SHE

CRUSTACEANS

General Introduction

Crustaceans are aquatic mandibulates with the second and third segments bearing antennae. They evolved from Trilobites. They are primarily aquatic arthropods living in sea water, freshwater and some are semi-terrestrial. They are the second largest arthropod group next to insects. There are about 26 000 (some authors claim 50, 000) species and include members such as:

i.              barnacles

ii.             crayfishes

iii.            woodlice

iv.           crabs

v.            shrimps

vi.           waterfleas, etc.

The crustaceans are divided into six sub classes namely:

a.       Branchiopoda (Fairy shrimp)

b.      Ostracoda (seed shrimp)

c.       Copepoda (eg Cyclops)

d.      Branchiura (Fish lice eg Argulus)

e.       Cirripedia (Barnacles)

f.       Malacostraca (Wood lice, pill bugs, Sand hoppers, Crabs, Crayfish, lobsters)

They are mostly free-living but some are parasitic (see post on Parasitic Crustaceans below)

Characteristics

1.      The body is divided into head and trunk. two major parts namely a Cephalothorax (head fused with thorax) and an Abdomen.

2.      The head is made of 6 segments while the trunk is made of between 2 and more than 65 segments which are distinct or variously fused.

3.      The head region bears five pairs of appendages (typically biaramous). The appendages of the head are

a.       1^st antennae (antennules) - Sensory in function

b.      2^nd antennae (antennae) – sensory in function

c.       Mandibles – For Feeding

d.      1^st maxillae (maxillules) – For feeding

e.       2^nd maxillae (maxillae) - Feeding

4.      The head is fused with up to eight segments of the trunk to form a cephalothorax.

5.      They bear two pairs of antennae on the head segment 2 and 3 of the head. This is a unique feature.

6.      They mostly possess a pair of compound eyes like other arthropods. However the compound eyes may be sessile or stalked. The eyes are sessile when they are borne directly on the head. They are stalked when they are borne on distinct movable stalks. In some species, only simple ocelli are present instead of compound eyes.

7.      They possess a cephalic shield called carapace. The carapace may grow forward beyond the head to form a rostrum.

8.      The trunk is divided into thorax and abdomen.

9.      The circulatory system is the open type and has no heart.

10.  The nervous system is in two parts – one is a ventral nerve system and the other is a ganglia system.

11.  Reproduction is sexual. The sexes are separate but some (e.g Barnacles) are hermaphrodites. They are oviparous. Fertilization is external. There are several metamorphosis. Development is through Nauplius larva.

An example of Crustaceans to discuss here is:

THE COMMON SHRIMP (Macrobrachium volenhovenii)

External Features

It has a body divided into three regions (tagmata), namely

i.                    The head (6 segments)                               Fused into Cephalothorax covered externally

ii.                  The thorax (8 segments)                            by a carapace

iii.                The trunk (6 segments)

Based on the number of segments in each body region the shrimp can be said to be built on the formula 6, 8, 6 for the head, thorax and trunk respectively. The head and the thorax fuse together on form a cephalothorax which is covered dorsally by a hard, calcite impregnated chitinous shield called a carapace. The head is however demarcated from the thorax by a cervical grove on the crustacean’s dorsal side. 

Figure: External features of Macrobrachium vollenhovenii

In the adults the first segment of the head is lost. The second one bears a pair of antennules while the third bears two pairs of antennae. The fourth segment bears the mandibles while the fifth and sixth segments bear the first and second maxillae. Thus all the present head segments function in bearing the appendages of the head

In the same fashion the thorax is of eight segments where the first three are accessory feeding appendages called maxillipedes. The fifth, sixth, seventh and eight segments bear the walking legs. The first pair of legs, borne on the fifth segment, is specialized for attack and defense; it is large and chellate. The female gonopores are borne on the coxopodite of the third pair of walking legs while the male gonopores are on the fifth pair of legs.

The abdominal segments form the trunk and ends in a telson. The first five appendages of this region are called pleopods while the sixth are the uropods. The pleropods carry eggs of the crustacean.

Feeding and digestion

They feed on anything dead or alive serves as the crustacean’s food.

The food is first reduced to usable sizes by the crustacean’s maxillae and mandibles. They are then made to pass through the gut.

They have a digestive tract made up of an oesophagus, stomach (cardiac and pyloric) and the intestine. Sized food materials reach the cardiac (anterior) stomach from the esophagus where they are first churned by a gastric mill composed of three gastric ossicles (movable chitinous teeth). The food then goes to the pyloric stomach, which serves as a filter chamber containing bristles. In this chamber the food is filtered as it passes through the fine bristle structure, consisting of several tubules that secrete digestive enzymes into the pyloric stomach. Food materials that are too large for these bristles to allow passage are passed into the intestine to repeat the process, or the food is regurgitated. Absorption of digested food takes place in the digestive gland, from where they enter the blood and are carried to tissues and cells where they are used. 

Respiration

Respiration is through the gills , which are thin-walled. The gills are also bridged. They are filamentous outgrowths of the body surface. They have a rich supply of blood. They are on either side of the thorax. The beating of expedites of the second maxillae directs a strong current of water over the gills from behind. Oxygen diffuses through the gill filaments into the bloodstream of the crustacean. 

Blood circulation

As in other arthropods, crustaceans have open circulatory system. The circulatory system is made mainly of a compact hexagonal heart, dorsally situated in the thorax, and to which seven main arteries are connected. The blood is colorless, being mainly of haemocyanin. Haemocyanin is a copper-containing respiratory pigment. Some haemoglobin, an iron-containing pigment, may be dissolved in the crustacean’s blood. The blood is capable of clotting by releasing a coagulant that resembles thrombin. The blood passes from the open ends of the finer branches of the arteries into the large sinuses where it bathes the various organs. Blood collects from all parts of the body to the gills for purification.

Reproduction

The sexes are separate. The testes or ovary is a bilobed organ lying dorsally in the thoracic region and is situated below the heart. In males the first and second pleopods are modified into corpulatory organs.

In females the first pair of pleopods are vestigial. The male’s gonopores open on the basal segment of the fifth walking legs. The third walking legs bear the opening of the female’s gonopores. The eggs laid by the females are borne on the swimmerets, so also are the larvae. The larvae are similar to the adult in many respects.

Economic Importance of crustaceans

(i)     The large-sized crustaceans such as shrimps, crabs, crayfish and lobsters serve as food of high nutrient value for man.

(ii)   The small planktonic crustaceans such as copepods, krill and waterfleas are important links in the aquatic food chain.

(iii) Some waterfleas serve as intermediate hosts for Guinea worm (Dracunculus medinensis) which takes a huge medical toll on infested humans.

(iv) Some crustaceans inhabiting leaf-litter such as the woodlouse play an active role in nutrient cycling. Some other crustaceans, such as crayfish are beneficial to Man as they serve as an agent of decomposition of decaying vegetables and animal bodies in water.

(v)   In America some crustaceans especially the soft-shelled crayfishes are used by fisherman for use as fish baits.

Parasitic Crustaceans

 Parasites are organisms that derive benefits from their hosts at the detriment of the host species. Among crustaceans there are two types of parasites namely:

i.                   Ectoparasites – Found outside (on) the host

ii.                 Endoparasites – Found inside (in) the host

 The following are examples

1.     Lernaea spp. This is a copepod with the common name, anchor worm. It is an ectoparasite of freshwater fish.  The larvae are free-living, and resemble other free-living copepods.  However, the 3^rd copepodid stage is the parasite.  Males and females mate, the males die, and the females undergo a remarkable metamorphosis to develop a prominent head which they use to burrow into the flesh of the fish.  The penetration site later becomes the site for secondary infections by bacteria and fungi respectively.

2.     Lernaeocera spp. is an ectoparasitic copepod found in the gills of marine fish.  The life cycle is similar to that of Lernaea spp. Presented above. 

3.      Ergasilus sp is a copepod ectoparasite of freshwater fishes.  It has elongated maxilla which fuse and encircle the attacked gill filament. It may be also be found on the skin, and fins apart from the gills of fishes.  They have also been implicated as vehicles for other fish diseases.

4.     Argulus spp (eg Argulus japonicum). Unlike the parasitic copepods mentioned above, known as Fish louse, are motile Branchiurid ectoparasite curstaceans which can detach from one host and swim to another.  Each individual possesses two adhesive disks with which it adheres to the hosts’skin.  There is a sharp stylus which it uses to pierce the skin and extract blood meal. It is an ectoparasite of freshwater fishes.

5.     Sacculina spp. is a very unusual Cirripid Crustacean (barnacle).  Although the larvae resemble free-living species, the female will attach onto crabs and develop in the gonads of the host crab, much like a tumor.  The host gonads are destroyed in the process of the parasite developing. Consequently, infection with Sacculina leads to a phenomenon of parasitic castration. The genus Sacculina is one of the Rhizocephalan barnacles that parasitizes crabs. Similar to other barnacles, Sacculina have a planktonic larval stage, the nauplius, and a settling stage, the cyprids. The adults, however, unlike other typical barnacles, are internal parasites (called the "interna"), cuticular tumors which grow inside their crustacean hosts. These tumors can develop a system of branching roots that ramify throughout their hosts’ bodies and absorb their nutrients. The life cycle of Sacculina, therefore, comprises two stages: the endo- and ecto-parasitic stage. Sacculina larvae are dioecious. The male larvae are often smaller than those of the females. The life cycle begins with the female cyprid invading the crabs and then developing into a parasite with an internal root system (interna). Once the interna matures, it will develop a reproductive body outside the crabs through the abdominal part called the virgin externa. Male cyprids will then enter the virgin externa, which give rise to a fertilized externa with the eggs brooding inside it. Larvae will then be released via the externa once the eggs became mature.

 Adapted from http://facstaff.cbu.edu/~seisen/ParasiticCrustaceans.htm,  Segun A. O,  2013 (Tropical Zoology) and Denloye , A. A., 2011 (Introduction to Common Arthropods).