_Hallaxa cryptica_,Gosliner & Johnson, 1994
This nudibranch truly deserves its specific name of _Hallaxa cryptica_ Gosliner & Johnson, 1994. The species is known from the Philippines and various atolls in the Marshall Islands. It comes in two distinct color forms, one purple and one white. The photo of the white form above shows it blending in very well on its normal substrate and food source, a species of sponge. The "demonic eyes" on the right side of the nudibranch are its rhinophores, so the animal is moving across the sponge from left to right, apparently leaving a grazed path of bare rock behind it. The sponge the purple form lives on and eats is apparently the same species as the one the white form eats; the only visible difference is that this sponge is the same shade of purple as the nudibranchs that live on it.
At Enewetak Atoll, Marshall Islands, both the purple and white color forms were approximately equally abundant. At Kwajalein Atoll, the purple one is rare while the white one is common--but then, at Kwajalein, the purple form of the sponge is also rare.
About the only time you see these nudibranchs off their prey sponges is when they are depositing egg masses, which is what the purple one is doing in the photo. Interestingly, the egg masses deposited by the purple color form are purple, while those of the white color form are white.
The sponge is apparently sensitive to light and is always found on the
undersurfaces of dead coral rocks and rubble; hence, that is the only place
you'll find the nudibranchs, which are typically 25mm or less in length.
Kwajalein, Marshall Islands
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