Thick fish feed with superior suction

Did you know fish eat with their abs?

Many fish use suction to eat. Instead of grabbing food directly with their jaws, fish open their mouths, causing a pressure change that draws water into their mouth, along with what the fish is trying to eat. It takes a lot of power to create a successful slurp. The suction forces that largemouth bass produce while feeding cannot be produced by the head muscles alone (Camp et al., 2015). Instead, the axial (body wall) muscles, known as epaxial (back) and hypaxial (belly) muscles, help produce enough force for these fish to use suction to feed! Certain head muscles are able to transmit these forces from the body muscles to the hyobranchial (aka feeding) elements in the head. One such muscle is the sternohyoideus, which acts as a bridge between the body and the head, transmitting hypaxial muscle power to help lower the floor of the mouth during suction feeding.

**Figure 1** Anatomy of the sternohyoideous muscle in the striped surfperch (E. lateralis) in lateral view. (A) Photograph of superficial dissection of a fresh specimen. (B) Drawing from the photograph demonstrating the length and height of the SH muscle. *From Lomax, Martinson et al., 2020*.

But that’s not the whole story!

In different species, the sternohyoideus (SH) muscle can be bigger or smaller relative to the rest of the body musculature. For example, in largemouth bass, the mass of the SH muscle is 1.7% of the axial muscle mass, in bluegill sunfish, the SH is 4.0%, and in striped surfperch the SH is 8.8% of the axial muscle mass. This difference in relative muscle mass sparked a new study led by Lomax and Martinson, which explores the relationship between SH size and function. The team hypothesized that the size of the SH muscle determines its function. This study predicted that smaller SH muscles, like those found in largemouth bass, simply transmit power between the hypaxial muscles and the head. While larger SH muscles, like those found in the striped surfperch, may have a duel function for transmission as well as power generation.

**Figure 2** VROMM animation of suction feeding in a striped surfperch. Three camera views with neurocranium, cleithrum, tracked markers (animated as white spheres), and body plane (dark blue) animated from body markers. *From Lomax, Martinson et al., 2020*.

New method for quantify fish feeding muscle function

If the SH muscle contributes power to feeding, the muscle should contract during the part of suction feeding when the fish’s mouth is open the widest, a point known as peak gape. To explore if larger SH muscles contribute power to fish suction feeding, the team developed a novel method for measuring muscle strain and shortening velocity in the SH muscle. In striped surfperch the fish’s skin is very tightly connected to the underlying bones and associated muscles. Researchers in this new study took advantage of this anatomy and sutured plastic craft beads to the skin, directly above parts of the skull that contribute to suction feeding, including where the SH muscle attaches to bone, and a general plane on the fish’s body to use as a reference. The team then filmed fish feeding events using high speed video cameras. From these videos they used Video Reconstruction of Moving Morphology, or VROMM, to measure how much the SH muscle shortened during feeding events, and how much power it could be contributing to suction feeding.

Figure 3 Neurocranial elevation (degrees) and retraction and depression of the urohyal and cleithrum (millimeter) markers from one feeding event. Left axis is neurocranial rotation in degrees relative to the body plane and right axis is retraction or depression (in millimeter) of the urohyal and cleithrum markers. Neurocranium, black; urohyal retraction, red dashed; urohyal depression, blue dashed; cleithrum retraction, red solid; cleithrum depression, blue solid. Time zero and gray dashed line correspond to peak gape. From *Lomax, Martinson et al., 2020.*

Larger Sternohyoideus muscles contribute power during suction feeding!

Measurements from this novel method found that the SH muscle shrinks to approximately 8% of the muscle’s original length during suction feeding. Additionally, the timing of the SH contraction, which moves the cleithrum and urohyal components of the feeding apparatus, coincides with the peak gape of the feeding event (Figure 3). While the power measured from this muscle may not contribute the majority of the power for suction feeding, these results support the hypothesis that larger SH muscles have a duel function of transmitting power from the body muscles *and* generating power for suction feeding! The team was also able to validate their new method of measuring muscle movements. Their method does underestimate the true muscle strain; however, it presents a lower cost option for measuring muscle strain in species with tight skin-muscle connections. This study extends previous work by making this science more accessible and supports the hypothesis that thick fish have superior suction… at least where the SH muscle is concerned!