Macro
Leaf sheep and blue dragons, Spanish dancers and blue-and-black nudibranchs, mantis shrimp and day octopus, giant clams and the coral-eating crown-of-thorns starfish.
Background photo: Megan Applegate (Palau)
Sea slugs
485 species recorded in the Marianas
A survey of the Mariana Islands recorded 485 sea slugs from Guam and the Northern Marianas, about half of them not yet described by science; much of the work was done at Bile Bay, and some species are known only from Apra Harbor (Carlson & Hoff 2003). The first five below are listed for Guam in that survey; the others were photographed at Guam in December 2022.
Key marks: Very large, up to 25 cm, with a red mantle that flares out like a skirt when it swims (Wikipedia).
At Guam: Recorded on Guam (Carlson & Hoff 2003); Spanish dancers are seen on muck dives at Outhouse Beach (Tim Rock, DivePhotoGuide).
Key marks: Three to six lumpy blue-grey ridges topped with yellow tubercles, and a black stripe on the sole of the foot (Wikipedia).
At Guam: Recorded on Guam, with a museum specimen (Carlson & Hoff 2003).
Key marks: A blue body bordered with black, and orange to yellow gills and rhinophores (Wikipedia).
At Guam: Recorded on Guam (Carlson & Hoff 2003).
Formerly Risbecia tryoni.
Key marks: Brown with white-ringed purple spots (Wikipedia).
At Guam: Recorded on Guam, as Risbecia tryoni (Carlson & Hoff 2003).
Key marks: A tawny-brown mantle with a dark blue edge; the name covers several look-alike species (Wikipedia).
At Guam: Recorded on Guam (Carlson & Hoff 2003); photographed at Guam in December 2022 (photo on this page).
Listed for Guam as Pteraeolidia ianthina (Carlson & Hoff 2003); that name is now kept for the East Australian slugs, and the others are treated as P. semperi, itself a complex of more than one species (Wikipedia).
Key marks: A translucent tan body with many cerata, purple to lavender or golden brown, and rhinophores and long head tentacles with dark purple bands. It keeps algae from the hydroids it eats in its gut, where they photosynthesise. The cerata can sting (Wikipedia).
At Guam: Photographed at Guam in December 2022 (photo on this page).
A sap-sucking sea slug, not a nudibranch: it keeps chloroplasts from the Avrainvillea algae it eats and can photosynthesise for a short time (Wikipedia).
Key marks: Only 5–10 mm long: two dark eyes, two rhinophores like a sheep’s ears, and rows of leaf-like cerata (Wikipedia).
At Guam: Photographed at Guam in December 2022 (photo on this page). Look on green algae on sand.
Formerly Chromodoris fidelis (Wikipedia).
Key marks: A creamy-white mantle with a scalloped edge, an orange or red outer margin with a burgundy line between the two colours, and greyish rhinophores and gills; up to about 25 mm (Wikipedia).
At Guam: Photographed at Guam in December 2022 (photo on this page).
Key marks: A pinkish-purple body; thick, pointed cerata with a deep purple ring and cream-yellow tips; and vivid orange rhinophores. About 30 mm (Sea Slugs of the World).
At Guam: Guam is within its range (Sea Slugs of the World). Photographed at Guam in December 2022 (photo on this page).
A flatworm, not a sea slug (Wikipedia).
Key marks: Brown to black with pink lines around close-set bright yellow spots, a ruffled edge and two small tentacles at the front; about 8–10 cm. It swims by rippling its body (Wikipedia).
At Guam: Recorded in the Northern Mariana Islands (Wikipedia). Photographed at Guam in December 2022 (photo on this page).
Muck dives at Outhouse Beach and in the Piti Channel also turn up juvenile flatheads, pom-pom boxer crabs, decorator crabs, seahorses and sea hares (Tim Rock, DivePhotoGuide).
Crustaceans and octopus
Look in holes and under ledges
Key marks: Mostly green, with orange legs and leopard-like spots on the front of the carapace (Wikipedia).
At Guam: Its range reaches the Marianas (Wikipedia), and it appears in GuamReefLife’s Mariana Islands gallery (GuamReefLife).
CHamoru: gamson, for octopus (University of Guam).
Key marks: Hunts by day, changing colour and texture all the time (Wikipedia).
At Guam: Photographed at Guam in December 2022 (photo on this page). A resident octopus lives on the Kitsugawa Maru wreck (Tim Rock, DivePhotoGuide); octopus are also reported at Gun Beach (Stars and Stripes Guam) and Barracuda Rock (Guam Visitors Bureau).
Key marks: Red and white bands and long white antennae (Wikipedia).
At Guam: In GuamReefLife’s Mariana Islands gallery (GuamReefLife).
Key marks: No claws, long spiny antennae, and yellow-green to blue-black colour with small white spots (Wikipedia).
At Guam: Lobster are reported in the caverns at Anae Island (Stars and Stripes Guam).
Key marks: Two colourful spiral crowns rising from living coral, usually Porites (Wikipedia).
At Guam: Reported at Coral Gardens, near Agat (Stars and Stripes Guam).
Giant clams
Two species left of four
CHamoru: hima, for giant clams (Guampedia).
Key marks: A blue, green or brown mantle between the shell valves (Wikipedia).
At Guam: The most common giant clam on Guam and in the Marianas. Two larger species, Tridacna gigas and Hippopus hippopus, have been lost from Guam to over-harvesting (Guampedia). A 2024 US proposal would list it as threatened because it looks like other, listed giant clams (ECO Magazine).
Commercial sale of giant clams is banned on Guam, and clams taken for personal use must be over 7 inches (18 cm). The sinahi, a crescent-shaped pendant, is made from giant clam shell (Guampedia).
Stars and urchins
Including the coral-eating crown-of-thorns
Native, not introduced, but its outbreaks kill coral (Pacific Island Times).
Key marks: Large, with many arms covered in venomous spines; it feeds on hard coral (Wikipedia).
At Guam: An outbreak in the early 1970s cut coral cover in some areas from 50–60% to under 1%, and in 2006 about 1,500 per hectare were counted at Tanguisson Point, where 30 per hectare counts as an outbreak (NOAA (Burdick et al.)). Divers have removed them at Gun Beach and Hap’s Reef; don’t remove them yourself, but report them to the Guam Coral Reef Response Team (Pacific Island Times). In 2020 navy divers removed 191 from the Orote Ecological Reserve (Pacific Island Times).
Key marks: Usually bright blue, with long tube-shaped arms and rounded tips (Wikipedia).
At Guam: In GuamReefLife’s Mariana Islands gallery (GuamReefLife).
Key marks: An inflated pentagon, like a pincushion (Wikipedia).
At Guam: In GuamReefLife’s Mariana Islands gallery (GuamReefLife).
Key marks: Thick, blunt spines, usually red, with white rings at the base (Wikipedia).
At Guam: In GuamReefLife’s Mariana Islands gallery (GuamReefLife).
Hands off
Don’t touch, move or hold marine life, including to set up a photo. Shell collecting is banned in Guam’s marine preserves.
Credits
Photographed at Guam, December 2022
The photos on this page are Megan Applegate’s own, taken at Guam in December 2022, © Megan Applegate, all rights reserved. Species without a photo link to photos online.
| Species | Photographer | Licence | Source |
|---|---|---|---|
| Girdled glossodoris | Megan Applegate | © Megan Applegate, all rights reserved | Photographer's own photo, taken at Guam, December 2022 |
| Blue dragon | Megan Applegate | © Megan Applegate, all rights reserved | Photographer's own photo, taken at Guam, December 2022 |
| Leaf sheep | Megan Applegate | © Megan Applegate, all rights reserved | Photographer's own photo, taken at Guam, December 2022 |
| Goniobranchus fidelis | Megan Applegate | © Megan Applegate, all rights reserved | Photographer's own photo, taken at Guam, December 2022 |
| Desirable flabellina | Megan Applegate | © Megan Applegate, all rights reserved | Photographer's own photo, taken at Guam, December 2022 |
| Persian carpet flatworm | Megan Applegate | © Megan Applegate, all rights reserved | Photographer's own photo, taken at Guam, December 2022 |
| Day octopus | Megan Applegate | © Megan Applegate, all rights reserved | Photographer's own photo, taken at Guam, December 2022 |