FLORHAM PARK, N.J. - Muhammad Wilkerson isn't the only noticeable absence on the Jets defensive line these days.

Sheldon Richardson, their big-talking but highly productive defensive tackle, has been a no-show during this week's organized team activities.

"I have no idea [why Richardson isn't here]. I mean, it is voluntary, so there's not much you can do," coach Todd Bowles said Wednesday after Day 2 of OTAs.

According to Bowles, Richardson was also a no-show on Tuesday, the first day of OTAs, and an explanation for his absence was not "conveyed."

The Jets coach also said he isn't sure when Richardson will attend practice.

"He's been in and out -- as have a lot of guys," Bowles said of Richardson, the 2013 NFL Defensive Rookie of the Year. "He's been here half the time, half the time he wasn't."

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But Bowles repeatedly stressed that these workouts are voluntary. The Jets' mandatory minicamp begins June 9.

"You hope everybody comes all the time, but unfortunately that's not the case," Bowles said. "Guys have things they do in the offseason that they've probably had scheduled that they can't afford to miss, so I understand that part as well."

Wilkerson, meanwhile, has been absent this offseason as his contract talks remain stalled. Under contract for 2015, the defensive lineman is seeking an extension and has not attended any offseason workouts. He is, however, expected to be here for mandatory minicamp.

Brandon Marshall also did not attend Wednesday's OTA session because he "had something to take care of," Bowles said. But the wide receiver was present on Tuesday and is expected to be at the facility on Thursday.

Notes & quotes: Slot receiver Jeremy Kerley has been nursing an Achilles and ankle injury "for a couple days now," according to Bowles.