So what's illuminating those barnacles on the waves? Why, it's our old pal the sun, of course. That all-powerful light doesn't let anything out of its sight, so to speak. Even if those "shafts" of light are "submerged," they're still getting through.
Notice all of the alliteration we have here? We've got "submerged," "shafts," "sun," "split," "spun"—phew. It's a regular snake dance going on in our mouths. But why all the S sounds?
For one thing, the "submerged shafts" back in stanza 2 set us up for all of the sun imagery we get in the beginning of the third stanza and accents the appearance of that all-powerful light. It's everywhere, literally.
And it also seems that the speaker is reminding us that nothing can hide in this giant ecosystem under the sea.
So what about the imagery of the sun's rays being "split"? We're not encouraging you to stare at the sun too long, but if you do, you'll notice that those rays look as if they're split into different parts. So we're getting Moore's ideas here even more that there's no hiding, even beneath the sea, since there's always a nearby ray to light things up.
And since we have a simile, comparing the rays to "spun glass," we know there's even more reflection and lighting up of hidden places. When glass catches the sun's rays, things tend to light up, and heat up, pretty quickly (thanks, Wilderness Survival 101).
And since the glass is "spun," we've got the added bonus of imagining blown glass with all those delicate threads of glass woven into each other.
Just like the previous stanzas, we have the same sort of rhyme scheme happening with another couplet ("sun" and "spun"). We can also assume Moore will be repeating that AABBC pattern throughout the poem, just with different rhymes. Again, it's all about symmetry, both in the appearance of the lines and sound of the words. (Check out "Form and Meter" and "Sound Check" for more.)
[…] move themselves with spotlight swiftness into the crevices- in and out, illuminating
Remember here that our speaker's still on about those shafts of sunlight, way back in the last stanza. So, light moves pretty quickly, right? (All you Trekkies out there know what we're talking about.) Those shafts of light are moving with "spotlight swiftness" (we get more S sound alliteration to emphasize the appearance of that swift spotlight). There's no escaping those shafts of light.
And not only are those rays swift, they're also getting into all those "crevices" that were previously hidden. So whatever's hanging out in those dark spots is suddenly getting the light shone on it.
That light's effective, too. It moves "in and out, illuminating"… well, it illuminates something, but once again we're cut off at the end of the stanza. Suffice it to say that this is one set of effective spotlights.
At this point we see that Moore has moved from the dark and cold imagery of "black jade" to something lighter and more illuminating, like "spun glass." We've done a 180 turn with all this bright language, but why?
For one thing, maybe the speaker is purposely breaking things up, both visually with those fishy looking lines and contextually with the differing language. So, just when we think we've got things down pat, the speaker dissolves it all and brings us into a new image or thought. And hey, you have to admit it's kind of fitting when we consider the theme of life and death announced by those "ash-heaps." Phenomena like "in and out," light and dark, life and death are constantly dissolving into one another, right?
Babies are born and older folks die—lots of dissolving and rebirth going on. (Go here for more on this idea of interconnection.)
So the ocean's ecosystem is part of this extended metaphor of life and death. Even the ocean's waves mimic the patterns we see in life, rising up and down with the tide.