Crayfish Anatomy

Phylum Arthropoda:

In this lab, you will become familiar with the external and internal anatomy of a crustacean arthropod, the crayfish, Cambarus sp.

Phylum Arthropoda (arthro = joint; poda = foot) is the most numerous phylum of all living organisms, both in number of species and in number of individuals. One, very conservative, estimate is that there are well over one million species of insects alone. In terms of number of individuals, there are more ants than anything else, and in terms of numbers of species, there are more kinds of beetles than anything else: 40 to 50% of all insect species are beetles. There are more species of insects than all other plants and animals together.

Characteristics of Phylum Arthropoda include:

1. jointed appendages, the primative condition being one pair of appendages per body segment (Most Crustacean appendages are of the primitive biramous (bi = two, ramous = a branch) form, from which other arthropod appendages are thought to have evolved, but note that the “pincers” on the tips have a different origin.) These jointed appendages include the antennae and mouthparts, but not the insect wings.
2. a segmented body, generally divided into three major regions — head, thorax, and abdomen (in some groups head and thorax are combined to form a cephalothorax),
3. a true head which exerts major control over the other parts of the body,
4. an exoskeleton, composed primarily of chitin (a polysaccharide whose monomer is an amino sugar), and often impregnated with mineral salts to form hardened or sclerotized areas (sclero = hard) — this exoskeleton serves as protection and provides places for muscle attachment. Arthropods must molt because their exoskeletons don’t grow with them.
5. an open circulatory system with a dorsal heart which collects blood from the body cavity and pumps it back into the body cavity again. In insects, the anterior portion of the heart (which is located in the abdomen) is extended into a tube that is called an aorta which directs the blood forward as it goes out into the body cavity.
6. a well-developed, ventral, solid nervous system of mesodermal origin and well-developed sense organs

Characteristics of Classes of Phylum Arthropoda:

Class      example(s)	Number of antennae	Number of legs & attachment	Number of body parts & attachment	Photo
Subphylum Trilobitomorpha
Class Trilobita      trilobites	These were most common during the Cambrian and Ordovician periods of geological history, and can be found preserved in a number of rock formations in the Cincinnati area. They are now extinct.
Subphylum Chelicerata
Class Merostomata      horseshoe crabs
Class Arachnida      spiders      scorpions      mites      ticks      daddy-long-legs	0	4 prs, att. to cephalothorax, (chelicerae & pedipalps are m.p.)	cephalothorax & abdomen
Class Pycnogonida      sea spiders
Subphylum Crustacea
Class Malacostraca      crayfish      crabs      pillbugs      etc.	2 pair	5 prs incl. cheliped att. to cephalothorax, and swimmerets, m.p., incl. mandibles	cephalothorax & abdomen
Note: there are quite a number of other classes of Crustacea not listed here.
Subphylum Atelocerata
Class Diplopoda      millipedes	1 pair	many, 2 pr per apparent segment bec of fused segm., m.p. incl. mandibles	head and “trunk” segments, every two segments fused into one apparent segment
Class Chilopoda      centipedes	1 pair	many, 1 pr per segment, m.p. incl. mandibles & poison claw on next segm.	head and “trunk” segments
Class Pauropoda      pauropods
Class Symphyla      symphylans
Class Hexapoda (Insecta)      insects	1 pair	3 pr, 1 pr. per thoracic segment, m.p. incl mandibles, some abdominal	head, three-segmented thorax, segmented abdomen (wings are not appendages)

Some further notes on various subgroups within Arthropoda:

• Subphylum Trilobita (tri = three) are all now extinct. Their bodies were divided sideways into three sections (the “lobes”) as well as a head and body regions.
• Subphylum Chelicerata (cheli = a claw, hoof) has the first pair of appendages modified as pincer-like mouthparts called chelicerae (these are the fangs in spiders). Their bodies are divided into a cephalothorax (cephalo = head) and an abdomen.
  • Class Xiphosura (xipho = a sword; ura = tail) are called horseshoe crabs because of the horseshoe-shaped carapace covering the cephalothorax. Their abdomen is thin, long, and pointed, hence the class name. They are marine, typically found in intertidal areas.
  • Class Arachnida (arachni = spider) includes scorpions, mites and ticks, daddy-long-legs, and spiders. They have simple eyes on top of their cephalothorax. The first pair of appendages is modified as chelicerae, small pincer-like mouthparts, which are further modified as fangs in spiders. The second pair of appendages is modified as pedipalps (pedi = foot; palpi = a feeler), which in spiders, look like small legs, but in scorpions are modified as large pincers (to capture food and for defense). Arachnids have four pairs of walking legs. Occasionally there are other appendages, like the spinnerets in spiders. Arachnids breathe via book lungs.
    More information on arachnids will follow in a subsequent lab.
• Subphylum Crustacea (crusta = crust, rind) includes crayfish and lobsters, crabs, pillbugs, and several other groups. They have gills, thus terrestrial pillbugs need to maintain a 100% humidity environment around their gills to be able to “breathe.” Crustaceans have the head and thorax combined into one body region, the cephalothorax, as well as an abdomen. They have two pairs of antennae, mandible-type mouthparts (of different evolutionary origin than mandibles in insects), and other mouthparts which include two pairs of maxillae and three pairs of maxillipeds, all of which are formed from modified appendages.
  • Class Malacostraca, Order Decapoda (deca = 10), which includes crabs, crayfish, and lobsters. They have five pairs of walking legs including a large pincer, the cheliped (ped = foot).
• Subphylum Atelocerata (formerly Mandibulata) includes insects, centipedes (centi = 100), and millipedes (milli = 1000). Centipedes and millipedes don’t really have 100 or 1000 legs, but they do have lots. Members of this subphylum have one pair of antennae, mandibles (mandibul = jaw), and two other pairs of mouthparts (either two pairs of maxillae or one pair of maxillae plus a labium), which are modified appendages. Note that their mandibles are modified appendages and are NOT homologous to our mouth or teeth.
  • Class Chilopoda (chilo = lip) is the centipedes. Their bodies are divided into head and trunk regions. The first pair of legs on the trunk is modified as poison jaws to capture and kill prey and assist mouthparts (also used for defense). On the trunk region, centipedes have one pair of appendages per segment. Centipedes are predatory/carnivores, and do have a poisonous “bite” to subdue their prey.
  • Class Diplopoda (diplo = double, two) is the millipedes. They also have head and trunk regions. Many of their trunk “segments” are actually two segments fused together into one apparent segment. Thus millipedes appear to have two pairs of legs per apparent segment. As their name would suggest, millipedes generally have more legs than centipedes, and the segmental pairing of legs is pretty obvious. Millipedes are scavengers, so it’s highly unlikely that one would bite if handled. Some millipedes are able to secrete small amounts of toxic chemicals like cyanide to protect themselves if threatened. While this is not enough to be a threat to humans, it has been reported that if an appropriate millipede is enclosed in a small jar and shaken, it will emit enough cyanide to kill itself and any insects placed in the jar with it (I do not recommend killing animals just to watch them die).
  • Class Hexapoda or Insecta (hexa = 6) is the insects. Insects have three body regions: head, three-segmented thorax (prothorax, mesothorax, and metathorax), and abdomen. Most also have both compound and simple eyes; mouthparts (covered by a labrum in front, and highly modified in some orders) which include mandibles, maxillae, and a labium; three pairs of walking legs, one on each of the three thoracic segments. Many insects have wings, which are NOT modified appendages, merely flaps of the exoskeleton of the meso- and metathoracic segments. There are about 28 to 30 orders of insects, depending on whose classification scheme you’re using.
    More information on insects will follow in a subsequent lab.

Origins of Arthropod Appendages:

It is thought that the early arthropod ancestors (descended from organisms that looked like marine worms or, later, Peripatus) looked sort of like a centipede: they had a number of body segments, each with a pair of jointed appendages. From there, some of these segments became fused to form a head and some of the appendages became modified to form mouthparts or antennae. Early on, there was an evolutionary split which led to the various modern subphyla and classes. Currently, three living subphyla are recognized, with trilobites representing an extinct fourth subphylum.

The theoretical origins of arthropod appendages from an earthworm-like ancestor are thought to include the following stages:

Earthworm-like Ancestor

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Clamworm-like Ancestor
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Peripatus-like Ancestor
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Various Arthropods
Generic Arachnid:

Generic Crustacean:
Generic Insect:

Note that, for example, the chelipeds (“pincers”) of crayfish and the chelicerae (“pincers”) of scorpions are not the same. The chelipeds of crayfish are appendages of segment #8, while the chelicerae of scorpions are appendages of segment #1. Note that in both the arachnids and the insects, segment #8 has no appendages at all. In crustaceans, segment #1 bears the antennae, and in insects, has no associated appendages.

Classification of Crayfish:

Even though crustaceans have mandibles, this is due to convergent evolution rather than a close relationship to other mandibulates. Thus crustaceans are placed in a separate subphylum from the other mandible-bearing arthropods. The technical explanation for this is that in crustaceans, the biting surface of the mandibles appears to have evolved from an inner (mesal) bulge, or endite lobe of the protopodite (note that the gills come from a similar outer (lateral) bulge, the exite lobe) similar to the origins of the mouthparts in the Trilobita and the Chelicerata). In all other mandibulates (insects, centipedes, millipedes) the biting surface is modified from the tips (the distal portion) of the endopodite.

Unlike the other mandibulates and similar to the Chelicerata, crustaceans have a cephalothorax (cephalo = head), although, unlike the chelicerates, the head and thorax are often somewhat delineated. They have a telson like the trilobites had. Unlike any other arthropods, they have two pairs of antennae: their first pair, the antennules, are homologous with the antennae on insects and other mandibulates, while their second pair, the antennae, have no homologous structures in insects and other mandibulates. As mentioned above, crustacean mandibles are slightly different in construction/origin from those of other mandibulates, and they have more accessory mouthparts than any other class.

Thus, crayfish are classified as:

Kingdom Animalia
         Phylum Arthropoda
                  Subphylum Crustacea
                           Class Malacostraca
                                    Order Decopoda
                                             Family Cambaridae
                                                      Genus Cambarus (including numerous species)

External Anatomy:

Obtain a dissecting tray and tools, as well as a preserved crayfish that has been rinsed.

Note the body regions: cephalothorax, covered by a “shell-like” carapace (carapac = a covering, shield) on top, and abdomen. Can you tell where the head and thorax join? Note the abdomen, and see if you can find all seven abdominal segments. The elongated, anterior portion of the crayfish’s cephalothorax is called the rostrum.

On the head, note the stalked, compound eyes. These fit into grooves in the front of the head, and are often recessed into them.

In many decapods (deca = ten, poda = foot: referring to the walking legs), each segment of the abdomen bears a pair of small appendages called swimmerets with the exception of the last segment, called the telson. In crayfish, the penultimate segment bears a pair of two-parted appendages called uropods (uro = tail). The order gets its name from the ten walking legs (five pairs) located on the thorax, including the cheliped (the pincer; cheli =claw, ped =foot).

Observe all the appendages by carefully removing those from the left side one at a time and arranging in order on the desk top or a piece of paper for further study. It is easiest to do this starting from the rear.

1. The telson is the last segment (#19) and does not have any appendages.


2. The second-last (penultimate) segment (#18) of many decapods has a broad, flat, two-part structure, the uropod (uro = tail, poda = foot).
   
   Note, in this and other labeled drawings, below, “exo” refers to the exopodite, “end” refers to the endopodite, and “pro” refers to the protopodite.
   
3. The other abdominal segments each bear a pair of small appendages called swimmerets. Can you find all five pairs? Are they all identical or are some different? In crayfish, the swimmerets of the first two abdominal segments (13 & 14) are larger in the male and modified for sperm transfer. Because of their use in sperm transfer, some texts also refer to these specialized swimmerets as gonopods. In females, the first two pairs of swimmerets are smaller and look more similar to the remaining three pairs.



4. The thoracic region has five pairs of walking legs (deca = ten for the ten “legs” of decapods), the front-most pair being modified as a cheliped (cheli = claw, hoof; ped = foot) or pincer.
   
   In some decapods, each of the other four walking legs has a small pincer at the tip, while in others, the walking legs have just one tip segment. Do the crayfish’s walking legs have one- or two-tipped ends? These legs are the appendages of segments 8-12.
   
   
   The openings of the female’s oviducts (ovi = egg) are located at the bases of the third pair of walking legs (segment #10) with a seminal receptacle (semin = seed, sperm, semen) between the bases of the fourth pair of legs (segment #11). On the males, the openings of the vasa deferentia (vasa = vessel, duct; deferens = carry away) are located by the bases of the fifth pair of walking legs (segment #12). As you remove these legs, note which have gills attached.


5. The next appendages are associated with the mouth. They are quite small, flattened, and closely packed. The easiest way to remove each one is first to determine exactly which is which, then grasp firmly at the base with a forceps and twist and pull it loose. From posterior to anterior, these are the maxillipeds (maxill = the jaw, jawbone) (third, second, first) on segments 7, 6, and 5 at the front of the thorax.
   Third Maxilliped (Segment #7)
   Second Maxilliped (Segment #6)
   First Maxilliped (Segment #5)
   Then, on segments 4 and 3 of the head, are the second and first maxillae, and on segment 2, the mandibles. The mandibles are heavy, grinding structures associated with the mouth. Note if any of these appendages have gills attached. Note in which direction the mandibles move.

Second Maxilla (Segment #4)
First Maxilla (Segment #3)
Mandible (Segment #2)

6. The appendages of segment 1 and the prostomium (pro = before, in front of; stoma = mouth) are, respectively, the antennae and the antennules (antenna = sailyard).
   
   
   Antenna (Segment #1)
   Antennules (Prostomium) The external openings of the green glands are located on the bases of the antennae (green gland will be studied inside) although may be difficult to see.

Carefully cut away part of the carapace on the left side to expose the gill chamber. Note the gills and the inner wall of the gill chamber (the wall of the thorax). Note how some of the gills are attached to the wall of the thorax and others to the bases of the appendages. In the crayfish, the gills resemble feathers in shape. Count the number of gills.

Internal Anatomy:

Beginning at the back edge of the carapace, cut two lines forward, about ¼ to ½ inch (about 1 cm, or so) to each side of the middle. Note: cut very shallowly because there are body organs located right below where you will be cutting. Gently lift off the flap, trying to remove only the chitinous exoskeleton. The underlying epidermis may adhere to the exoskeleton or it may remain covering the internal organs. If it remains, you will need to gently remove it also. You may need to widen the opening further to view all the organs. Also, continue these slits into the abdomen as far as feasible and carefully remove the flap of exoskeleton.

In the thorax, the first thing you should see dorsally is the heart. You may be able to see small openings, ostia (os = mouth), through which the blood enters the heart. At several points, you may be able to see arteries leaving the heart. The circulatory system is an open one, thus once the blood leaves the arteries, it circulates throughout the body cavity. The membranous chamber in which the heart lies is the pericardial sinus (peri = around; cardio = heart).

If your crayfish is a female, much of the thorax will probably be filled with eggs (brownish). Below the heart, behind the stomach are the reproductive organs. As mentioned, if the ovaries (ova, ovo = egg) of the female are filled with eggs, they will be easily recognizable, although the ovary walls will be “invisible.” Those of a female without eggs as well as the testes of the male may be difficult to distinguish from the digestive glands and may even be partially imbedded in the glands. Generally, the reproductive organs (the ovaries at least) are a darker, orange-brown color and tougher than the digestive glands. The female’s oviducts extend toward the bases of the third pair of walking legs while the male’s vasa deferentia open between the fifth walking legs. Check a crab/crayfish of the opposite sex to see the differences.

Intact, Unopened Stomach/Gizzard

Opened Gizzard with One
Grinding Structure Removed Forward of (and perhaps slightly below) the heart is the stomach. To see this, you may gently need to remove some of the eggs and the heart. You may be able to insert a blunt probe through the mouth and short esophagus (eso = within, inward; phago = eat) into the stomach. In the crayfish, the stomach is divided into anterior and posterior sections (crop and gizzard) by a constriction bearing grinding structures to shred the food as it passes. From the stomach, the intestine continues to the end of the abdomen, but may be hidden in the thorax by other organs. A pair of large, soft, yellowish structures around the stomach (under the eggs) are the digestive glands. You may remove the heart (and eggs), if not already removed, to see the digestive glands better. They are connected to the intestine and aid in digestion and absorption of food.

Side View of Nerve Cord Going Around Esophagus

Rear View of Brain and Nerve Cord Going Around Esophagus The nerve cord is located ventrally. It is a pair of thin, whitish, fairly tough “strings” that can best be located by first looking for the two halves that go around the esophagus. To follow the nerve cord posteriorly, you may have to gently and carefully remove abdominal muscles and/or push aside or remove the digestive organs in the thorax. Note that there is a nerve ganglion in each segment and that they are connected by the double-stranded nerve cord, resulting in the “ladder-like” appearance of the nerve cord. In the thorax, there are chitinous structures over the cord which will need to be cut away to see it. Each of the posterior thoracic segments bears its own ganglion, but those of the anterior thoracic and posterior head segments (segments 3 through 7) are fused to form one subesophageal (“under the esophagus”; sub = under, beneath) ganglion (gangli = a knot on a string, swelling). The two halves of the nerve cord continue anteriorly around each side of the esophagus to the supraesophageal (“over the esophagus”; supra = above, over, beyond) ganglion or “brain,” from which nerves radiate to the eyes, antennae, etc.

Examine the various thoracic and abdominal nerve ganglia to determine whether the those ganglia are the same or different in size. How do you think the size of a ganglion correlates to the functions being performed in/by that segment of the body?

In the anterior part of the body cavity, between the esophagus, supraesophageal ganglion, and attachment of the antennae, are the green glands. These excretory organs function similarly to our kidneys in that they remove waste from the blood and discharge it to the outside.

Other Things to Include in Your Notebook

Make sure you have all of the following in your lab notebook:

• all handout pages (in separate protocol book)
• all notes you take as you read through the Web page and/or during the introductory mini-lecture
• all notes and data you gather as you perform the lab
• labeled drawings (yours!) of
  • overall external anatomy (dorsal and/or side view)
  • all appendages, including antennules, antennae, mouthparts, walking legs, swimmerets, uropods
  • difference between specially-adapted male and female swimmerets (gonopods)
  • overall internal anatomy (dorsal view)
  • heart
  • ventral nerve cord and brain (dorsal view)
• answers to all discussion questions, a summary/conclusion in your own words, and any suggestions you may have
• any returned, graded pop quiz