Sea Spiders - Types, Characteristics and Habitat

As stated in the introduction, the sea spider is not a true spider. It is an arthropod that is related to the true spider and may even be closer than to insects (class Insecta). While they have some similarities to true spiders, it is very important we distinguish them. There are no marine true spiders that can breathe underwater, but there are spiders of the family Desidae living in intertidal zones and trap air in barnacles to stay under water for long periods.

The classification of sea spiders is as follows:

• Kingdom: Animalia
• Phylum: Arthropoda
• Subphylum: Chelicerata
• Class: Pycnogonida
• Order: Pantopoda

The name of the order Pantopoda means ‘all feet’. While they are grouped in the subphylum Chelicerata with arachnids and horseshoe crabs, there is some scientific argument over their taxonomic grouping. Some evidence suggests that sea spiders evolutionarily separated from these other arthropods millions of years ago. This makes them a type of basal chelicerate, distancing them from true spiders and horseshoe crabs.

We have learned a little about the scientific categorization of sea spiders, something that is determined by their characteristics. The following are some of the characteristics of sea spiders:

• Size: sea spider size can vary. Some have bodies of about 0.5 mm and legs of 1.5 mm, while giant species can have a body as large as 8 cm with an extended legspan of 75 cm. This is something very particular within the group known as sea spiders.
  
  
• Legs: the number of legs is usually 4 pairs, but this also varies. There are species with 5 or even 6 pairs of legs.
  
  
• Breathing: respiration does not occur in the same way as in other chelicerates through book lungs or tracheals. Sea spiders intake the gases through their legs and spread oxygen by diffusion through to the rest of the body.
  
  
• Proboscis: an elongated appendage at the front of the head, the proboscis in sea spiders is unusual. It can vary greatly in both shape and size between species. The true mouth is located between the chamber of the proboscis and the esophagus.
  
  
• Body: unlike many arthropods, the body is not clearly divided into tagmas. They have an anterior region where the proboscis is located. They also have 3 to 4 pairs of appendages, including the chelicerae, the palps, the ovigers and the first pair of legs. Species that have ovigers use them to care for eggs, clean themselves and in mating. After the first body segment, there are others that form a kind of trunk, each having a pair of legs that they use to move. The terminal region has a dorsally projected tubercle and the anus.
  
  
• Gonopores: sea spiders are unique with the presence of various gonopores which are located on the legs of the second body segment. These correspond to the genital anatomy of these animals.
  
  
• Muscles: in the smallest species, the muscles are a single cell which is surrounded by connective tissue.
  
  
• Skeleton: sea spiders have an exoskeleton which is non-calcareous, meaning it is not made of calcium carbonate.
  
  
• Eyes: there are species that do not have eyes, especially those that live in the depths of the ocean, but in others there are up to two pairs of simple eyes that are located dorsally.
  
  
• Circulation: like other arthropods, it has an open circulatory system and a hemolymph that distributes oxygen throughout the body, as well as other essential compounds. Learn more about hemolymph with our article on whether insects have blood.
  
  
• Heart: they have a tiny elongated heart which carries out a strong impulse of hemolymph throughout the body. This is except in the rear legs, where circulatory movement occurs due to intestinal peristalsis that extends to these extremities.
  
  
• Digestion: excretion occurs in two ways, either through the digestive tract or when they molt.
  
  
• Nervous system: the nervous system of sea spiders is simple, consisting of what would be a brain that connects to specific ventral nerve cords. Learn more about brain-like organs with our article on whether insects have a brain.

We might ask this question because we know there are many species of venomous true spiders. This may lead us to associate sea spiders with types of venomous animals, but these marine animals do not have the ability to envenomate.

They do not have glands for the production of venom, nor the fangs or stingers which would allow it to be used. However, the proboscis of many sea spiders have oral structures which can mechanically pierce prey suck out tissues from within. Despite this, sea spiders are not poisonous, nor are they venomous.

There are around 1,300 identified species of sea spiders, which are grouped into two suborders:

• Eupantopodida
• Stiripasteride

In turn, superfamilies, families and the respective genera and species are grouped in each suborder. Let's look at some examples or types of sea spiders:

Anoplodatylus nanus

A member of the Phoxichilidiidae family, it is one of the smallest types of sea spiders, with a body that measures just 0.5 mm and legs 1.5 mm. It is a type of sea spider that lives on the Spanish coasts.

Anoplodactylus evansi

It is also grouped within the Phoxichilidiidae family, present on the coasts of Australia, New South Wales and Tasmania. It has a length of 1 cm, legs of 3 cm and a weight between 50 to 80 gr. It is red and blue in color, with a yellow hue at the joints.

Colossendeis acuta

This sea spider is included in the Colossendeidae family, it is native to the depths of the Antarctic Pacific. The body measures around 1 cm, but the legs can reach 24 cm. They are usually brown in color.

Colossendeis colossea

Like the previous species, it is classified within the Colossendeidae family. It stands out for being the largest sea spider recorded, some referring to it as a giant sea spider. The body area measures 3.5 cm, but the legspan reaches 75 cm when extended. It develops in the northernmost areas of the Pacific and Atlantic oceans, as well as in the Arctic, but can extend to the Iberian Peninsula, Bay of Biscay and Gibraltar.

Pycnogonum litorale

Corresponds to the Pycnogonidae family, with a distribution that includes the northern Atlantic Ocean, the North Seas, the Western Mediterranean and the English Channel. The trunk measures up to 1 cm, reaching up to about 25 cm with the legs. The color varies between shades of brown and yellow, with males being darker than females. This means there is some sexual dimorphism in this animal.

Callipallene brevirostris

This species is classified in the Callipallenidae family, being typical of the Gulf of Mexico at about 1,701 m deep. Two subspecies are recognized. It measures around 1.5 mm but can grow up to about 5 mm, with a yellowish and transparent color.

Sea spiders have a wide distribution in the world's oceans. Depending on the species, they can be located in Australia, New Zealand, Europe, the Pacific coasts of North America, as well as in seas such as the Caribbean and Mediterranean, among others.

Sea spiders are often located in shallow areas of the coast, associated with spaces such as coral reefs or rocky areas with a diversity of algae. They use the different types of coral and rocks to camouflage themselves. There are also species that are not restricted to this sea level, since some can be present at great depths, reaching around 7,000 meters.

Sea spiders have separate sexes, although there is a description of a hermaphroditic species in the scientific literature. The sexual organs include the ovaries in females. The testicles of males are in the dorsal region. Both males and females can be with several partners and monogamy has not been reported.

Fertilization occurs externally. The couple aligns their legs where the genital pores are located. The female then releases the eggs and the male the sperm, allowing fertilization to occur. A peculiarity in sea spiders is that it is the males who take care of the eggs and the young. These take the fertilized eggs in their ovigers, until the moment of hatching.

Sea spiders hatch as larvae, the type of which varies depending on the species. During the first juvenile stage the larvae continue to be cared for by the males. In the larval stage, sea spiders have a caecum, a head and only 3 pairs of cephalic appendages. Its development begins at this point and the other parts of the body emerge until it becomes an adult.

Depending on the larval form, after the first stage when they no longer depend on the father, sea spiders can be free-living or be a type of parasitic cyst. The maintain this form until adulthood.

Sea spiders do not have a long life expectancy. It can is often around a year since they are animals that are highly preyed upon by seabirds, rays and other fish, as well as crabs. For this reason, pycnogonids, usually have a single reproductive cycle if they are lucky enough to reach adulthood.

Now you know about these odd-looking marine creatures, you may want to discover some more. You can do so with our article on the weirdest-looking fish in the sea.

If you want to read similar articles to Sea Spiders - Types, Characteristics and Habitat, we recommend you visit our Facts about the animal kingdom category.