Pleurocera acuta

General Body Structures:


The shell is a form of external skeletal structure for several animals, it serves as an important protective covering, weight support structure, and attachment site for muscles (Hickman, 2009).  Exoskeletons are characteristic of Arthropods and Mollusks.  In mollusks, the shell continues to grow as the organism increases in size, eliminating the need to molt (Hickman, 2009). This adaptation comes with both benefits and downfalls.  One benefit is the avoidance of vulnerability and energy costs associated with molting. During molting, an organism is in an extremely vulnerable state, and must spend much of its energy to molt and create a new shell (Hickman, 2009). The downfall of not molting is that the organism must dedicate some of its daily energy resources to building a new larger shell.  If resources are limited or the snail somehow cannot regenerated new shell, their body size is limited (Hickman, 2009).

Shell Coiling:

The shell of Pleurocera acuta varies in color from pale to deep brown averaging 29.9 mm long and 11.3 mm wide (Dazo, 1965).  The shell of Pleurocera acuta is related to other Gastropods by having torsion, the twisting of internal body cavity 180 degrees so that the anus is in the same opening as the mouth (Hickman, 2009).   Pleurocera acuta also shares the characteristic of having a single shell that is coiled.  Coiling, also known as spiral winding of the shell, is a separate characteristic that evolved before torsion (Hickman, 2009).  Each separate spiral can also be referred to as a whorl (Benson, 2011)Pleurocera acuta can contain anywhere from 9 to 14 whorls (Benson, 2011).  According to research referenced by Hickman, early Gastropods had bilateral symmetry but have evolved to shift their shells posterior and upward for a more balanced body structure.  Still, the weight of the first whorl was distributed unevenly to the right side of the body resulting in the gills, auricle, and kidney organs of that side being lost.  Because of this Pleurocera acuta and similar Gastropods are classified as having bilateral asymmetry (Hickman, 2009).

Shell Torsion:

 Torsion is also present in the body structure of Pleurocera acuta.  Torsion is an evolutionary characteristic that distinguishes Gastropods from the other Mollusk groups (Hickman, 2009).  Torsion is the twisting of the internal body structure 180 degrees so that the anus is located anterior to the head (Hickman, 2009).  This adaptation is thought to be created for protection by decreasing the weak openings to the inside of the shell.  As a result of torsion the left gill, kidney, and heart atrium  move from the left to the right side of the body and the nerve cord is twisted to form a figure eight (Hickman, 2009).  But this arrangement brings with it a set of obstacles to overcome, the major one being fouling.  Fouling is the presence of waste washing over the gills and contaminating the body cavity (Bay, 1999).  To avoid fouling snails have developed some interesting tactics.  The first comes from the sensory system of the snail.  Sensory organs in the mantle cavity called osphradia sense the direction of the water currents so that the waste is drawn away from the head when released (Dazo, 1965; Hickman, 2009).   The snail also possesses two independent incurrent and excurrent water siphons that are separated by space (Hickman, 2009).   Another adaptation that can also be coupled with coiling is the loss of the right gill, which creates a one way path for water through the body (Hickman, 2009).  Water enters the incurrent siphon on the left side of the mantle cavity and out the excurrent siphon on the right side (Hickman, 2009). 

The mantle is a thin, white, almost transparent layer of tissue that acts as a buffer between the hard protective shell and the soft inner visceral mass of Pleurocera acuta (Hickman, 2009; Dazo, 1965).  It is well supplied with blood vessels and nerves and contains the gills, osphradium, and anus (Dazo, 1965).  One of the most important functions of the mantle is to secrete a layer that will form the shell (Hickman, 2009).

Shell Layers:

The shell is made up of three distinct layers. The innermost layer is called the nacreous layer.  It is iridescent in color and composed of calcium carbonate which is secreted by the mantle (Hickman, 2009).  The middle layer is the prismatic later which is composed of calcium carbonate crystals in a protein matrix (Hickman, 2009). The outermost layer is called the periostracum.  It is pigmented and composed of a protein called conchin which gives it strength and resistance to dilute acidity (Hickman, 2009)


The operculum is a characteristics specific to the Gastropod subgroup Prosobranchia  (Hickman, 2009).  The operculum acts as a type of door to the shell.  It protects the visceral mass from external elements by holding water and other nutrients inside for extended periods of time (Hickman, 2009).  The operculum of Pleurocera acuta is reddish brown oval structure made of chiton that consists of three whorls in adults (Hickman, 2009; Dazo, 1965).  In a study done by B.C. Dazo, it was noted that some of the snails lost their operculum's because of parasites. Once lost, the operculum was regenerated completely in three to four months (Dazo, 1965).


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