Notes on Horse-Shoe Crab:- 1. Habit and Habitat of Horse-Shoe Crab 2. External Structures of Horse-Shoe Crab 3. Integumentary System and Endoskeleton 4. Digestive System 5. Respiratory System 6. Circulatory System 7. Excretory System 8. Nervous System 9. Reproductive System 10. Breeding and Life History 11. Affinities.
Contents:
- Habit and Habitat of Horse-Shoe Crab
- External Structures of Horse-Shoe Crab
- Integumentary System and Endoskeleton of Horse-Shoe Crab
- Digestive System of Horse-Shoe Crab
- Respiratory System of Horse-Shoe Crab
- Circulatory System of Horse-Shoe Crab
- Excretory System of Horse-Shoe Crab
- Nervous System of Horse-Shoe Crab
- Reproductive System of Horse-Shoe Crab
- Breeding and Life History of Horse-Shoe Crab
- Affinities of Horse-Shoe Crab
1. Habit and Habitat of Horse-Shoe Crab:
These marine animals are bottom dwellers in shallow water. During breeding season, both male and female horse-shoe crabs come on land and dig holes at the upper limit of high tides to lay eggs. It can dig by the help of cephalothorax and may remain completely buried. The full-grown horse-shoe crabs usually crawl by using the legs, while the young’s can swim invertedly with the help of abdominal appendages.
2. External Structures of Horse-Shoe Crab:
As its name indicates the body is horse- shoe-shaped. It is divisible into two parts cephalothorax or prosoma and abdomen or opisthosoma (Fig. 18.102). The abdomen extends posteriorly as caudal spine.
Both the thorax and abdomen are un-segmented and are covered dorsally by exoskeleton. The abdomen remains movably articulated with the cephalothorax. In contrast with the hard dorsal exoskeleton, the ventral side contains soft sclerites in between the appendages.
Cephalothorax (Prosoma):
The cephalothorax or Prosoma is formed by the fusion of cephalic region and six appendage-bearing thoracic segments. Lines of segmentation are absent and the thoracic appendages are closely approximated. The dorsal exoskeletal covering, carapace, is more or less semicircular in outline. It is drawn at the posterolateral ends into pointed projections.
Following structures are seen in the cephalothorax:
(1) Spiny ridges:
One median and two lateral spiny ridges are longitudinally disposed on the dorsal side. The median ridge has two grooves, one on its either side, which run longitudinally to continue along the abdomen.
(2) Eyes:
Two pairs of eyes, median and lateral are present on the carapace. The median eyes are small and simple eyes. These are placed anteriorly, one on each side of the median ridge. The lateral eyes are comparatively larger and compound in nature. Each lateral eye is placed on the outer side of each lateral ridge.
(3) Mouth:
This aperture is present within a depression on the ventral side and bounded on all sides by the processes of cephalo-thoracic appendages.
(4) Frontal organ:
It is a wart-like structure in front of the mouth. In larva, it acts as photoreceptor, but its function in adult is unknown.
(5) Appendages:
Six pairs of tube-like distinct appendages are arranged like a ring on the ventral side. The first pair is known as cheliceras. In adult, it is a pre-oral appendage. Each chelicera is slender, trisegmented and situated in the anterior border of the mouth.
The distal article is chelated. The second pair is called pedipalpi. It is more or less leg-like and jointed. The proximal segment is spiny and the distal one is chelate. It helps in food capturing and locomotion. The third to sixth pairs of appendages are walking legs.
The walking legs serve both the functions of locomotion and food procurement. For the purpose of ingestion, the bases of the legs are drawn into spiny lobes, called gnathobases (Fig. 18.103) which remain radially arranged around the mouth. The last pair, in both sexes, is non-chelate and bears four movable spines at the distal tip.
These spines act as shovel for removing loose sand. In addition, the outer border of each coxa of the fifth or last pair of legs bears a spatulate process, called flabellum, which helps to direct the dorsal water current over the gills of the abdomen. A pair of small, flat, coxa-like appendages called chilaria is present. It remains directed vertically downwards and represents the degenerated seventh pair.
Abdomen (Opisthosoma):
The abdomen or opisthosoma looks like a hexagon. At the anterior end, the three sides of the hexagon form a lens-shaped hinge to articulate with the cephalothorax. In the posterior end of the abdomen lies a notch for tine articulation of caudal spine. On the dorsal groove of the abdomen there are six pairs of depressions and six pairs of laterally projected spines.
Such depressions and spines denote the position of segments. Six pairs of appendages are present in the ventral side of the abdomen. Each appendage is lameller and consists of a narrow inner process and a wide outer plate. The first pair of abdominal appendage is without gill.
It is united at the middle to form genital operculum which carries genital pore. The remaining five pairs are free from one another and each carries a gill or branchial lamella. The abdominal appendages beat slowly but continuously. At the time of swimming the speed becomes accelerated.
Caudal spine:
It is a long posteriorly projected pointed structure, which remains movably articulated with the abdominal notch and acts as a lever in pushing and balancing during locomotion. It does not represent the telson but is considered as an appendage of the telson.
3. Integumentary System and Endoskeleton of Horse-Shoe Crab:
As in other arthropods, the thick exoskeleton consists of epicuticle and chitinous cuticle. The cuticle is devoid of calcareous material. Posteriorly there are six pairs of apodemes for the attachment of abdominal muscles.
These are mesodermal cartilaginous plates inside the body cavity. In the cephalothorax, there is an endosternite for the attachment of leg muscles, but in abdomen there are six pairs of dorsal and six pairs of ventral endosternites.
4. Digestive System of Horse-Shoe Crab:
It includes:
(1) Alimentary canal and
(2) Digestive glands.
(1) Alimentary canal:
The alimentary canal begins from a slit-like mouth between the gnathobases of 2- 5th cephalothoracic appendages. It leads into a cuticularised pharynx. The pharynx forms a loop and opens into a dilated proventriculus or gizzard. The wall of the gizzard is muscular and its inner part is ridged and raised as denticles.
The gizzard communicates to a short and straight intestine and the opening is guarded by a funnel-like valve. From the intestine arise two pairs of branched caeca, which occupy major part of the cephalothorax. The intestine ends in a short rectum. The rectum opens to the exterior by an aperture, called anus. The anus is present near the base of caudal spine.
(2) Digestive glands:
The intestinal caeca serve as the digestive glands. The digestive juice is alkaline and contains various enzymes to split the complex food.
Mechanism of nutrition:
The horse-shoe crab hunts at night. The foods include worms and molluscs. The chela of the legs brings the food to the gnathobases, which in turn forces it to enter into the mouth. The food is swallowed intact. The gnathobases of the last appendage may break the shell. The chilaria pushes the food forward.
The chewing takes place within the gizzard, from where accepting bones and shells other substances enter into the intestine. The unwanted shells and bones are again regurgitated from the mouth. The digestion takes place mostly in the caeca and nutritive substances are also absorbed in its wall. The residual matters pass through the anus as faeces, which may be 5-10 cm long.
5. Respiratory System of Horse-Shoe Crab:
The respiration takes place in aquatic medium. The respiratory organs are known as gills or branchial lamellae. These are present in association with the last five pairs of abdominal appendages. Each lamella is borne on the exopodite part of each leg and consists of 150-200 delicate, richly vascullarised leaf-like structures, called lamellae. These lamellae are the organs fogas exchange.
These leaf-like gas exchange surfaces are called book-gills. The abdominal appendages move constantly to flow current of water which bathes the vascularised leaves of the book-gills and these gills also help in swimming as they move during swimming.
6. Circulatory System of Horse-Shoe Crab:
The circulatory system includes heart, arteries, sinuses and the circulating fluid blood.
The heart is present on the dorsal side of the intestine (Fig. 18.104). It is fusiform in appearance and extends almost the entire length of the body. It is enclosed within a pericardial sinus. Heart communicates with this pericardial sinus by means of eight pairs of ostia.
A median ventral artery and a pair of lateral arteries arise from heart and run anteriorly. The median artery supplies the dorsal side of the anterior part and the lateral arteries supply its ventral side.
These two lateral arteries unite and form a spacious blood sinus, within which lies the brain and sub-oesophageal ganglion. This sinus extends posteriorly to enclose the ventral nerve cord and its ganglia. From the adjoining regions of the first four pairs of ostia, four pairs of lateral arteries originate.
These lateral arteries of each side join to form a lateral vessel. These two lateral vessels unite posteriorly and continue as sub-abdominal artery into the caudal spine.
From each lateral vessel, near the region of second pair of ostia, an artery arises. This artery splits into one caecal artery to supply the caeca and a marginal artery to continue along the outer border of the cephalothorax. The two marginal arteries finally unite with the anterior median artery.
The arteries open within haemocoelomic spaces, called lacunae. Several lacunae unite to form small sinuses. From these lacunae and sinuses, deoxygenated blood is finally collected in three large longitudinal sinuses.
One such longitudinal sinus encloses the intestine and the other two are placed ventrally. From these ventral sinuses, the blood is sent to the gills for aeration. From gills, aerted blood is carried to the pericardial sinus by several laterally placed dorsoventral sinuses.
The blood is a fluid with dissolved pigment, haemocyanin. Very few cells are seen in it.
7. Excretory System of Horse-Shoe Crab:
The excretory organs are known as coxal glands. For its bright colour, it is also known as brick-red gland. Each gland is four lobed and occupies the ventral side of the endosternite in the cephalothorax.
A convoluted nephridial canal begins blindly from each gland and unites to form a common passage. It then runs up to the base of the fourth walking leg and forms a vesicle there. From this vesicle arises a common excretory canal which opens to a raised papilla, lying between the coxae of fifth and sixth appendages.
8. Nervous System of Horse-Shoe Crab:
It includes central nervous system, peripheral nervous system and sense organs:
(i) Central nervous system:
It consists of Sub-pharyngeal ganglion or Brain, Sub-pharyngeal ganglion and a double Ventral nerve cord. The brain is oval, compact and divided into two lateral lobes. It is placed anterior to the sub-pharyngeal ganglion and forms a ring around the oesophagus. The sub-pharyngeal ganglion is formed by the fusion of ganglia belonging to the second to eighth segments.
From the sub-pharyngeal ganglion arises ventral nerve cord. It runs posteriorly and carries four ganglia in its path, each representing the ninth to fourteenth segments. The last ganglion is formed by the fusion of ganglia belonging to the last three segments.
(ii) Peripheral nervous system:
Several peripheral nerves are given out from different ganglia. The optic nerve, which comes from the eye, opens to the brain.
(iii) Sense Organs:
The sense organs of horseshoe crab are:
(1) Sensory cells,
(2) Frontal organ and
(3) Eyes.
(1) Sensory cells:
These cells are scattered all over the masticatory processes of the thoracic limbs. The sensory cells which are present on the distal article of the walking legs are chemoreceptors.
(2) Frontal organ:
This is a specialised hairy area having a diameter of 5-8 mm. The function is not known in adult but serves as photoreceptor in larvae.
(3) Eyes:
Among the two kinds of eyes the lateral eyes are better developed than median eyes.
Each median eye is a simple eye and consists of:
(a) Lens—large, spherical and cuticular structure,
(b) Corneagen cells—present underneath the lens and formed by the hypo-dermis,
(c) Retina—it includes a central rhabdome and 6-8 irregular sensory cells.
The lateral eyes are compound eyes. Each lateral eye (Fig. 18.105) has:
(a) Lens:
It is a thick but transparent cuticle, formed by the downward conical projections of hypodermal cells.
(b) Ommatidia:
Beneath the lens, the hypodermal layer is modified as ommatidia. Each ommatidium consists of 10-15 cells, called retinulae, the inner parts of which are modified to form a central rhabdome. A central ganglion cell is present immediately beneath the rhabdome.
The ommatidia are separated from each other by single layer of columnar cells. From the structure of lateral eye it is evident that these are of much simpler design than the compound eye of other arthropods. The median eyes appear to be degenerated structures.
9. Reproductive System of Horse-Shoe Crab:
Sexes are separate and sexual dimorphism is present. The males are small in size and have hooks at the terminal ends of second and third cephalothoracic appendages.
Both the reproductive rogans—testis and ovary extend within the cephalothorax and abdomen. The middle part of the gonad is placed on the dorsal side of the intestine and the anterior part is branched within the lobes of the liver. Through paired ducts, the reproductive cells pass towards genital operculum, where they open through a slightly projected aperture.
10. Breeding and Life History of Horse-Shoe Crab:
During breeding season, both the sexes perform nuptial visit to the shore. The male fixes itself to the carapace of the female. The fertilized eggs are deposited in holes which are prepared by the female for this purpose in the region which is slightly above the high tide area. The size of the eggs varies from species to species and ranges from 1.7 to 3.5 mm in diameter.
After early development, the young horseshoe crab comes out of the case. It has only three pairs of abdominal appendages and is without caudal spine. It resembles the trilobite in its appearance (Fig. 18.106) and is called trilobite stage of horse-shoe crab. It swims freely in water by its abdominal appendages. After a series of moulting it gains the features of adult Limulus.
11. Affinities of Horse-Shoe Crab:
Attempts have been repeatedly made to establish relationship between horse-shoe crab and other groups of animals.
Large volumes of available information may be discussed under two broad heads:
(A) Affinities with fossil forms and
(B) Affinities with living forms.
(A) Affinities with Fossil Forms:
1. With Hemiaspidae (Fossil crustaceans):
Following are the similarities with the fossil crustaceans:
(i) Large head is covered with dorsal shield.
(ii) A telson is present.
(iii) Two compound eyes are placed laterally.
(iv) Larval form of horse-shoe crab after first moult resembles the members of the family Hemiaspidae.
2. With Trilobites:
Following similarities are noted between the two groups:
(i) Cephalothorax with lateral eyes is present.
(ii) Appendages are biramous.
(iii) The body is longitudinally divided into three parts by two furrows.
(iv) Presence of lateral pleural spine.
(v) Trilobite stage in the development of Limulus is highly suggestive of a relationship between the two groups; so far the structural peculiarities are concerned.
But at the same time, points of dissimilarities are also evident:
(i) Trilobites have distinct dorsal segmentation and trilobation of the body which are absent in Limulus.
(ii) Absence of antennae in Limulus.
(iii) Structure of abdominal appendages and genital operculum also differs.
3. With extinct Eurypterida:
Similarities:
(i) The body in both has three regions— prosoma, mesosoma and metasoma.
(ii) In both, the cephalothoracic appendages correspond in number and position.
(iii) Appearance of limb, presence of telson, presence of median and lateral eyes is the similar features in both.
Dissimilarities:
(i) Nature of abdomen in Eurypterida is different and shows resemblance with that of scorpion.
(ii) Nature of cephalothorax is also scorpion-like.
Remarks:
There are, however, many other points of differences between the groups under discussion, but these two groups, according to Woods (1946), are bridged by some palaeozoic forms of Limulus, thus making the connection closer.
(B) Affinities with Living Forms:
Among the living forms, horse-shoe crab has close resemblance with crustaceans and arachnids.
Some aspects of their similarities and dissimilarities are given below:
1. With Crustaceans:
Following are the similar features:
(i) Aquatic habits and identical appearance of abdominal appendages appear to be similar features.
(ii) Presence of simple median and less complicated compound eyes and
(iii) Possession of endosternite (like Triops) provides strong evidences in favour of the affinity.
But the two groups differ on following points:
(i) The respiratory organ, book-gill of horse-shoe crab has no parallel in any crustaceans.
(ii) Nauplius stage during development, a most distinguished feature of crustacea, is not seen in horse-shoe crab.
2. With Arachnids:
The xiphosurids and arachnids have many common features. For this reason, both are included under the subphylum Chelicerata. But still the two groups differ.
Some similarities and dissimilarities are mentioned below:
(a) Similarities:
(i) A broad carapace covers the cephalothorax.
(ii) Cephalothorax bears six pairs of limbs and paired median and lateral eyes.
(iii) The caudal spine of Limulus resembles the post-abdominal part of scorpion.
(iv) Presence of structures like endosternite, genital operculum and telson in both the groups.
(v) Book-gills of Limulus are supposed to have evolved from book-lungs of scorpion.
(vi) The suctorial pharynx, symmetrical liver, rudimentary genital glands illustrate other similar features.
(b) Dissimilarities:
In spite of strong similarities with arachnids, the horse-shoe crab differs from arachnids in two important features:
(i) Absence of Malpighian tubules an
(ii) Structure of book-gills.
Remarks:
It is evident that horse-shoe crab possesses more arthropod features and has closer relationship with Arachnida. It appears that the horseshoe crab, together with trilobites, originated from some common arthropod ancestor but remained universally isolated.
A. With vertebrates with Ammocoetes larva of Petromyzontia:
Similarities (Gaskell, 1908):
(i) Presence of median and lateral eyes.
(ii) Branchial appendages of Limulus may be compared with those of Ammocoetes larva.
(iii) The auditory apparatus is comparable to that of Ammocoetes larva of Petromyzon.
Dissimilarities are many and more pronounced.
Remark:
Gaskell (1908) in his article “The Origin of Vertebrates”, tried to correlate Limulus with Ammocoete larva of Petromyzon to trace evolution of vertebrates from the Limulus, but modern authors find it difficult to draw any such relationship.
B. With Ostracoderms:
Similarities:
(i) Dermal skeleton of both may be comparable.
(ii) Presence of both median and lateral eyes.
(iii) Gills are comparable in both groups.
(iv) Endosternite may be comparable to the primitive skull of ostracoderms.