Characters/anatomy

Page summary:

• General anatomy
• Appendages/feeding apparatus
• Eyes/respiration/circulatory system
• Moulting/swimming
• Male female variation/reproduction

General anatomy:

Xiphosurans were large invertebrates that could reach up to 60 cm in length (females) and weigh up to 5kg. The body of the horseshoe crab had a dorsal covering of a hardened unmineralised exoskeleton of chitinous cuticle called the carapace, which was separated via a hinge into two segments. The exoskeleton was shed periodically as the organism grew. The domed shovel shape allowed the horseshoe crab to dig for invertebrate prey or burrow to evade predation and burry eggs. The body was divided into three segments, with the prosoma consisting of the head and thorax in the anterior (cephalothorax), the abdomen (opisthosoma) laying medially and to the posterior the telson (tail spine). The cephalothorax and the opisthosoma were attached via the same hinge that separated the carapace. The opisthosoma possessed six spines along the margin for additional protection. The telson was attached to the abdomen at the terminal base. While the telson might resemble the poison sting in the xiphosurans close relations - the scorpions, it was actually harmless, being only used for steering and righting itself if the organism became inverted. Depressions and ridges occured on the dorsal surface of the carapace such as the ophthalmic ridge. These were sites for muscle attachment on the internal side of the exoskeleton.

The nervous system in modern horseshoe crabs can be said to be comparable to fossil xiphosurans. It consists of a circum-oesophageal brain with a ventral nerve cord, giving rise to five sets of ganglia from which nerves radiate throughout the body. There is also a pair of longitudinal nerve cords controlling the heart. Fossil xiphosuran morphology varied very little in 250 million years with the most variation occuring in the opisthosoma. Ancestral forms had free (unfused) opisthosomal segments which varied in number. Some species had post-opisthosomal segmentation at the abdominal base, anterior to the telson. Most fossil forms had movable posterior spines used for protection. The main evolutionary trend within the order Xiphosura and Synziphosura is the tagmatisation (fusing) of the opisthosomal segments from unfused in ancestral forms to completely fused in descendant forms (modern horseshoe crab). The prosomal region varied very little.

Appendages/feeding apparatus:

Xiphosurans are characterised as having 6 pairs of uniramous, segmented appendages attached to the ventral side of the prosoma, which were completely covered by the carapace as protection from predators. Antennae were absent. The first pair were modified into feeding structures called chelicerae, which is common to all members within the Chelicerata including the arachnids and scorpions. The chelicera's (claws) primary utilisation was for food handling, directing food particles towards the mouth. The subsequent set of four appendages with the first pair often called the pedipalps, had a small claw on the anterior segment (at the tip), being used for locomotion and food handling. At the base of each leg the coxae were covered with inward pointing spines called gnathobases. Xiphosurans, as all Chelicerata, lacked jaws with which to macerate food thus in conjunction with the movement of the whole appendage, the gnathobases teared and shredded food particles. Chemoreceptors in the form of tiny hairs were located on the gnathobases. Xiphosuran stomachs did not contain acid thus the horseshoe crab had a gizzard containing sand and small grit debris to aid digestion. The last pair of appendages (6th pair) located most posteriorly had a flat and paddle shaped terminal segment. These were used for pushing/swimming during locomotion but also for clearing away sediments during burrowing and laying eggs.

The opisthosoma (abdomen) was hinged to the cephalothorax and can be divided into two parts, the anterior mesosoma and the posterior metasoma. The mesosoma is characterised by broader and thin segments called operculum covering the genital and reproductive organs hence are known as genital operculum. The metasoma are narrower and cover the gill lamellae thus are known as the gill operculum.

Eyes/respiration/circulatory system:

Xiphosurans had a total of 10 eyes/photoreceptors used for finding food, mates and sensing light. The two dominant eyes were the obvious 2 lateral compound eyes. These were primarily used for finding mates during the reproductive stages. Each compound eye had about 1,000 receptors or ommatidia (cone-like structures similar to those in mammalian eyes however a lot larger). The ommatidia were photo sensitive thus altered their functional mode with fluctuating photic levels. At low illumination, the lateral eyes increased the sensitivity of the ommatidia allowing continued function in near total darkness. Horseshoe crabs had an additional five eyes on the dorsal surface of the carapace in the prosomal region. Additionally, directly behind each lateral eye was a rudimentary lateral eye. At the anterior of the prosomal carapace was a small central ridge with three dark spots. The two more posterior structures constitute the median eyes, the third being the endoparietal eye. Each of these eyes was a photoreceptor rather than an "eye", detecting ultraviolet (UV) light from the sun and reflected light from the moon. This allowed xiphosurans to follow the lunar cycle, an important component in the reproductive cycle which peaked on during the lunar cycle. Two ventral eyes were located near the mouth however their function is unknown. Additionally, multiple photoreceptors were located on the telson. It is hypothesised that these aided synchronisation with the photic cycle (nocturnal behaviour).

Xiphosuran respiration removed oxygen from water using five paired gills that were located under the gill operculum on the opisthosoma. Each pair of gills consisted of a "stack" of thin and delicate membranes called lamellae with the gill operculum as protection against damage. Gaseous exchange occured on the surface of the lamellae as the gills were in motion. Opercular motion (metachronal rhythm) aided the replenishment of fresh water and removed waste-saturated water. Each gill contained approximately 150 lamellae, stacked horizontally and resembling pages in a book and are thus commonly called book gills. Book gills were a characteristic feature that is shared by all the Chelicerata including the terrestrial arachnids and scorpions. Opercular motion additionally functioned as paddling to propel juvenile xiphosurans through the water.

Xiphosurans had a highly developed circulatory system. A long tubular heart ran medially down the cephalothorax and opisthosoma joining with arteries and two veins that passed through the gills (aided via opercular motion). The "blood", a respiratory pigment called haemocyanin (blue colour instead of the red colour of haemoglobin) flowed into the book gills where it was oxygenated in the lamellae of each gill. Metabolic wastes were extracted from the blood by two pairs of coxal-glands, which created urine that passed to a bladder and was excreted through a special pore at the base of the fourth pair of prosomal appendages.

Moulting/swimming:

Like all organisms with an exoskeleton, a horseshoe crab had to moult (shed) its exoskeleton as it grew. Before moulting (ecdysis), a new exoskeleton began to form partly re-using components/chemicals from the old carapace. Once the new exoskeleton was ready, the horseshoe crab absorbed water through its book gills, inflating itself, thus increasing its volume. The increasing pressure on the smaller, older carapace caused it to break along the exuviation suture on the ventral anterior section allowing the horseshoe crab to crawl out of the front, leaving the old exoskeleton behind. This process took approximately 24 hours, during which the new soft exoskeleton hardened. Each moult increased the horseshoe crab's size by an estimated 25-30%.

It is hypothesised that xiphosurans swam upside down. Hydrodynamic stability tests using xiphosurid casts reveals that a vortex formed under the carapace of the dorsal plate, producing lift. Young and immature animals tended to swim more frequently.

Male/female variation and reproduction:

There were distinctive variations between males and females, however this varied with each species and is hard to examine/apply in extinct species. In the extant Limulus polyphemus, maturity is reached after 9-10 years (average life expectancy is 20-40 years). At this stage moulting ceases in the male, however the female will moult an additional one or two times, and the result is that the female is considerably larger than the male (reproductive reasons). The mature male develops a modified first pair of walking legs (pedipalps) with a hook-like terminal structure. The modified leg's function is to clasp onto the shell of the female during reproduction. Prior to reaching maturity, alternative phenotypic differences (genital pores) are used to identify males from females. The pores are located behind the first gill operculum at the base of the first pair of book gills. The genital pores of males are firm pointed structures and white in colour whereas the female genital pores are broad convex structures. Fertilisation is external, the female lays her eggs and the male releases his sperm onto them.
In the breeding season males and females migrate to certain shorelines in order to reproduce. The female must deposit her eggs (15 cm deep hole) in the moist warm sand within the tidal zone. The female will lay up to 3600 eggs and following fertilisation she will cover them up and leave. The larvae require 6 weeks to hatch and pass through as many as 16 moults before adulthood is attained. The early larval stages resemble trilobites and are therefore sometimes referred to as "trilobite larvae".

Comparative analysis of xiphosuran and eurypterid reproductive strategies reveals that eurypterid reproduction was more advanced. Eurypterids had internal fertilisation using spermatophores stored in a horn organ. Conversely, xiphosuran reproduction relied on the female laying eggs and the male releasing sperm to fertilise the eggs.

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Author: Andrew Przewieslik
Last updated: 21/11/05
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