Yes – there are spider-like animals in the ocean!
Globally, more than 1,300 species have been identified.
But, even though they are jointed-legged invertebrates (arthropods) and most have 8 legs, they are not true spiders (arachnids) nor are they crustaceans. They are classified into the a distinct group of arthropods (the chelicerate subphylum). The name of the order to which this species belongs to is the “pantapoda” which is Latin for “all legs”, a good label as they have almost no abdomen.
They have a mouth part called a proboscis, a flexible tube that they use to mix digestive chemicals with their food and then suck it up.
Some species have additional leg-like appendages near their mouths. Often only the male has these structures so that they can take care of the eggs by carrying them.
The species pictured here are those I see most often around northeast Vancouver Island and they are only about 1 cm across. They have been given the common name “Yellow Hairy Sea Spider” (Tanystylum anthomasti). The hairy parts are believed to help the animals feel and sense chemicals.
I have only ever seen this sea spider species on Red Soft Coral colonies (Gersimia rubiformis). They appear to feed on the bushy polyps of the soft coral. As defence, the polyps can retract and have stinging cells but this seems to do little to deter the sea spiders.
One of the things I find fascinating about sea spiders is that they have a very thin external skeleton (exoskeleton) and as a result don’t need a respiratory system; they can “breath” through their skins.
Nudibranch species that also feed on Red Soft Coral include the Diamondback Nudibranch (Tritonia festiva to 10 cm) and the Orange Peel Nudibranch (Tochuina gigantea to 30 cm).
As an aside, there is also a fascinating association between Red Soft Coral and Basket Stars (Gorgonocephalus eucnemis). See below. Basket Star embryos develop INSIDE the polyps of the soft coral! It’s also thought the embryos feed on the soft coral’s eggs which brood inside the parent. When juvenile Basket Stars emerge from the coral’s polyps, they hang onto the outside till about 3 mm in disk diameter. Then, they crawl onto an adult Basket Star, shuffling off when approx. 5 cm. When adult Basket Stars’ 5 seeming infinitely branched arms are fully outstretched, width is up to 75 cm. Age is up to 35 years.
- Berkley University; Introduction to Pycnogonida.