Anyway I think golden apples are a better description of this mandala than barnacles, which was my first thought.
First of all, all my joins are invisible joins. I don't start with a slip stitch and I don't end with one. I start with no loop at all. I start the first stitch with nothing on the hook. Insert hook, YO and pull through foundation only, then one more ch for sc and 2 more ch for dc. Then after I complete the last stitch, I cut the yarn to about 6 inches, pull the loop on the hook out, thread the yarn onto a darning needle, wrap it around the top of the first stitch, and thread it back through where I pulled it out. If the round is sc then I just carry the yarn to the back, tie it to the start piece with an overhand knot (first half of a square knot), and weave the end in. Then I weave in the other end. Other ways are possible, but that's how this was made. In the case of dc, I have a tutorial here about how I make the invisible join. It's my own idiosyncratic method, so of course there are other ways to do it.
Yarns used: Vanna's choice linen, white, charcoal grey, silver grey, toffee, grey marble, and mustard, and a couple of scrap bits that I have no idea what they were originally.
Directions for this mandala are:
Round 1: Puce (a bit of yarn I've had forever) Start with magic loop, ch3, then 11 (dc, ch1), ending with invisible join around 3rd ch of ch3. Pull loop tight. 12 dc and 12 ch1 sp.
Round 2: Gold (also yarn I've had forever, in fact the first yarn I ever bought for myself) 2 dc in each ch1 space. 24 dc.
Round 3: (Linen) 1 hdc in each dc. LOOSELY. This stitch was not the best choice, because it was too tight, but if crocheted loosely, the next round will spread the stitches out and flatten them. 24 hdc
Round 4: (White) 2 dc in each hdc (48 dc).
Round 5: (Charcoal grey) 1 sc in each dc (48 sc).
Round 6: (Silver grey) dc 2 tog, ch 2 around. (24 ch2 sp).
Round 7: (Toffee) 3 dc in each sp. (72 dc).
An alternative is to make Wink's standard mandala up to round 6. Round 6 ends with 72 stitches. Her directions also illustrate magic ring and another way to do an invisible join.
Then make the shells. I used grey marble. Because of the way I join, I start with half a shell. Pull a loop through, ch2, 2 dc. Then (skip 2 st, sc in next, skip 2 st, 6 dc in next) 11 times. skip 2 st, sc in next, skip 2 st, 3 dc in first st, end with invisible join.
One more time, remove the hook and pull the loop through the next st of the previous circle. Complete the sc and then sc around, joining to the start of the round. Repeat the previous step for circles 4 to 11.