Once a pair, Apple and Fitbit have called it quits on their relationship. A search Monday came up empty for consumers seeking Fitbit in the Apple online store.

According to the blog website, 9to5Mac, some Apple retail locations still had Fitbit devices for sale, but it also said those devices might not be there much longer.

"There's only a limited amount of shelf space in retail and that's the same for brick and mortar as it is for the web," said Patrick Moorhead, founder and principal analyst at Moor Insights and Strategy.

Fitness-tracking devices from Nike, Jawbone and Mio are currently available at the Apple online store. The Jawbone UP and Jawbone UP24 both support Apple's Health app and are featured in Apple's HealthKit section in the online store.

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Apple's HealthKit allows developers to make apps more useful by allowing them to access your health data. For example, you can allow the data from your blood pressure app to be automatically shared with your doctor. A few weeks ago, Fitbit announced it "currently has no plans to integrate with HealthKit."

"As there aren't commentary from either company, Fitbit could have been asking for better terms from Apple to get on-shelf. On the other hand, Apple could very well be providing more shelf space for products that support HealthKit. That would make a lot of sense," said Moorhead.

Fitbit did not return CNBC's request for a comment, and Apple responded by stating it does not comment on any of its vendors.