Sea urchins are typically small, rounded, spiny creatures found on shallow rocky marine coastlines. The primary hazard associated with sea urchins is contact with their spines.
Biology and Identification
Sea urchins are echinoderms, a phylum of marine animals shared with starfish, sand dollars and sea cucumbers. Echinoderms are recognizable because of their pentaradial symmetry (they have five rays of symmetry), which is easily observed on a starfish. This symmetry corresponds with a water vascular system used for locomotion, transportation of nutrients and waste, and respiration. Sea urchins have tubular feet called pedicellariae, which enable movement. In one genus of sea urchin — the flower sea urchin — some of the pedicellariae have evolved into toxic claws. In this species, the spines are short and harmless, but these toxic claws can inflict an envenomation.
Sea urchins feed on organic matter in the seabed. Their mouth is located on the base of their shell, and their anus is on the top. The color of sea urchins varies depending on the species — shades of black, red, brown, green, yellow and pink are common.
There are species of sea urchins in all oceans, from tropical to arctic waters. Most incidents between humans and sea urchins occur in tropical and subtropical waters.
Mechanism of Injury
Sea urchins are covered in spines, which can easily penetrate divers’ boots and wetsuits, puncture the skin and break off. These spines are made of calcium carbonate, the same substance that comprises eggshells. Sea-urchin spines are usually hollow and can be fragile, particularly when it comes to extracting broken spines from the skin.
Injuries usually happen when people step on them on while walking across shallow rocky bottoms or tide pools. Divers and snorkelers are often injured while swimming on the surface in shallow waters as well as when entering or exiting the water from shore dives.
Although little epidemiological data is available, sea-urchin puncture wounds are common among divers, particularly when in shallow waters, near rocky shores or in close proximity to wrecks and other hard surfaces. The DAN Medical Information team receives at least one call a week regarding sea-urchin injuries, typically from divers and snorkelers swimming in very shallow waters near rocky shores.
Signs and Symptoms
Injuries are typically in the form of puncture wounds, often multiple and localized. Skin scrapes and lacerations are also possible. Puncture wounds are generally painful and associated with redness and swelling. Pain ranges from mild to severe depending on several factors, including the species of sea urchin, the body area of the wound, joint or muscular layers compromised, number of punctures, depth of puncture, and the individual’s threshold for pain. Multiple puncture wounds may cause limb weakness or paralysis, particularly with the long-spined species of the genus Diadema. On very rare occasions, immediate life-threatening complications may occur.
- Be observant while entering or exiting the water from shore dives, particularly when the bottom is rocky.
- If swimming, snorkeling or diving in shallow waters, near rocky shores or in close proximity to wrecks and other hard surfaces, maintain a prudent distance and buoyancy control.
- Avoid handling these animals.
There is no universally accepted treatment for sea-urchin puncture wounds. Both first aid and definitive care is symptomatic.
- Apply heat. Immerse the affected area in hot water (upper limit of 113°F/45°C) for 30 to 90 minutes. If you are assisting a sting victim, try the water on yourself first to assess tolerable heat levels. Do not rely on the victim’s assessment, as pain may impair his ability to evaluate tolerable heat levels. If you cannot measure water temperature, a good rule of thumb is to use the hottest water you can tolerate without scalding. Note that different body areas have different tolerance to heat, so test the water on the same area where the diver was injured. Repeat if necessary.NOTE: Very few species of sea urchins contain venom. If venom is present, hot-water immersion may also help denature any superficial toxins.
- Remove any superficial spines. Tweezers can be used for this purpose; however, sea-urchin spines are hollow and can be very fragile when grabbed from the sides. Your bare fingers are a softer alternative to hard tweezers.NOTE: Do not attempt to remove spines embedded deeper in the skin; let medical professionals handle those. Deeply embedded spines may break down into smaller pieces, complicating the removal process.
3. Wash the area thoroughly, but avoid forceful rubbing and scrubbing if you suspect there may still be spines embedded in the skin.
4. Apply antiseptic solutions or over-the-counter antibiotic ointments if available.
5. Do not close the wound with tape or glue; this might increase the risk of infection.
NOTE: Deep puncture wounds are the perfect environment to culture an infection, particularly tetanus.
6. Regardless of any first aid provided, always seek a professional medical evaluation.
Contrary to popular belief, very few species of sea urchins are actually toxic. Pain and swelling is often the result of the body’s reaction to myriad antigens present on the surface of the spines.
Spines are usually covered with strong pigments, so individual puncture wounds are often clearly visible and may cause suspicion that each puncture contains a fragment of a spine. Though this is possible, it may not necessarily be the case. It is easier to assess each individual puncture once the acute inflammatory process has started to recess.
The decision of whether surgical removal of retained spines is necessary is usually based on joint or muscular layer involvement and whether there is pain with movement or signs of infection. Spines will usually encapsulate in a short time, but they may not always dissolve. A reactive granuloma is a common reaction to remaining small foreign bodies. Radiological localization, fluoroscopy or an ultrasound might be useful to avoid a blind surgical extraction that may cause further spine fracture.
The use of anti-inflammatories and physical therapy is often key for managing these injuries, particularly when they involve small joints as a prolonged inflammatory process may result in fibrosis, which may limit range of motion.
If signs of infection are present, the doctor may prescribe antibiotics and a tetanus booster.
Flower Sea Urchin
The flower sea urchin (Toxopneustes spp.) is the most toxic of all sea urchins. Its short spines are harmless, but its pedicellariae, which look like small flowers, are tiny claws (Toxopnueustes means “toxic foot”). These claws contain a toxin that can cause severe pain similar to that of a jellyfish sting, faint giddiness, difficulty breathing, slurred speech, generalized weakness, and numbness of the lips, tongue and eyelids.