relatively limited nervous system and reduced cephalisation due to the loss of a head structure. Instead of a centralised nervous system, sense organs are scattered around peripheral regions with many bivalves having them located on the muscular foot, siphons and mantle margins (Barnes et al, 2004). These organs are controlled by a bilaterally symmetrical nervous, which is composed of three pairs of ganglia and nerve cords. The two cerebropleural ganglia are located dorso-laterally within bivalves and control statocysts, labial palps, anterior gut, cerebral eyes, anterior mantle and anterior abductor muscle. The paired pedal ganglia are located on the midline near the gap between the visceral mass and the foot. They maintain control over retractor muscles and the foot controlling the movement and digging ability of the bivalve (Barnes et al, 2004). The visceral ganglia are the final pair, and are located the surface of the posterior abductor muscle. These control the posterior abductor muscle, siphons, mantle gills, posterior pedal retractors, osphradia, gills and the majority of the viscera (Barnes et al, 2004).

 

Lamellibranch gills (Eulamellibranch):

One of the key defining characteristics of bivalves is the loss of the radula. This is due to their dramatically altered feeding behaviour to filter feeding in which the gills have been evolutionarily adapted to capture suspended food particles whilst preforming gas exchange. Figure .. shows this change in gill structure over time. This is known as a lamellibranch gill structure and consists of a series of W shaped filaments that are aligned in a row, attached to the mantle wall. Along the adjacent edges of each filament sit rows of lateral cilia (Biotech, 2014). These rows generate the currents that pull water through in through the ostia (between filaments for non-eulamellibranchs), along the axis of the gill and out, causing the current that drives gas exchange. Another type of cilia known as lateral-frontal cilia are modified into stiff bundles (known as Ciri) that form a net like structure, which captures any food particles as water, passes between adjacent filaments. A final type of cilia known as frontal cilia act as a conveyor belt transporting food from the lateral-frontal cilia to food grooves located in the vertices and corners of W-shaped filaments. Food grooves transport these particles from filament to filament and eventually to the palps, which pass it to the mouth (Barnes et al, 2004). In figure … a diagram of a Lamellibranch gills system is visually represented and figure… shows the three types of cilia in relation to water flow.

 

Eulamellibranch Gill:

There are three major types of Lamellibranch gills, filobranch, psuedolamellibranch, and eulamellibranch and as a species of Heterodonta; K. scalarina possesses the later (Barnes et al, 2004) (Coan & Valentich-Scott, 2006). This type is the most specialised of the three due to a number of advancements:

·       The interlamellar and interfillamentar junctions are highly developed permanent tissue connections (replacing filament connections of filobranchs)

·       The exhalant chamber is divided into vertically water tubes by solid lamellar junctions that extend the entire length of the lamellae

·       Extended sheets of tissue extend between adjacent filaments made by interfilamtar junctions. These sheets contain ostia (holes) that allow inflow of water

·       Inhalant side of lamella are distinctly ridged, with grooves being an interfilamentar junction. Lateral cilia direct water flow along grooves until water enters water tubes through ostia. This is where oxygenation of blood occurs.

In figure … you can see the distinct difference between the eulamellibranch gill and filobranch gill clearly highlighted.

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Figure 3
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Figure 4

Biogeographic Distribution

The Stepped venus has a spotted distribution throughout the indo pacific region, with some populations being recorded as far north as Mauritius and as far east as the islands of the coast of Africa (Atlas of Living Australia). The species is spread sparsely between latitudes of 13.5 and -43.1 degrees and longitudes of 57.5-153.1 degrees, however there are many concentrated populations found along the coastlines of Australia (Discover life). K. scalarina populations can be found in abundance along sandy beaches throughout the southern region of Australia, in parts of Western Australia, along the east coast as far north as Brisbane, and in particularly high abundance throughout Tasmania (Maguire et al, 1998). All recorded findings of the Stepped Venus can be seen in figure 1, with figure 2 showing a more focused map of the sightings within Australia, (atlas of living Australia) (discover Life). An interactive map of sightings around the globe can be found at the Discover life website http://www.discoverlife.org/mp/20m?kind=Katelysia+scalarina.

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Figure 5
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Figure 6

Evolution and Systematics

The Mollusca phylum is one of the most successful on the planet, second only to Arthropoda, and like most other phyla of the animal kingdom it arose within the early Cambrian (Barnes et al, 2004). The Cambrian explosion brought about a massive diversification of the Mollusca taxon, giving rise to all 7 orders including Bivalves. Bivalvia is thought to have arisen from extinct ancestors that resembled laterally compressed monoplacaphorans, known as rostroconchs. The relationships of the mollusc classes can be seen in below. The highly derived body plan of bivalves ensured their steady radiation, which gave rise to 106 extant families (Taylor & Zardus, 2017). K. scalarina is a member of one of the largest families of bivalves, Veneridae. The relationships of the Venus family still require gentic analysis but a comprehensive study preformed in 2006 gave the most parsimonious results placements, including the position of the Katelysia genus and included species (Bieler & Kappner, 2006), seen in the tree below.

 

 

Systematic classification (Atlas of living Australia):

Ø  Kingdom: Animalia

Ø  Phylum: Mollusca

Ø  Class: Bivalvia (Subclass: Autobranchia)

Ø  Order: Cardiida (Super Order: Cardiiformii, Sub Order: Cardiidina)

Ø  Family: Veneridae (Super Family: Veneroidea, Sub Family: Tapetinae)

Ø  Genus: Katelysia

Ø  Species: Katelysia scalarina

 

Synonyms:

·       Venus scalarina

·       Venus conularis

·       Venus strigosa

·       Venus aphrodina

·       Venus aphrodinoides

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Figure 7
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Figure 8

Conservation and Threats

Commercial Use:

Commercially sold clams in Australia mostly come from wild populations (Cropp et al, 1998). The Katelysia genus is one of the most heavily exploited species within southern waters as it is one of the most shallow water bivalves in the area, and is used as both food and bait (Maguire et al, 1998). Between 1994 and 1996 23-45 tons per year of K. scalarina were caught, and if this massive number does not decrease, or aquaculture is further introduced the species could be decimated (Maguire et al, 1998).

 

Other Threats:

Lepsiella paivae is a small gastropod that can be found along a sandy beach in Shoal bay, Western Australia. This gastropod feeds almost entirely on K. scalarina which is the dominate resident bivalve. L. paivae is able to effectively by pass bivalves protective shell by using its modified radula in a drill like fashion to prey on the soft tissue beneath (Morton, 2005). A study conducted on this particular population found that there were almost no K. scalarina juveniles present on the sandy beach, and that this was due to the population of L. paivae being specialised at hunting juveniles (Morton, 2005). This could potentially pose a great threat to the population of K. scalarina, as a huge rise and spread of L. paivae could decimate numbers by physically reducing the population and by targeting the young restrict recovery of the population over time.

 

Another potentially devastating threat to this species, and many others, is the introduction of non-indigenous species. Introduced species can provide competition and predation pressure into ecosystems. A threatening species to K. scalarina populations is the European green crab (Carcinus maenas). This crab preys upon clams such as the stepped venus, and a study published in 2002 predicts that the invasion of this species will have significant negative impacts on K. scalarina populations (Mackinnon et al, 2002). If the spread of this invasive species continues the stepped venus could see dramatic reductions in population size causing a massive financial blow to Australian fisheries, just like the Canadian oyster and European anchovies (Mackinnon et al, 2002).

 

Aquaculture:

A student in the 2017 class undertook an investigation into the potential use of K. scalarina for bioremediation of aquaculture waste and runoff. The results of this study can be found by searching for Loup Paitard in the class section of this site.

References

·       Atlas of Living Australia. Katelysia scalarina (Lamark 1818)(n.d.). Retrieved from http://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:a310df30-28a4-47c3-9168-02726901fd18#.

·       Barnes, D. B., Fox, R. S., & Ruppert, E. E. (2004). Invertebrate Zoology: A Functional Evolutionary Approach. Thompson Brooks/Cole, 7. 102 Dodds Street, Southbank, Victoria 3006, Australia.

·       Bellchambers, L. (1998). Ecology and Ecophysiology of Katelysia scalarina (Bivalvia: Veneridae), a commercially exploited clam. University of Tasmania, 1-17.

·       Maguire, G. B., SOH, S. W. L., & Ward, R. T. (1998). Genetic studies of the Venerid clam genus Katelysia. Journal of Shellfish Research, 4, 1057-1064.

·       Cropp, M., Frankish, K., John, M., Kent, G. N., & Maguire, G. B. (1998). Broodstock conditioning, spawning induction, and larval rearing of the stepped venerid, Katelysia scalarina (Lamarck 1818). Journal of Shellfish Research, 4, 1065-1070.

·       MacKinnon, C., Proctor, C., Rodriguez, L. F., Ruiz, G. M., & Walton, W. C. (2002). Effect of an invasive crab upon a marine fishery: green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia). Journal of Experimental Marine Biology and Ecology, 272, 171-189.

·       Morton, B. (2005). Predator-Prey interactions between Lepsiella (Bedeva) paivae (Gastropoda:Muricidae) and Katelysia scalarina (Bivalvia: Veneridae) in Princess Royal Harbour, Western Australia. Journal of Mollusca Studies, 4, 371-378.

·       Discover Life. Distribution map [image] (n.d.). Retreived from http://www.discoverlife.org/mp/20m?map=Katelysia+scalarina.

·       Biotech (2014). Bivalve Molluscs. Retrieved from http://cronodon.com/BioTech/bivalves.html

·       Coan, E. V., & Valentich-Scott, P. (2006). Chapter 27 Marine Bivalves. American Malacological Society, 341-347.

·       Bieler, R., Collins, T. M., Combosch, D. J., Giribet, G., Glover, E. A., Graf, D. L., Harper, E. M., Healy, J. M., Kawauchi, G. Y., Lemer, S., Mclntyre, E., Mikkelsen, P. M., Strong, E. E., Taylor, J. D., & Zardus, J. D. (2017). A family-level Tree of Life for bivalves based on Sanger-sequencing approach. Molecular Phylogenetics and Evolution, 107, 191-208.

·       Bieler, R., & Kappner, I. (2006). Phylogeny of venus clams (Bivalvia: Venerinae) as inferred from nuclear and mitochondrial gene sequences. Molecular Phylogentics and Evolution, 40, 317-331.

·       Goard, L. J., Heasman, M. P., Nell, J. A., & O’Connor, W. A. (1994). Hatchery production of the venerid clam Katelysia rhytiphora (Lamy) and the Sydney cockle Anadara trapezia (Deshayes). Aquaculture, 119, 149-156.

·       Nell, J. A., & Paterson, K. J. (1997). Effect of different growing techniques and substrate types on the growth and survival of the clams Tapes dorsatus (Lamarck) and Katelysia rhytiphora (Lamy). Aquaculture Research, 28, 707-715.

·       Gribben, P. E., & Wright, J. T. (2013). Habitat-former effects on prey behaviour increase predation and non-predation mortality. Journal of Animal Ecology, 83, 388-396.

·       Morse, P. (1987). Comparative Functional Morphology of the Bivalve Excretory System. American Zoologist, 27, 737-746.

·       Learn About Clams and Relatives. Reproduction: development & settlement [Image](n.d). Retrieved from http://www.asnailsodyssey.com/LEARNABOUT/CLAM/clamRepr.php.

·       Jenson, P. (2000). Seashells of South-East Australia. Capricornia Publications. Box 345, Linfield, NSW 2070, Australia.

·       Wilson, B. (2002). A Handbook To Australian Seashells On Seashores East to West and North to South. New Holland Publishers. 14 Aquatic Drive, Frenches Forrest, NSW 2086, Australia.