DOI: 10.5061/dryad.612jm641f
title: Resource allocation to a structural biomaterial: induced production of byssal threads decreases growth of a marine mussel

description: 1. Specialized mechanical structures produced by organisms provide crucial
 fitness advantages. The energetic cost associated with producing these
 structural materials and the resulting energetic trade-off with growth,
 however, is rarely quantified. We evaluate resource allocation to
 structural material production within the context of an energetic
 framework by combining an experimental manipulation with an energetic
 model. 2. Mytilid bivalves produce byssus, a network of collagen-like
 threads that tethers individuals to hard substrate. We hypothesized that a
 manipulation that induces the production of more byssal threads would
 result in increased energetic cost and decreased growth. 3. In month-long
 field experiments in spring and autumn, we severed byssal threads across a
 range of frequencies (never, weekly, daily), and measured shell and tissue
 growth. We then quantified the costs associated with the production of
 byssal threads using a Scope for Growth model. 4. We found that byssal
 thread removal increased byssal thread production and decreased growth.
 The cost calculated per byssal thread was similar in the spring and autumn
 (~1 J/thread), but energy budget calculations differed by season, and
 depended on thread quantity and seasonal differences in assumptions of
 metabolic costs. 5. This work demonstrates that the cost of producing a
 structural material has a substantial effect on mussel energetic state.
 The energetic cost of producing byssal threads was 2-8% percent of the
 energy budget in control groups that had low byssal thread production, and
 increased 6 to 11-fold (up to 47%) in mussels induced to produce threads
 daily. 6. We propose that characterizing the trade-off between the cost of
 biomaterial production and growth has implications for understanding the
 role of trade-offs in adaptive evolution, and improved natural resource
 management and conservation practices.
description: The effect of thread severing frequency on thread production rate
 and growth of <i>Mytilus trossulus</i> was investigated in a
 field setting over 1 month in autumn 2013 (mid October to mid November)
 and in spring 2014 (late April to late May). <i>Mytilus
 trossulus,</i> ranging approximately 2-3 cm length (~80 to 200 mg
 dry weight tissue), were collected from Argyle Creek on San Juan Island,
 WA (Lat 48.521652°N and Long 123.014061°W) and transported to Friday
 Harbor Laboratories (Lat. 48.525350°N, Long. 123.012521°W). The
 pre-existing byssal threads were severed from each mussel using scissors
 before the mussel was placed in a flexible mesh enclosure (10 cm x 22 cm,
 HDPE vexar plastic, 1 cm<sup>2</sup> mesh size) suspended from
 a floating dock at ~1m depth. The three treatments differed in the
 frequency at which the newly produced byssal threads were severed during
 the experiment: never, weekly or daily (or 0, 1 and 7 times per week,
 respectively). Fifteen mussels were distributed evenly among three
 replicate enclosures for each treatment. Mussels in the
 “daily” treatment group were monitored for byssus production every day by
 counting and severing newly produced byssal threads. New byssal threads
 were also counted and severed for mussels in the “weekly” treatment group,
 but at a lower frequency of once per week. New byssal threads of the
 mussels in the “never” treatment group were not severed and were counted
 at the end of the 4-week experimental period. Thread production of each
 individual mussel was counted in all treatments, however in the “never”
 control group in spring only the total number of byssal threads produced
 by the group was recorded. This value was divided by sample size to obtain
 an average thread production for each individual in this single
 group. Shell growth was calculated as the change in
 shell length, measured with calipers (±0.1 mm). Buoyant weight (±0.001 g)
 was determined at the beginning and end of the experiment. At the end of
 the experiment, gonad and somatic tissues were removed from the shell and
 dried at 60°C to a constant weight, and the dried shell weight was
 measured (±0.01 g). Condition index (CI) was calculated by dividing dry
 tissue weight (g) by shell length cubed
 (mm<sup>3</sup>).
description: In the spring experiment seven mussels either were lost from the
 field enclosure or died during this experiment and were not included in
 the analysis (ID's 11, 31, 3, 44, 54, 88, 19). In the autumn
 experiment two mussels died and their final length and tissue mass at 29
 days was estimated from their 14 - 21 day growth rate, and the
 relationship between tissue weight and length cubed for the
 population (ID's 22, 37). In the spring experiment a single thread
 count was determined for the 'never' treatment population rather
 than for each individual (ID's 9 - 59,
 "thread_count").