Composition follows habitat and depth in cryptobenthic fish assemblages: indicator value for Mediterranean coastal habitats

Imagine the ocean floor not as a flat, empty stage, but as a bustling, multi-layered city with distinct neighborhoods. Some districts are rocky and craggy, others are soft and sandy, and some are covered in ancient, colorful coral structures. Just like people in a city, the tiny creatures living there have strong preferences for where they hang out. In the world of marine biology, scientists study "cryptobenthic" fish—these are the shy, small residents that hide in the cracks and crevices of the seabed, often going unnoticed by casual observers. Because these fish are so picky about their specific "apartments" (the type of rock or sand they live on) and are very sensitive to changes in their environment, they act like living mood rings for the ocean. If the neighborhood changes, the fish change too. This makes them incredibly useful for scientists trying to figure out if a coastal area is healthy or in trouble, much like how a canary in a coal mine warns miners of danger. Understanding who lives where helps us map the hidden life of the sea and protect the delicate balance of these underwater communities.

Now, let's dive into a new study that decided to take a closer look at these hidden fish in the Mediterranean Sea, specifically in two bays in Türkiye: Saros Bay and Gökova Bay. The researchers acted like underwater detectives, swimming along timed paths between 0 and 40 meters deep (that's about the height of a 13-story building) during the summer and autumn months of 2022 through 2024. They were looking for the "who's who" of the fish world, focusing on four main families of small, bottom-dwelling fish: Gobiidae, Blenniidae, Tripterygiidae, and Gobiesocidae. In total, they spotted 38 different species. The most common "residents" they found in both bays were two specific types of gobies: Gobius geniporus and Gobius vittatus.

The big question was: what determines which fish live where? The study found that the fish community is organized like a well-planned city, but with a twist. The layout is shaped by two main things: the type of habitat (the "neighborhood" defined by the EUNIS classification system) and the depth (how deep down you go). However, depth turned out to be the stronger boss. It's as if the depth of the water is the main zoning law that decides who can live there, more so than the specific type of rock or sand. Interestingly, the study found that the number of different fish species (richness) and how evenly they were spread out didn't change much between different habitats or depths. Instead, the "vibe" of each neighborhood was defined by which specific species were there and how many of them there were, rather than just the total count of fish.

The researchers also tried to find "indicator species"—specific fish that act like a signature for a particular habitat, similar to how a specific bird might tell you you're in a pine forest. They identified 13 potential candidates, but here's the catch: the indicators were specific to each bay. A habitat group in Saros Bay might be signaled by one set of fish, while the exact same type of habitat in Gökova Bay was signaled by a completely different set. It's like how two different cities might have different "famous foods" that represent the same type of neighborhood.

When it came to where the fish were most abundant, the results varied by location. In Gökova Bay, the fish were least common on bare, soft sand and most common in the complex, rocky "coralligenous" habitats. In Saros Bay, the richest communities were found on the rocky "infralittoral" zones. Finally, the study checked if having a Marine Protected Area (MPA)—a zone where fishing is restricted—changed the fish mix. Surprisingly, the paper found that the fish composition did not differ between the protected and unprotected areas. The fish didn't seem to care about the legal boundaries in this specific study; they were just following the rules of their habitat and depth.