With Shark Week nearing its tail end yesterday, I had a notion to photograph all the zoo’s toothy finned predators: the sand-tiger sharks and white-tip reef shark of the Discovery Bay shark tank; the small leopard and horn sharks of D-Bay’s estuary; the big zebra shark (aka the other leopard shark) and the small epaulet and brown-banded bamboo sharks of the Tropics trail’s coral reef. It didn’t quite work out that way, though. As usual, the reef was photo-friendliest — especially on a sunny afternoon, with natural buttery light bathing the artificial coral. (At 60 feet long, the tank’s sheer size makes real coral impractical in there.) I wound up parked on a comfy bench, watching little boys marvel at the big zebra (or leopard) shark. (September update: The reef has closed for renovations, to reopen in February 2012 before the zoo’s annual Tropical Beach Party.)

It’s tempting to call this toothy guy a leopard shark because of his spots. And we would, except for the inevitable confusion with D-Bay’s smaller estuary dwellers of the same name, which live in water 20 degrees cooler off the California coast. This tropical zebra shark starts life as a pup with vertical yellow stripes and stays that way until the stripes dissolve into spots when he reaches a length of two or three feet. And like most sealife in this exhibit, he hails from IndoPacific waters. (So does the white-tip reef shark, but because he was a little too predatory in here, that sleek gray shark now lives with larger Atlantic-Caribbean creatures in Discovery Bay.)

My favorite thing to say about sharks is that of the 450 species, only about 10 are dangerous to humans. (My favorite shark-related phrase: “nictitating membrane,” a protective transparent third eyelid.) Though large, this zebra shark is not an aggressive species, though it’s said to bite quite painfully in the wild if threatened or provoked. This species is a nocturnal bottom-feeder, so it was a treat to see him circling at or above eye level yesterday, showing off his spots from various angles.

The shark question we volunteers get asked the most: Why don’t the sharks eat the other fish in the tank? The short answer: gel diet. A new zoo shark is kept in temporary solitude and fed its new mealtime staple, a cubed mixture of seaweed, ground-up seafood and vitamins, until it realizes there’s no need to “hunt.” Zookeepers sit on the rocks above the reef at 3 p.m. daily and feed the sharks cubes of gel diet on spears. It’s fun to watch the sharks’ sucking mouths approach the water’s surface, seeking their midafternoon lunch.

Even on an extra-crowded zoo day like yesterday, the coral reef is a reliable refuge: a place to take a load off and chat with a fellow volunteer while background music plays and children cry “Dory!” each time the blue palette tang from “Finding Nemo” swims into view. And here, a large circling shark can be seen as an educational diversion, not a threat. And in February, it will be again. Until then:

A forest of coral, with beautiful bivalves

I started volunteering at the zoo in November 2003, in weather much like this dark, rainy, gusty April we’ve been having. On the darkest days, I’d find myself standing in front of the live-coral tank in Discovery Bay, drinking in all the color and light of the underwater rainforest.

When that tank sprang a leak and went away, I had to go without my coral fix for months — until this bigger, better replacement arrived a year or two back. It takes time to build up a saltwater aquarium into this resplendent spectacle, and it’s always evolving. Veteran inhabitants like our “Finding Nemo”-style clownfish — see him floating in that central cave? — and his fishy sidekick Dory the blue tang share space with newer arrivals like my new favorite clam, the azure fellow down in the lower left corner. (There’s at least one more of him elsewhere in the aquarium, if you look closely.)

Like so much sealife, this beautiful bivalve has multiple names: The zoo labels it a boring clam because it bores down into the rock by releasing acids, then anchors itself to a chosen spot. But it’s also known as a burrowing, crocea or crocus clam, and the species’ coloration can range into oranges and golds. (Detailed information about Tridacna crocea is surprisingly scarce online, but I eventually found some from an aquarium store and reef hobbyists.) The crocea requires “intense” lighting and feeds itself via photosynthesis, like a plant. This one must have been “burrowing” as I watched it last week; using its byssal muscle and attaching itself to the floor with byssal threads or filaments, it opened and closed like a butterfly’s wings, almost appearing to breathe.

I can’t even count all the corals, fish and other creatures living in here, but one of my favorites is hiding leftward of the crocus clam in the photo below: the blue-dot or blue-spotted jawfish, which has dug itself a hole and is peering out, as is this species’ wont. (Here’s a closeup, too.) We have three new jawfish in here now, digging burrows and sometimes even swimming.

The world’s oceans, and this aquarium, contain countless varieties of coral with a host of melodious names: both the rocklike hard corals (staghorn, velvet stone, pineapple brain, trumpet) and the plantlike soft corals (mushroom, toadstool, bubble). Like its relative the floral-looking anemone, a coral is an animal, symmetrically arranged around a central mouth. Unlike anemones, individual coral polyps press together to form ever-growing colonies — ever-growing, at least, until environmental factors lead to coral bleaching and threaten the entire ecosystem of creatures that take shelter in the reef. Home to a quarter of all marine life, these reefs are dwindling in tropical seas worldwide. It’s not just their beauty that makes them worth saving.

There be dragons … and lions

Way back on Halloween, when my parents were visiting from southern Wisconsin, we stopped by my volunteer stomping grounds to check out the newer stuff: Faces of Africa in Tropics and the newly visible baby dolphin (she got her name this week, by the way: Taijah, pronounced Tay-sha). But the star attraction of this particular visit — along with all the small trick-or-treaters in animal costumes — had to be the weedy and leafy sea dragons in Discovery Bay.

Here we see one little weedy between two leafies — note his slimmer snout and less spectacular camouflage. Both varieties of dragon live in Australian waters and feed on tiny mysid shrimp, which they slurp through their strawlike mouths. Cousins of the seahorse, these dragons have fins but no skeletons; they control their buoyancy via a large swim bladder and change direction by nodding their heads. In the past year or so, they moved into the newest, bluest tank in D-Bay¬† — a roomier, more visually striking showcase than the smaller tank where a few weedies used to live. That tank is now home to this fearsome creature:

These impressively toxic spines make lionfish one of the most venomous fish in the ocean. Our old weedy-dragon tank contains two types: a spotfin (this guy, I’m pretty sure) and a dwarf fuzzy. The spines are used for defense, not offense, and the fish comes at you upside-down. Such an attack will rarely kill you, but its effects include nausea, extreme pain and even convulsions. Like sea dragons, lionfish come from Indo-Pacific waters. But they’ve made their way into the Atlantic and Caribbean, where a lack of predators has transformed them into an invasive species. Release by a fed-up home aquarist, or from Florida aquariums shattered by Hurricane Andrew in 1992, explains their presence. I’m just happy to be on the opposite side of the glass. Underwater, lions are far more terrifying than dragons.