Sea spider

Pantopoda collage

Callipallene brevirostris (YPM IZ 077244) 003

20200205 Pycnogonida Pantopoda morphology

Tanystylum californicum 104577432

Achelia spinosa (YPM IZ 077366) 004

Nymphon-leptocheles

Sea spiders, also known as Pycnogonida, are marine arthropods of the class Pycnogonida. They are cosmopolitan, found in oceans around the world, including the Arctic and Antarctic regions. Despite their common name, sea spiders are not true spiders, which belong to the class Arachnida. Instead, they form a distinct group within the Chelicerata, a subphylum that also includes spiders, scorpions, and horseshoe crabs.

Description

Sea spiders are characterized by their slender bodies and long legs, in contrast to the majority of other arthropods, which have more robust bodies. They can range in size from a few millimeters to over 90 centimeters in leg span in the case of the largest species. Their bodies are divided into an cephalothorax and abdomen, but unlike most arthropods, their abdomens are small and often reduced to a point where it is hardly visible externally. They possess a proboscis that allows them to suck nutrients from soft-bodied invertebrates, which constitutes their primary diet.

Anatomy and Physiology

The anatomy of sea spiders includes several unique features. They have a simple circulatory system, where the heart is reduced or absent, and their digestive system extends into their legs. This adaptation allows for the direct absorption of nutrients into their limbs. Sea spiders also exhibit a phenomenon known as external digestion, where they release enzymes to break down the body tissues of their prey before sucking up the resulting nutrients.

Respiration is carried out through the surface of their body, as they lack specialized respiratory organs. This is sufficient due to their small size and the high oxygen content of their marine environments. Additionally, sea spiders have a unique excretory system that relies on structures called coxal glands, located at the base of their legs.

Reproduction and Development

Sea spiders exhibit a variety of reproductive strategies, with some species showing care for their offspring. Males often carry the eggs, attached to special appendages until they hatch. The development of sea spiders is direct, meaning they do not go through a larval stage; the offspring resemble miniature adults when they emerge.

Ecology

Sea spiders are found in a variety of marine environments, from shallow waters to the deep sea. They are mostly predators or scavengers, feeding on soft-bodied invertebrates such as sponges, cnidarians, and worms. Their ecological role includes controlling the population of their prey species and contributing to the marine food web.

Taxonomy

The class Pycnogonida is divided into several orders and families, with over 1300 described species. The taxonomy of sea spiders has been a subject of debate, with some researchers suggesting they are closely related to the ancestors of all living arthropods.

Conservation

Currently, there are no specific conservation efforts directed at sea spiders. They are not considered to be under threat, largely due to their wide distribution and adaptability to different marine environments. However, like many marine organisms, they could be indirectly affected by global issues such as ocean acidification and climate change.

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