Reproduction
Acanthochitona sp. like almost all polyplacophorans is dioecious, meaning it has separate sexes, male and female (Ruppert et al 2004, Kaas et al 1998). Most polyplacophorans are broadcast spawners, where there is a synchronous release of sperm and eggs and fertilisation occurs externally in the water column, without copulation (sexual intercourse)(Ruppert et al 2004). Most polyplacophorans are also indirect developers, meaning they possess a larval stage, whereas some species brood eggs in the mantle cavity and direct development occurs (lacking a larval stage)(Ruppert et al 2004). The family Acanthochitonidae is one of two families within Polyplacophora where no brooding species are known (Pearse 1979) and the only known brooding species in Australia occurs in the family Ischnochitonidae (Kaas et al 1998). Therefore, Acanthochitona sp. does not brood and reproduces through external fertilisation and has indirect development.
Polyplacophoran gametes are transported out of the body by two gonoducts, they move into the papillae groove and are moved out into the water column by the exhalant current and aperture (Kaas et al 1998, Ruppert 2004). The eggs are usually help together by a gelatinous substance or accumulate in a loose sack at the end of the female (Kaas et al 1998). If broadcast spawning is to be successful, then it is essential that synchronous release of gametes occur to maximise the change of fertilization occurring. Polyplacophoran males are said to be the first to spawn, releasing sperm into the water column which stimulate females to release their gametes also, these gametes most likely being planktonic (floats in the water column) to increase dispersal capability (Kaas et al 1998).
Eggs and Sperm
Features that are useful to determine phylogenies within polyplacophora and useful in taxonomy are the morphologies of egg hulls and sperm (Kaas et al 1998, Bucklands-Nick 2008 and Ituarte et al 2010). A hull is an extra-cellular outer covering of the egg, and has projections that are very elaborate in free-spawning polyplacophorans (Ituarte et al 2010). The sub order Acanthochitonina is characterised by having sperm that have a fibrous complex (area of fibrous material) on the flagellum (locomotorary appendage) and the mitochondria of the sperm occurs anteriorly (Buckland-Nicks 2008)(Figure 1). Another characteristic of this order is that the eggs have cupsules on the hull (cup-like projections) and the base is wide (Buckland-Nicks 2008). At the start of oogenesis (creation on an egg cell/ovum), the ooplasm (egg cell) forms strange lobules on the surface from infolding of the egg membrane, which is characteristic of Acanthochitonina (Ituarte et al 2010)(Figure 2). A characteristic feature of the genus Acanthochitona is that eggs have closed cupsules on the hull, rather than open ones, the centrioles (cylindrical structures that are distributed to daughter cells after cell division) in the eggs are fused laterally and fertilization occurs between the cupsules (Buckland-Nicks 2008). Acanthochitona sp. would therefore possess these characteristics that are exclusive to higher taxonomic ranks.
 Figure 1: Sperm of Acanthochitona uridus, demonstrating the fibrous complex on the flagellum. Adapted from Buckland-Nicks 2008.
Figure 2: Plaxiphora aurata, from the sub order Acanthochitonina, demonstrating lobulations in the ooplasm during the early stages of oogenesis. Adapted from Ituarte et al 2010.
Larval stage
Molluscs are trochozoans, meaning they produce trochophore larvae that have a prototroch, which is a girdle of cilia that surrounds the larvae's middle. Indirect developing polyplacophorans produce a lecithitrophic planktonic trophophore, which is a larvae that feeds off a yolk reserve, floats in the water column and has the features typical of trochophore larvae (Ruppert et al 2004)(Figure 3). The larvae will eventually settle and develop into an adult, but the time this takes varies greatly between species.
Embryotic development of different polyplacophorans have demonstrated that the general morphology is very similar, just with different rates, and as Acanthochitona sp. is an indirect developer, it is likely to have a similar larval stage (Kaas et al 1998, Pearce 1979).
Figure 3: Lateral view of an example of a polyplacophoran larvae (lecithotrophic planktonic trochophore) from an Ischnochiton species.
Development
During development in polyplacophorans, cleavage of the egg is spiral, determinate and holoblastic, which is characteristic of a generalized mollusc (Ruppert et al 2004, Kaas et al 1998). In polyplacophorans, the first two cleavages of the egg is equal, with a third uneven cleavage, resulting in eight cells. Development is then a lot more rapid after this stage and when 64 cells are present a molluscan cross is seen, which is a typical arrangement of cells, common in mollusc development (Kaas et al 1998). Gastrulation and the blastopore forms and as polyplacophorans are protostomes, the blastopore becomes the mouth (Ruppert et al 2004). Many features of the larvae (Figure 3) develop and this is preceded with the larvae hatching, approximately 20 hours after fertilisation. Metamorphosis usually occurs 5 days after fertilisation where the body flattens, segmentation occurs, the grooves in the epidermis secrete the first seven valves with the tail valve developing last (Figure 4). The tegmentum of the valves forms before the articulamentum (Figure 4). Typical larvae characteristics disappear and adult features such as the mouth are observed (Kaas etal 1998). As previously mention, development is similar across most polyplacophorans, and Cryptochiton sterelli, from the same family as Acanthochitona sp. (Acanthochitonidae), demonstrated a short larval period and development of a non-feeding larvae, similar to other non-brooding polyplacophorans (Lord 2010). This demonstrates that Acanthochitona sp. development would be similar to that described but perhaps with variation in developmental rates.
Figure 4: Development of a polyplacophoran larvae. The number on top of the larval stage relates to the age in days. D=Dorsal view. V=Ventral view. S=Side view. Adapted from Kaas et al 1998. |