After clawing her way to the top at Red Lobster, Edna Morris is out as the chain’s president for letting hungry customers eat too much of its all-you-can-eat crab dinners.

She tried to boost profits with a $22.99 dinner promotion at Red Lobster, but instead lost millions by bungling her calculations on how many times patrons would refill their plates with pricey crab and the fixings.

“It wasn’t the second helping on all-you-can-eat but the third,” company Chairman Joe R. Lee told analysts.

“And maybe the fourth,” added Chief Operating Officer Dick Rivera, who’s taking over from Morris.

The crab fiasco was blamed for triggering a sell-off of shares in the parent, Darden Restaurants, wiping out $405.9 million of stock value in a single session.

Insiders said that Morris, who joined the seafood chain two years ago, had previously headed a major steak house chain, Quincy’s Family Steakhouse, where red meat was more quickly filling in all-you-can eat promos.

She didn’t anticipate that crab was much lighter, requiring more bites to fill up a customer.

The “Endless Crab” promotion ran for several months at the 679 eateries, but disappointed company brass and lopped off $3.3 million in first-quarter profits of Darden Restaurants from a year earlier.

Besides unexpected gorging by customers, the wholesale price of crabs skyrocketed to eat deeper into profits, the company explained.

The company’s more prosperous sister chain of 521 Olive Garden restaurants posted record sales of its Italian fare and offset the crab washout.

The crab gluttony helped bloat first-quarter food costs by more $31 million from a year earlier.

The chain says that weak sales will cause company profits to fall short in the next quarter. It expects per-share profits of between 15 and 18 cents, lower than the 22 cents earlier anticipated.

The bad news sent Darden’s stock plunging 12 percent, dropping to $18.90, down $2.47, in heavy trading of 8.7 million shares.

Two analysts – CIBC’s John Glass and RBC Capital Markets’ David Geraty – also downgraded the stock yesterday, accelerating the rout.