TOKYO — Guam officials Friday afternoon ordered a commercial ship from South Korea to leave port after thousands of spiders poured out of cargo intended for a construction site where a facility is to be built to house crews that will one day build new U.S. military bases on the island.

The spiders were discovered Wednesday evening, shortly after the ship arrived, officials said. It left the port soon after but moored off the island as officials investigated.

By Friday afternoon, it remained unclear what species the spiders were and whether or not they were venomous or posed any danger to the island. No injuries or bites had been reported, officials said.

The about-face of the M.V. Altavia was issued after the Guam Department of Agriculture ruled the spiders an “invasive species” too numerous to destroy or contain, according to Joseph Torres, the department’s director.

“It was because of the quantity,” Torres said of the spiders, the largest of which had bodies as big as quarters. “It behooved us to take the most extreme measure.”

It’s not uncommon to find unwelcome life among ships and planes, cargo and passengers, who arrive at Guam’s seaport and airport, Torres said.

But the numbers and size of the spiders sent more than shivers through the island’s spine. It signaled another example of the challenges the island faces as the military works to turn Guam into a military hub for the Pacific in coming years.

“There’s only two animals that curl my skin,” Torres said during a phone call Friday afternoon from Guam. “Spiders and snakes.”

Both have been unwelcome visitors on the 212-square-mile island over the years. The brown tree snake came after World War II and has eaten away many of the island’s native bird species, according to the U.S. Department of Agriculture. Later came the coqui frog, which barks as loud as a dog. Four years ago, the coconut rhinoceros beetle showed up and began munching its way through the island’s palm trees.

Now, the military is working to implement an agreement with Japan to move more than 8,000 U.S. Marines from Okinawa to Guam by 2014. At the same time, the Pentagon has plans to add an Army air defense unit and expand berthing capabilities on the island so that aircraft carriers can pull into Apra Harbor for three weeks at a time.

All the construction equipment needed for the projects will flow through Apra Harbor, the island’s only seaport. It’s an influx that has some worried about what animals, plants and marine life may come along.

“I certainly brought that up as a major point of concern,” said Dennis Santo-Tomas, the director of the Guam Customs and Quarantine Agency.

He’s not the only one. The military acknowledged that the introduction of invasive species could have a “significant impact” on Guam in its draft environmental impact statement released last year. That document — which listed the buildup’s effects on everything from water supplies to population growth — drew sharp criticism from local and federal officials for its lack of details about mitigating problems the military may introduce onto the island.

A final copy of that statement is due out this month, and military officials have said they intend to slow down the construction pace to address some of the concerns.

Still, Santo-Tomas is bracing for the changes. He thinks his current staff of 100 customs officers — whose responsibility includes inspecting all incoming vessels at the port, Guam International Airport, Andersen Air Force Base, and Naval Base Guam — will need to increase by at least 30 officers each year during the construction rush.

The military’s construction for the buildup has not begun yet, but private companies like Younex have begun making investments. Younex is building the Ukudu Workforce Village, a housing area for expected construction workers on the northwestern side of the island, near the proposed site for the Marines’ new home.

It’s there that the M.V. Altavia’s cargo was headed on Wednesday, according to Bernadette Meno, spokeswoman for the Port Authority of Guam. Calls to local contacts for the Altavia on Friday went unanswered. A phone call to Younex’s Guam office on Friday afternoon was not returned.

The Altavia pulled in Wednesday, and customs officers checked its containers, Meno and Santo-Tomas said. Once Port Authority workers began moving the crates, the spiders began spilling out.

One container made it to the dock before officials realized the problem, Torres said, and it was quickly restacked. The ship was sent back out to moor and await further instructions. Both customs and agriculture officials inspected the boat on Thursday to see whether extermination was a possible solution.

By Friday afternoon, the answer was no. Meno said the port authority would work with the crew to resupply the ship for its journey.

Torres said the ship faces no penalties for the incident, and he cautioned not to blame the military buildup for the incident.

“Whether the military comes or not, it will happen,” he said of the invasive species scare.

When asked if the crew was safe for the trip home, Torres first laughed and admitted he wouldn’t want to be aboard. Then he said he really didn’t know.

“They must be safe,” he said finally, “because (the spiders) were on board already when it was coming (here).”