DOI: 10.5061/dryad.1ns1rn93z
title: Data from: Large parasites in a crowded space: variation in prevalence and volumetric effects of <em>Sarcotaces arcticus</em> (Collett, 1874) in two host rockfish (<em>Sebastes</em> spp.) species.

description: Sarcotaces arcticus (Collett, 1874) is a relatively large, internal
 parasitic copepod that infects many marine fishes. Although its large size
 suggests it may have a negative effect on host reproduction by reducing
 space available in the abdominal cavity (i.e., volumetric effect), such
 quantitative aspects of host-parasite relationships for S. arcticus have
 never been documented. We compared the prevalence and the ratio of live to
 dead parasites among sizes and sexes of two species of rockfish hosts
 (Sebastes ciliatus, Tilesius, 1813, dark rockfish; and Sebastes
 variabilis, Pallas, 1814, dusky rockfish) and quantified the reduction of
 internal space available in infected hosts. Samples were collected in
 southeast Alaskan waters where the two host rockfish species coexist in
 sympatry. Both total prevalence and prevalence of live S. arcticus were
 significantly higher in S. variabilis compared to S. ciliatus, because of
 higher prevalence in female S. variabilis. The relationship between body
 cavity volume, volume available for reproduction, and total length was
 isometric for both host species combined. An average live S. arcticus with
 a volume of 8.1 milliliters occupied about 45% in smaller hosts and about
 5% in larger hosts of the volume available for reproductive organs. The
 high prevalence and large size of this parasite could significantly reduce
 fecundity and fitness of rockfish hosts through reductions in internal
 volume available for reproduction.
description: To assess the prevalence and effects on internal volume of hosts
 of <em>S. arcticus</em>, we surveyed two species of rockfish:
 <em>S. variabilis</em> and <em>S. ciliatus.</em>
 <em>Sebastes ciliatus</em> and <em>S.
 variabilis</em> are sister species that are commonly caught in the
 same areas. Both species are pelagic, occur in large aggregations, and
 feed on pelagic crustaceans in the water column. They are sympatric and
 syntopic in the areas we surveyed, such that fishing yields a mix of both
 species and sexes, and the depth at which both species are caught does not
 differ. We caught both species via hook and line in Frederick Sound, near
 Admiralty Island, AK, USA (56°57′14.76″ N, 134°10′36.19″ W). We were not
 able to collect individuals from the smallest size class (&lt;240 mm
 TL; ages 1–4 yr) of either host fish species. This study focuses on
 individuals larger than about 240 mm TL, which are the smallest
 individuals available in the large aggregations where we fished.
 <strong>Study system</strong> We
 caught a total of 182 <em>S. ciliatus</em> and 135
 <em>S. variabilis</em> from 7–10 July 2017. In addition to
 capturing the two host species, we caught 31 <em>S.
 ruberrimus</em>, 29 <em>S. maliger</em>, and small
 samples of other co-occurring rockfish species (<em>S.
 brevispinis</em>, <em>S. caurinus</em>, <em>S.
 flavidus</em>, <em>S. melanops</em>, <em>S.
 miniatus</em>, and <em>S. nigrocinctus</em>). On the
 same day the fish were caught, we dissected each fish for internal
 observation and detection of <em>S. arcticus</em> parasites.
 We recorded total length and standard length to the nearest millimeter of
 the host fish, determined the sex, collected muscle tissues and otolith
 samples, recorded whether each fish was parasitized by <em>S.
 arcticus</em>, and counted the total number of galls of <em>S.
 arcticus</em>. Only *S. ciliatus *and <em>S.
 variabilis</em> were parasitized by <em>S.
 arcticus</em>; therefore, none of the other
 <em>Sebastes</em> species were used for further analysis. We
 identified <em>S. arcticus</em> parasites in <em>S.
 ciliatus</em> and <em>S. variabilis</em> by the presence
 of an internal gall(s) near the anal vent of the fish. Then, we carefully
 extracted the galls to prevent rupture of the parasites and preserved them
 in 90% ethyl alcohol. In the lab, we dissected the gall and characterized
 each parasite by life stage. We separated <em>S. arcticus</em>
 into three life stages at the time of the collection: live small, live
 large, and dead. We classified stage one as a gall, up to 10 mm in length,
 containing a female but no black fluid (hemosiderin). Stage two was
 classified as a gall (&gt;10 mm in length) containing a female and
 hemosiderin, as well as the presence of eggs or nauplii. We classified
 stage three as the “scar” or dead stage, which we identified by a
 relatively small and hard gall, with no identifiable female, often with
 the presence of solidified hemosiderin (ranging from 5 to 20 mm in
 length). Additionally, we classified each fish into one of three size
 classes based on total length: small (fish between 24 to 37 cm; 5–10 years
 old), intermediate (fish between 37 to 44 cm;10–15 years old), and large
 (44+ cm; 15+ years old) [11]. In 2018, we collected 21
 <em>S. ciliatus</em> and 10 <em>S. variabilis</em>
 to assess the volumetric effects of <em>S. arcticus</em>
 infection. To measure available space, we extracted the organs of each
 fish and measured their volume in milliliters via water displacement.
 Major organs such as the heart, kidneys, and stomach were included in the
 measurement of organ volume. The stomachs were always empty due to
 barotrauma-induced extrusion during capture. The swim bladder was excluded
 from our measurements, since its volume is highly variable depending on
 the location of the fish in the water column. If the individual was
 reproductively mature, we removed their enlarged gonads before calculating
 organ volume. We did not remove gonads of reproductively immature
 individuals because they were only small, thread-like structures that
 would not significantly affect the volume available. After all the organs
 were removed, we measured the total volume of each fish’s body cavity in
 milliliters. To adjust the volume of the nonreproductive internal organs
 to account for the swim bladder, we took the volume of nonreproductive
 organs and added an estimated 20% of the total cavity volume to represent
 the swim bladder. We then divided nonreproductive organ volume by total
 cavity volume (×100) to get the percentage of total cavity volume occupied
 by nonreproductive organs. Further, to calculate volume
 available for reproduction, we subtracted nonreproductive organ volume
 from total cavity volume. From a separate set of host individuals, we
 collected 19 galls of <em>S. arcticus</em>. The volume of
 these galls was measured in milliliters via water displacement to
 establish an average volume occupied by a single <em>S.
 arcticus</em> infection. We then divided the average volume of a
 single <em>S. arcticus</em> individual by the calculated
 volume available for reproductive organs (×100) to get the percentage of
 normally available volume for reproductive organs that would be occupied
 by an individual <em>S. arcticus</em>.
 <strong>Analysis</strong> We
 calculated prevalence of <em>S. arcticus</em> for both species
 of host rockfish in relation to sex and size class. Prevalence was
 measured as the number of infected hosts divided by the total number of
 hosts examined for the specified group [14]. We compared prevalence
 of <em>S. arcticus</em> between species and sexes, and among
 size classes within species by sex combinations using contingency table
 analysis in Proc FREQ in SAS (SAS version 9.4, SAS Inc., Cary, NC, USA).
 Because some categories had an expected value &lt; 5, we used a
 one-tailed version of Fisher’s exact test for comparison. We calculated
 prevalence of live <em>S. arcticus</em> in relation to sex and
 size class of host in the same way as total prevalence using contingency
 table analysis in Proc FREQ in SAS (SAS version 9.4, SAS Inc., Cary, NC,
 USA). To determine differences in the distribution of live and dead
 parasites, we mapped parasite mortality structure (live or dead) among
 size classes of the host fishes by comparing the number of parasites that
 were live to the number that were dead within each host species by sex and
 by size class of host fishes. We used a contingency table analysis to test
 for deviation from the overall expected proportion of live and dead
 parasites among host groups using Proc FREQ in SAS (SAS version 9.4, SAS
 Inc., Cary, NC, USA). We performed an Analysis of
 Covariance (ANCOVA) to evaluate how total cavity volume and
 nonreproductive organ volume differed between host species and with total
 length (Table 1; R software, version 4.4.0). Because there was no
 significant species effect nor a significant species by total length
 interaction, we combined species for further analyses. We used linear
 regression to test against an expected slope of 3 (isometry for a
 length/volume comparison) between the (ln transformed) total length (mm)
 and (ln transformed) host cavity, and (ln transformed) nonreproductive
 organ volume. In addition, we used linear regression to determine if the
 percent of cavity volume available for reproduction occupied by an
 individual <em>S. arcticus</em> varied with host body size, as
 well as the percent of organ volume out of total cavity volume.
 We estimated the volume of the two dead parasite galls as 4
 milliliters each (half of the average live parasite volume) and subtracted
 them from the total measured gall volumes. For fish with multiple live
 parasites, we evenly distributed the volume to separate them into
 individual parasites. Subsequently, we determined the mean, minimum, and
 maximum percent of the potential reproductive volume an average
 <em>S. arcticus</em> occupied in the host species
 combined. We estimated the volume of the two dead
 parasite galls as 4 milliliters each (half of the average live parasite
 volume) and subtracted them from the total measured gall volumes. For fish
 with multiple live parasites, we evenly distributed the volume to separate
 them into individual parasites. Subsequently, we determined the mean,
 minimum, and maximum percent of the potential reproductive volume an
 average <em>S. arcticus</em> occupied in the host species
 combined.
description: # Large parasites in a crowded space: variation in prevalence and
 volumetric effects of *Sarcotaces arcticus* (Collett, 1874) in two host
 rockfish (*Sebastes* spp.) species
 [https://doi.org/10.5061/dryad.1ns1rn93z](https://doi.org/10.5061/dryad.1ns1rn93z) ## Description of the data and file structure Large parasites in a crowded space: variation in prevalence and volumetric effects of *Sarcotaces arcticus* in two host rockfish (*Sebastes* spp.) species ### Files and variables #### File: Sarcotaces_arcticus_dataset_Sept_2025.xlsx | TAB: All Fish Data, Columns  | Description | | :----------------------------------- | :------------------------------------------------------------------------------------------------------------------------------------------- | | Data collected from July 7–10, 2017 | | | Caught near Admiralty Island, Alaska | | | column 1: Fish ID | Fish identification number | | column 2: Species | Species name | | column 3: Total Length (mm) | Total length of fish in millimeters  | | column 4: Sex | F= Female, M=Male, J= Juvenile, N/A = unable to determine  | | column 5: Size Class | Size class of fish according based on total length. 1= 0-370 millimeters, 2= 370-440 millimeters, 3=440-480 millimeters, 4= 480+ millimeters | | column 6: Parasite/s Present | Yes= Parasite present, No= No parasite present  | | column 7: Number of Parasites | Number of parasites within fish  | | TAB: Parasite Data, Columns | Description | | :----------------------------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Data collected from July 7–10, 2017 | | | Caught near Admiralty Island, Alaska | | | column 1: Fish ID | Fish identification number | | column 2: Fish Species | Fish species name | | column 3: Fish Sex | F= Female, M=Male, J= Juvenile  | | column 4: Fish Size Class | Size class of fish based on total length. 1= 0-370 millimeters, 2= 370-440 millimeters, 3=440-480 millimeters, 4= 480+ millimeters | | column 5: Parasite TL | Total length of parasite in millimeters; "missing" means the measurement was not available, typically because the parasite cyst had been damaged during the fileting of the fish. | | column 6: Parasite TW | Total width of parasite in millimeters; "missing" means the measurement was not available, typically because the parasite cyst had been damaged during the fileting of the fish. | | column 7: Parasite Stage | 1= Stage 1; &gt; 10 millimeters containing a female but no hemosiderin,  2= Stage 2; &gt;10 millimeters containing a female and hemosiderin as well as eggs or nauplii, 3= Stage 3; dead stage identified by a small and hard gall, with no identifiable female often with the presence of solidified hemosiderin ( 5 - 20 millimeters in length). | | TAB: Volume Data, Columns | Description | | :---------------------------------------------- | :-------------------------------------------------- | | Data collected in 2018 | | | Caught near Admiralty Island, Alaska | | | column 1: Fish ID | Fish identification number | | column 2: Species | Species name | | column 3: Total Length (mm) | Total length of fish in millimeters  | | column 4: Sex | F= Female, M=Male, J= Juvenile  | | column 5: Cavity Volume (mL) | Volume of fish cavity in milliliters | | column 6: Organ volume without Swimbladder (mL) | Organ volume without the swimbladder in milliliters | | TAB: Parasite Volume, Columns | Description | | :----------------------------------- | :--------------------------------- | | Data collected in 2018 | | | Caught near Admiralty Island, Alaska | | | Column 1: Number of Parasites | Total number of parasites | | column 2: Volume (mL) | Volume of parasites in milliliters | | column 3: Number of Live | Total number of live parasites | | column 4: Number of Dead | Total number of dead parasites | ## Code/software Excel file